hugetlb: introduce nr_overcommit_hugepages sysctl

[linux-2.6] / mm / hugetlb.c

/*
 * Generic hugetlb support.
 * (C) William Irwin, April 2004
 */
#include <linux/gfp.h>
#include <linux/list.h>
#include <linux/init.h>
#include <linux/module.h>
#include <linux/mm.h>
#include <linux/sysctl.h>
#include <linux/highmem.h>
#include <linux/nodemask.h>
#include <linux/pagemap.h>
#include <linux/mempolicy.h>
#include <linux/cpuset.h>
#include <linux/mutex.h>

#include <asm/page.h>
#include <asm/pgtable.h>

#include <linux/hugetlb.h>
#include "internal.h"

const unsigned long hugetlb_zero = 0, hugetlb_infinity = ~0UL;
static unsigned long nr_huge_pages, free_huge_pages, resv_huge_pages;
static unsigned long surplus_huge_pages;
unsigned long max_huge_pages;
static struct list_head hugepage_freelists[MAX_NUMNODES];
static unsigned int nr_huge_pages_node[MAX_NUMNODES];
static unsigned int free_huge_pages_node[MAX_NUMNODES];
static unsigned int surplus_huge_pages_node[MAX_NUMNODES];
static gfp_t htlb_alloc_mask = GFP_HIGHUSER;
unsigned long hugepages_treat_as_movable;
int hugetlb_dynamic_pool;
unsigned long nr_overcommit_huge_pages;
static int hugetlb_next_nid;

/*
 * Protects updates to hugepage_freelists, nr_huge_pages, and free_huge_pages
 */
static DEFINE_SPINLOCK(hugetlb_lock);

static void clear_huge_page(struct page *page, unsigned long addr)
{
        int i;

        might_sleep();
        for (i = 0; i < (HPAGE_SIZE/PAGE_SIZE); i++) {
                cond_resched();
                clear_user_highpage(page + i, addr + i * PAGE_SIZE);
        }
}

static void copy_huge_page(struct page *dst, struct page *src,
                           unsigned long addr, struct vm_area_struct *vma)
{
        int i;

        might_sleep();
        for (i = 0; i < HPAGE_SIZE/PAGE_SIZE; i++) {
                cond_resched();
                copy_user_highpage(dst + i, src + i, addr + i*PAGE_SIZE, vma);
        }
}

static void enqueue_huge_page(struct page *page)
{
        int nid = page_to_nid(page);
        list_add(&page->lru, &hugepage_freelists[nid]);
        free_huge_pages++;
        free_huge_pages_node[nid]++;
}

static struct page *dequeue_huge_page(struct vm_area_struct *vma,
                                unsigned long address)
{
        int nid;
        struct page *page = NULL;
        struct mempolicy *mpol;
        struct zonelist *zonelist = huge_zonelist(vma, address,
                                        htlb_alloc_mask, &mpol);
        struct zone **z;

        for (z = zonelist->zones; *z; z++) {
                nid = zone_to_nid(*z);
                if (cpuset_zone_allowed_softwall(*z, htlb_alloc_mask) &&
                    !list_empty(&hugepage_freelists[nid])) {
                        page = list_entry(hugepage_freelists[nid].next,
                                          struct page, lru);
                        list_del(&page->lru);
                        free_huge_pages--;
                        free_huge_pages_node[nid]--;
                        if (vma && vma->vm_flags & VM_MAYSHARE)
                                resv_huge_pages--;
                        break;
                }
        }
        mpol_free(mpol);        /* unref if mpol !NULL */
        return page;
}

static void update_and_free_page(struct page *page)
{
        int i;
        nr_huge_pages--;
        nr_huge_pages_node[page_to_nid(page)]--;
        for (i = 0; i < (HPAGE_SIZE / PAGE_SIZE); i++) {
                page[i].flags &= ~(1 << PG_locked | 1 << PG_error | 1 << PG_referenced |
                                1 << PG_dirty | 1 << PG_active | 1 << PG_reserved |
                                1 << PG_private | 1<< PG_writeback);
        }
        set_compound_page_dtor(page, NULL);
        set_page_refcounted(page);
        __free_pages(page, HUGETLB_PAGE_ORDER);
}

static void free_huge_page(struct page *page)
{
        int nid = page_to_nid(page);
        struct address_space *mapping;

        mapping = (struct address_space *) page_private(page);
        BUG_ON(page_count(page));
        INIT_LIST_HEAD(&page->lru);

        spin_lock(&hugetlb_lock);
        if (surplus_huge_pages_node[nid]) {
                update_and_free_page(page);
                surplus_huge_pages--;
                surplus_huge_pages_node[nid]--;
        } else {
                enqueue_huge_page(page);
        }
        spin_unlock(&hugetlb_lock);
        if (mapping)
                hugetlb_put_quota(mapping, 1);
        set_page_private(page, 0);
}

/*
 * Increment or decrement surplus_huge_pages.  Keep node-specific counters
 * balanced by operating on them in a round-robin fashion.
 * Returns 1 if an adjustment was made.
 */
static int adjust_pool_surplus(int delta)
{
        static int prev_nid;
        int nid = prev_nid;
        int ret = 0;

        VM_BUG_ON(delta != -1 && delta != 1);
        do {
                nid = next_node(nid, node_online_map);
                if (nid == MAX_NUMNODES)
                        nid = first_node(node_online_map);

                /* To shrink on this node, there must be a surplus page */
                if (delta < 0 && !surplus_huge_pages_node[nid])
                        continue;
                /* Surplus cannot exceed the total number of pages */
                if (delta > 0 && surplus_huge_pages_node[nid] >=
                                                nr_huge_pages_node[nid])
                        continue;

                surplus_huge_pages += delta;
                surplus_huge_pages_node[nid] += delta;
                ret = 1;
                break;
        } while (nid != prev_nid);

        prev_nid = nid;
        return ret;
}

static struct page *alloc_fresh_huge_page_node(int nid)
{
        struct page *page;

        page = alloc_pages_node(nid,
                htlb_alloc_mask|__GFP_COMP|__GFP_THISNODE|__GFP_NOWARN,
                HUGETLB_PAGE_ORDER);
        if (page) {
                set_compound_page_dtor(page, free_huge_page);
                spin_lock(&hugetlb_lock);
                nr_huge_pages++;
                nr_huge_pages_node[nid]++;
                spin_unlock(&hugetlb_lock);
                put_page(page); /* free it into the hugepage allocator */
        }

        return page;
}

static int alloc_fresh_huge_page(void)
{
        struct page *page;
        int start_nid;
        int next_nid;
        int ret = 0;

        start_nid = hugetlb_next_nid;

        do {
                page = alloc_fresh_huge_page_node(hugetlb_next_nid);
                if (page)
                        ret = 1;
                /*
                 * Use a helper variable to find the next node and then
                 * copy it back to hugetlb_next_nid afterwards:
                 * otherwise there's a window in which a racer might
                 * pass invalid nid MAX_NUMNODES to alloc_pages_node.
                 * But we don't need to use a spin_lock here: it really
                 * doesn't matter if occasionally a racer chooses the
                 * same nid as we do.  Move nid forward in the mask even
                 * if we just successfully allocated a hugepage so that
                 * the next caller gets hugepages on the next node.
                 */
                next_nid = next_node(hugetlb_next_nid, node_online_map);
                if (next_nid == MAX_NUMNODES)
                        next_nid = first_node(node_online_map);
                hugetlb_next_nid = next_nid;
        } while (!page && hugetlb_next_nid != start_nid);

        return ret;
}

static struct page *alloc_buddy_huge_page(struct vm_area_struct *vma,
                                                unsigned long address)
{
        struct page *page;
        unsigned int nid;

        /* Check if the dynamic pool is enabled */
        if (!hugetlb_dynamic_pool)
                return NULL;

        /*
         * Assume we will successfully allocate the surplus page to
         * prevent racing processes from causing the surplus to exceed
         * overcommit
         *
         * This however introduces a different race, where a process B
         * tries to grow the static hugepage pool while alloc_pages() is
         * called by process A. B will only examine the per-node
         * counters in determining if surplus huge pages can be
         * converted to normal huge pages in adjust_pool_surplus(). A
         * won't be able to increment the per-node counter, until the
         * lock is dropped by B, but B doesn't drop hugetlb_lock until
         * no more huge pages can be converted from surplus to normal
         * state (and doesn't try to convert again). Thus, we have a
         * case where a surplus huge page exists, the pool is grown, and
         * the surplus huge page still exists after, even though it
         * should just have been converted to a normal huge page. This
         * does not leak memory, though, as the hugepage will be freed
         * once it is out of use. It also does not allow the counters to
         * go out of whack in adjust_pool_surplus() as we don't modify
         * the node values until we've gotten the hugepage and only the
         * per-node value is checked there.
         */
        spin_lock(&hugetlb_lock);
        if (surplus_huge_pages >= nr_overcommit_huge_pages) {
                spin_unlock(&hugetlb_lock);
                return NULL;
        } else {
                nr_huge_pages++;
                surplus_huge_pages++;
        }
        spin_unlock(&hugetlb_lock);

        page = alloc_pages(htlb_alloc_mask|__GFP_COMP|__GFP_NOWARN,
                                        HUGETLB_PAGE_ORDER);

        spin_lock(&hugetlb_lock);
        if (page) {
                nid = page_to_nid(page);
                set_compound_page_dtor(page, free_huge_page);
                /*
                 * We incremented the global counters already
                 */
                nr_huge_pages_node[nid]++;
                surplus_huge_pages_node[nid]++;
        } else {
                nr_huge_pages--;
                surplus_huge_pages--;
        }
        spin_unlock(&hugetlb_lock);

        return page;
}

/*
 * Increase the hugetlb pool such that it can accomodate a reservation
 * of size 'delta'.
 */
static int gather_surplus_pages(int delta)
{
        struct list_head surplus_list;
        struct page *page, *tmp;
        int ret, i;
        int needed, allocated;

        needed = (resv_huge_pages + delta) - free_huge_pages;
        if (needed <= 0)
                return 0;

        allocated = 0;
        INIT_LIST_HEAD(&surplus_list);

        ret = -ENOMEM;
retry:
        spin_unlock(&hugetlb_lock);
        for (i = 0; i < needed; i++) {
                page = alloc_buddy_huge_page(NULL, 0);
                if (!page) {
                        /*
                         * We were not able to allocate enough pages to
                         * satisfy the entire reservation so we free what
                         * we've allocated so far.
                         */
                        spin_lock(&hugetlb_lock);
                        needed = 0;
                        goto free;
                }

                list_add(&page->lru, &surplus_list);
        }
        allocated += needed;

        /*
         * After retaking hugetlb_lock, we need to recalculate 'needed'
         * because either resv_huge_pages or free_huge_pages may have changed.
         */
        spin_lock(&hugetlb_lock);
        needed = (resv_huge_pages + delta) - (free_huge_pages + allocated);
        if (needed > 0)
                goto retry;

        /*
         * The surplus_list now contains _at_least_ the number of extra pages
         * needed to accomodate the reservation.  Add the appropriate number
         * of pages to the hugetlb pool and free the extras back to the buddy
         * allocator.
         */
        needed += allocated;
        ret = 0;
free:
        list_for_each_entry_safe(page, tmp, &surplus_list, lru) {
                list_del(&page->lru);
                if ((--needed) >= 0)
                        enqueue_huge_page(page);
                else {
                        /*
                         * Decrement the refcount and free the page using its
                         * destructor.  This must be done with hugetlb_lock
                         * unlocked which is safe because free_huge_page takes
                         * hugetlb_lock before deciding how to free the page.
                         */
                        spin_unlock(&hugetlb_lock);
                        put_page(page);
                        spin_lock(&hugetlb_lock);
                }
        }

        return ret;
}

/*
 * When releasing a hugetlb pool reservation, any surplus pages that were
 * allocated to satisfy the reservation must be explicitly freed if they were
 * never used.
 */
static void return_unused_surplus_pages(unsigned long unused_resv_pages)
{
        static int nid = -1;
        struct page *page;
        unsigned long nr_pages;

        nr_pages = min(unused_resv_pages, surplus_huge_pages);

        while (nr_pages) {
                nid = next_node(nid, node_online_map);
                if (nid == MAX_NUMNODES)
                        nid = first_node(node_online_map);

                if (!surplus_huge_pages_node[nid])
                        continue;

                if (!list_empty(&hugepage_freelists[nid])) {
                        page = list_entry(hugepage_freelists[nid].next,
                                          struct page, lru);
                        list_del(&page->lru);
                        update_and_free_page(page);
                        free_huge_pages--;
                        free_huge_pages_node[nid]--;
                        surplus_huge_pages--;
                        surplus_huge_pages_node[nid]--;
                        nr_pages--;
                }
        }
}


static struct page *alloc_huge_page_shared(struct vm_area_struct *vma,
                                                unsigned long addr)
{
        struct page *page;

        spin_lock(&hugetlb_lock);
        page = dequeue_huge_page(vma, addr);
        spin_unlock(&hugetlb_lock);
        return page ? page : ERR_PTR(-VM_FAULT_OOM);
}

static struct page *alloc_huge_page_private(struct vm_area_struct *vma,
                                                unsigned long addr)
{
        struct page *page = NULL;

        if (hugetlb_get_quota(vma->vm_file->f_mapping, 1))
                return ERR_PTR(-VM_FAULT_SIGBUS);

        spin_lock(&hugetlb_lock);
        if (free_huge_pages > resv_huge_pages)
                page = dequeue_huge_page(vma, addr);
        spin_unlock(&hugetlb_lock);
        if (!page)
                page = alloc_buddy_huge_page(vma, addr);
        return page ? page : ERR_PTR(-VM_FAULT_OOM);
}

static struct page *alloc_huge_page(struct vm_area_struct *vma,
                                    unsigned long addr)
{
        struct page *page;
        struct address_space *mapping = vma->vm_file->f_mapping;

        if (vma->vm_flags & VM_MAYSHARE)
                page = alloc_huge_page_shared(vma, addr);
        else
                page = alloc_huge_page_private(vma, addr);

        if (!IS_ERR(page)) {
                set_page_refcounted(page);
                set_page_private(page, (unsigned long) mapping);
        }
        return page;
}

static int __init hugetlb_init(void)
{
        unsigned long i;

        if (HPAGE_SHIFT == 0)
                return 0;

        for (i = 0; i < MAX_NUMNODES; ++i)
                INIT_LIST_HEAD(&hugepage_freelists[i]);

        hugetlb_next_nid = first_node(node_online_map);

        for (i = 0; i < max_huge_pages; ++i) {
                if (!alloc_fresh_huge_page())
                        break;
        }
        max_huge_pages = free_huge_pages = nr_huge_pages = i;
        printk("Total HugeTLB memory allocated, %ld\n", free_huge_pages);
        return 0;
}
module_init(hugetlb_init);

static int __init hugetlb_setup(char *s)
{
        if (sscanf(s, "%lu", &max_huge_pages) <= 0)
                max_huge_pages = 0;
        return 1;
}
__setup("hugepages=", hugetlb_setup);

static unsigned int cpuset_mems_nr(unsigned int *array)
{
        int node;
        unsigned int nr = 0;

        for_each_node_mask(node, cpuset_current_mems_allowed)
                nr += array[node];

        return nr;
}

#ifdef CONFIG_SYSCTL
#ifdef CONFIG_HIGHMEM
static void try_to_free_low(unsigned long count)
{
        int i;

        for (i = 0; i < MAX_NUMNODES; ++i) {
                struct page *page, *next;
                list_for_each_entry_safe(page, next, &hugepage_freelists[i], lru) {
                        if (count >= nr_huge_pages)
                                return;
                        if (PageHighMem(page))
                                continue;
                        list_del(&page->lru);
                        update_and_free_page(page);
                        free_huge_pages--;
                        free_huge_pages_node[page_to_nid(page)]--;
                }
        }
}
#else
static inline void try_to_free_low(unsigned long count)
{
}
#endif

#define persistent_huge_pages (nr_huge_pages - surplus_huge_pages)
static unsigned long set_max_huge_pages(unsigned long count)
{
        unsigned long min_count, ret;

        /*
         * Increase the pool size
         * First take pages out of surplus state.  Then make up the
         * remaining difference by allocating fresh huge pages.
         *
         * We might race with alloc_buddy_huge_page() here and be unable
         * to convert a surplus huge page to a normal huge page. That is
         * not critical, though, it just means the overall size of the
         * pool might be one hugepage larger than it needs to be, but
         * within all the constraints specified by the sysctls.
         */
        spin_lock(&hugetlb_lock);
        while (surplus_huge_pages && count > persistent_huge_pages) {
                if (!adjust_pool_surplus(-1))
                        break;
        }

        while (count > persistent_huge_pages) {
                int ret;
                /*
                 * If this allocation races such that we no longer need the
                 * page, free_huge_page will handle it by freeing the page
                 * and reducing the surplus.
                 */
                spin_unlock(&hugetlb_lock);
                ret = alloc_fresh_huge_page();
                spin_lock(&hugetlb_lock);
                if (!ret)
                        goto out;

        }

        /*
         * Decrease the pool size
         * First return free pages to the buddy allocator (being careful
         * to keep enough around to satisfy reservations).  Then place
         * pages into surplus state as needed so the pool will shrink
         * to the desired size as pages become free.
         *
         * By placing pages into the surplus state independent of the
         * overcommit value, we are allowing the surplus pool size to
         * exceed overcommit. There are few sane options here. Since
         * alloc_buddy_huge_page() is checking the global counter,
         * though, we'll note that we're not allowed to exceed surplus
         * and won't grow the pool anywhere else. Not until one of the
         * sysctls are changed, or the surplus pages go out of use.
         */
        min_count = resv_huge_pages + nr_huge_pages - free_huge_pages;
        min_count = max(count, min_count);
        try_to_free_low(min_count);
        while (min_count < persistent_huge_pages) {
                struct page *page = dequeue_huge_page(NULL, 0);
                if (!page)
                        break;
                update_and_free_page(page);
        }
        while (count < persistent_huge_pages) {
                if (!adjust_pool_surplus(1))
                        break;
        }
out:
        ret = persistent_huge_pages;
        spin_unlock(&hugetlb_lock);
        return ret;
}

int hugetlb_sysctl_handler(struct ctl_table *table, int write,
                           struct file *file, void __user *buffer,
                           size_t *length, loff_t *ppos)
{
        proc_doulongvec_minmax(table, write, file, buffer, length, ppos);
        max_huge_pages = set_max_huge_pages(max_huge_pages);
        return 0;
}

int hugetlb_treat_movable_handler(struct ctl_table *table, int write,
                        struct file *file, void __user *buffer,
                        size_t *length, loff_t *ppos)
{
        proc_dointvec(table, write, file, buffer, length, ppos);
        if (hugepages_treat_as_movable)
                htlb_alloc_mask = GFP_HIGHUSER_MOVABLE;
        else
                htlb_alloc_mask = GFP_HIGHUSER;
        return 0;
}

#endif /* CONFIG_SYSCTL */

int hugetlb_report_meminfo(char *buf)
{
        return sprintf(buf,
                        "HugePages_Total: %5lu\n"
                        "HugePages_Free:  %5lu\n"
                        "HugePages_Rsvd:  %5lu\n"
                        "HugePages_Surp:  %5lu\n"
                        "Hugepagesize:    %5lu kB\n",
                        nr_huge_pages,
                        free_huge_pages,
                        resv_huge_pages,
                        surplus_huge_pages,
                        HPAGE_SIZE/1024);
}

int hugetlb_report_node_meminfo(int nid, char *buf)
{
        return sprintf(buf,
                "Node %d HugePages_Total: %5u\n"
                "Node %d HugePages_Free:  %5u\n",
                nid, nr_huge_pages_node[nid],
                nid, free_huge_pages_node[nid]);
}

/* Return the number pages of memory we physically have, in PAGE_SIZE units. */
unsigned long hugetlb_total_pages(void)
{
        return nr_huge_pages * (HPAGE_SIZE / PAGE_SIZE);
}

/*
 * We cannot handle pagefaults against hugetlb pages at all.  They cause
 * handle_mm_fault() to try to instantiate regular-sized pages in the
 * hugegpage VMA.  do_page_fault() is supposed to trap this, so BUG is we get
 * this far.
 */
static int hugetlb_vm_op_fault(struct vm_area_struct *vma, struct vm_fault *vmf)
{
        BUG();
        return 0;
}

struct vm_operations_struct hugetlb_vm_ops = {
        .fault = hugetlb_vm_op_fault,
};

static pte_t make_huge_pte(struct vm_area_struct *vma, struct page *page,
                                int writable)
{
        pte_t entry;

        if (writable) {
                entry =
                    pte_mkwrite(pte_mkdirty(mk_pte(page, vma->vm_page_prot)));
        } else {
                entry = pte_wrprotect(mk_pte(page, vma->vm_page_prot));
        }
        entry = pte_mkyoung(entry);
        entry = pte_mkhuge(entry);

        return entry;
}

static void set_huge_ptep_writable(struct vm_area_struct *vma,
                                   unsigned long address, pte_t *ptep)
{
        pte_t entry;

        entry = pte_mkwrite(pte_mkdirty(*ptep));
        if (ptep_set_access_flags(vma, address, ptep, entry, 1)) {
                update_mmu_cache(vma, address, entry);
        }
}


int copy_hugetlb_page_range(struct mm_struct *dst, struct mm_struct *src,
                            struct vm_area_struct *vma)
{
        pte_t *src_pte, *dst_pte, entry;
        struct page *ptepage;
        unsigned long addr;
        int cow;

        cow = (vma->vm_flags & (VM_SHARED | VM_MAYWRITE)) == VM_MAYWRITE;

        for (addr = vma->vm_start; addr < vma->vm_end; addr += HPAGE_SIZE) {
                src_pte = huge_pte_offset(src, addr);
                if (!src_pte)
                        continue;
                dst_pte = huge_pte_alloc(dst, addr);
                if (!dst_pte)
                        goto nomem;
                spin_lock(&dst->page_table_lock);
                spin_lock(&src->page_table_lock);
                if (!pte_none(*src_pte)) {
                        if (cow)
                                ptep_set_wrprotect(src, addr, src_pte);
                        entry = *src_pte;
                        ptepage = pte_page(entry);
                        get_page(ptepage);
                        set_huge_pte_at(dst, addr, dst_pte, entry);
                }
                spin_unlock(&src->page_table_lock);
                spin_unlock(&dst->page_table_lock);
        }
        return 0;

nomem:
        return -ENOMEM;
}

void __unmap_hugepage_range(struct vm_area_struct *vma, unsigned long start,
                            unsigned long end)
{
        struct mm_struct *mm = vma->vm_mm;
        unsigned long address;
        pte_t *ptep;
        pte_t pte;
        struct page *page;
        struct page *tmp;
        /*
         * A page gathering list, protected by per file i_mmap_lock. The
         * lock is used to avoid list corruption from multiple unmapping
         * of the same page since we are using page->lru.
         */
        LIST_HEAD(page_list);

        WARN_ON(!is_vm_hugetlb_page(vma));
        BUG_ON(start & ~HPAGE_MASK);
        BUG_ON(end & ~HPAGE_MASK);

        spin_lock(&mm->page_table_lock);
        for (address = start; address < end; address += HPAGE_SIZE) {
                ptep = huge_pte_offset(mm, address);
                if (!ptep)
                        continue;

                if (huge_pmd_unshare(mm, &address, ptep))
                        continue;

                pte = huge_ptep_get_and_clear(mm, address, ptep);
                if (pte_none(pte))
                        continue;

                page = pte_page(pte);
                if (pte_dirty(pte))
                        set_page_dirty(page);
                list_add(&page->lru, &page_list);
        }
        spin_unlock(&mm->page_table_lock);
        flush_tlb_range(vma, start, end);
        list_for_each_entry_safe(page, tmp, &page_list, lru) {
                list_del(&page->lru);
                put_page(page);
        }
}

void unmap_hugepage_range(struct vm_area_struct *vma, unsigned long start,
                          unsigned long end)
{
        /*
         * It is undesirable to test vma->vm_file as it should be non-null
         * for valid hugetlb area. However, vm_file will be NULL in the error
         * cleanup path of do_mmap_pgoff. When hugetlbfs ->mmap method fails,
         * do_mmap_pgoff() nullifies vma->vm_file before calling this function
         * to clean up. Since no pte has actually been setup, it is safe to
         * do nothing in this case.
         */
        if (vma->vm_file) {
                spin_lock(&vma->vm_file->f_mapping->i_mmap_lock);
                __unmap_hugepage_range(vma, start, end);
                spin_unlock(&vma->vm_file->f_mapping->i_mmap_lock);
        }
}

static int hugetlb_cow(struct mm_struct *mm, struct vm_area_struct *vma,
                        unsigned long address, pte_t *ptep, pte_t pte)
{
        struct page *old_page, *new_page;
        int avoidcopy;

        old_page = pte_page(pte);

        /* If no-one else is actually using this page, avoid the copy
         * and just make the page writable */
        avoidcopy = (page_count(old_page) == 1);
        if (avoidcopy) {
                set_huge_ptep_writable(vma, address, ptep);
                return 0;
        }

        page_cache_get(old_page);
        new_page = alloc_huge_page(vma, address);

        if (IS_ERR(new_page)) {
                page_cache_release(old_page);
                return -PTR_ERR(new_page);
        }

        spin_unlock(&mm->page_table_lock);
        copy_huge_page(new_page, old_page, address, vma);
        spin_lock(&mm->page_table_lock);

        ptep = huge_pte_offset(mm, address & HPAGE_MASK);
        if (likely(pte_same(*ptep, pte))) {
                /* Break COW */
                set_huge_pte_at(mm, address, ptep,
                                make_huge_pte(vma, new_page, 1));
                /* Make the old page be freed below */
                new_page = old_page;
        }
        page_cache_release(new_page);
        page_cache_release(old_page);
        return 0;
}

static int hugetlb_no_page(struct mm_struct *mm, struct vm_area_struct *vma,
                        unsigned long address, pte_t *ptep, int write_access)
{
        int ret = VM_FAULT_SIGBUS;
        unsigned long idx;
        unsigned long size;
        struct page *page;
        struct address_space *mapping;
        pte_t new_pte;

        mapping = vma->vm_file->f_mapping;
        idx = ((address - vma->vm_start) >> HPAGE_SHIFT)
                + (vma->vm_pgoff >> (HPAGE_SHIFT - PAGE_SHIFT));

        /*
         * Use page lock to guard against racing truncation
         * before we get page_table_lock.
         */
retry:
        page = find_lock_page(mapping, idx);
        if (!page) {
                size = i_size_read(mapping->host) >> HPAGE_SHIFT;
                if (idx >= size)
                        goto out;
                page = alloc_huge_page(vma, address);
                if (IS_ERR(page)) {
                        ret = -PTR_ERR(page);
                        goto out;
                }
                clear_huge_page(page, address);

                if (vma->vm_flags & VM_SHARED) {
                        int err;
                        struct inode *inode = mapping->host;

                        err = add_to_page_cache(page, mapping, idx, GFP_KERNEL);
                        if (err) {
                                put_page(page);
                                if (err == -EEXIST)
                                        goto retry;
                                goto out;
                        }

                        spin_lock(&inode->i_lock);
                        inode->i_blocks += BLOCKS_PER_HUGEPAGE;
                        spin_unlock(&inode->i_lock);
                } else
                        lock_page(page);
        }

        spin_lock(&mm->page_table_lock);
        size = i_size_read(mapping->host) >> HPAGE_SHIFT;
        if (idx >= size)
                goto backout;

        ret = 0;
        if (!pte_none(*ptep))
                goto backout;

        new_pte = make_huge_pte(vma, page, ((vma->vm_flags & VM_WRITE)
                                && (vma->vm_flags & VM_SHARED)));
        set_huge_pte_at(mm, address, ptep, new_pte);

        if (write_access && !(vma->vm_flags & VM_SHARED)) {
                /* Optimization, do the COW without a second fault */
                ret = hugetlb_cow(mm, vma, address, ptep, new_pte);
        }

        spin_unlock(&mm->page_table_lock);
        unlock_page(page);
out:
        return ret;

backout:
        spin_unlock(&mm->page_table_lock);
        unlock_page(page);
        put_page(page);
        goto out;
}

int hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
                        unsigned long address, int write_access)
{
        pte_t *ptep;
        pte_t entry;
        int ret;
        static DEFINE_MUTEX(hugetlb_instantiation_mutex);

        ptep = huge_pte_alloc(mm, address);
        if (!ptep)
                return VM_FAULT_OOM;

        /*
         * Serialize hugepage allocation and instantiation, so that we don't
         * get spurious allocation failures if two CPUs race to instantiate
         * the same page in the page cache.
         */
        mutex_lock(&hugetlb_instantiation_mutex);
        entry = *ptep;
        if (pte_none(entry)) {
                ret = hugetlb_no_page(mm, vma, address, ptep, write_access);
                mutex_unlock(&hugetlb_instantiation_mutex);
                return ret;
        }

        ret = 0;

        spin_lock(&mm->page_table_lock);
        /* Check for a racing update before calling hugetlb_cow */
        if (likely(pte_same(entry, *ptep)))
                if (write_access && !pte_write(entry))
                        ret = hugetlb_cow(mm, vma, address, ptep, entry);
        spin_unlock(&mm->page_table_lock);
        mutex_unlock(&hugetlb_instantiation_mutex);

        return ret;
}

int follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
                        struct page **pages, struct vm_area_struct **vmas,
                        unsigned long *position, int *length, int i,
                        int write)
{
        unsigned long pfn_offset;
        unsigned long vaddr = *position;
        int remainder = *length;

        spin_lock(&mm->page_table_lock);
        while (vaddr < vma->vm_end && remainder) {
                pte_t *pte;
                struct page *page;

                /*
                 * Some archs (sparc64, sh*) have multiple pte_ts to
                 * each hugepage.  We have to make * sure we get the
                 * first, for the page indexing below to work.
                 */
                pte = huge_pte_offset(mm, vaddr & HPAGE_MASK);

                if (!pte || pte_none(*pte) || (write && !pte_write(*pte))) {
                        int ret;

                        spin_unlock(&mm->page_table_lock);
                        ret = hugetlb_fault(mm, vma, vaddr, write);
                        spin_lock(&mm->page_table_lock);
                        if (!(ret & VM_FAULT_ERROR))
                                continue;

                        remainder = 0;
                        if (!i)
                                i = -EFAULT;
                        break;
                }

                pfn_offset = (vaddr & ~HPAGE_MASK) >> PAGE_SHIFT;
                page = pte_page(*pte);
same_page:
                if (pages) {
                        get_page(page);
                        pages[i] = page + pfn_offset;
                }

                if (vmas)
                        vmas[i] = vma;

                vaddr += PAGE_SIZE;
                ++pfn_offset;
                --remainder;
                ++i;
                if (vaddr < vma->vm_end && remainder &&
                                pfn_offset < HPAGE_SIZE/PAGE_SIZE) {
                        /*
                         * We use pfn_offset to avoid touching the pageframes
                         * of this compound page.
                         */
                        goto same_page;
                }
        }
        spin_unlock(&mm->page_table_lock);
        *length = remainder;
        *position = vaddr;

        return i;
}

void hugetlb_change_protection(struct vm_area_struct *vma,
                unsigned long address, unsigned long end, pgprot_t newprot)
{
        struct mm_struct *mm = vma->vm_mm;
        unsigned long start = address;
        pte_t *ptep;
        pte_t pte;

        BUG_ON(address >= end);
        flush_cache_range(vma, address, end);

        spin_lock(&vma->vm_file->f_mapping->i_mmap_lock);
        spin_lock(&mm->page_table_lock);
        for (; address < end; address += HPAGE_SIZE) {
                ptep = huge_pte_offset(mm, address);
                if (!ptep)
                        continue;
                if (huge_pmd_unshare(mm, &address, ptep))
                        continue;
                if (!pte_none(*ptep)) {
                        pte = huge_ptep_get_and_clear(mm, address, ptep);
                        pte = pte_mkhuge(pte_modify(pte, newprot));
                        set_huge_pte_at(mm, address, ptep, pte);
                }
        }
        spin_unlock(&mm->page_table_lock);
        spin_unlock(&vma->vm_file->f_mapping->i_mmap_lock);

        flush_tlb_range(vma, start, end);
}

struct file_region {
        struct list_head link;
        long from;
        long to;
};

static long region_add(struct list_head *head, long f, long t)
{
        struct file_region *rg, *nrg, *trg;

        /* Locate the region we are either in or before. */
        list_for_each_entry(rg, head, link)
                if (f <= rg->to)
                        break;

        /* Round our left edge to the current segment if it encloses us. */
        if (f > rg->from)
                f = rg->from;

        /* Check for and consume any regions we now overlap with. */
        nrg = rg;
        list_for_each_entry_safe(rg, trg, rg->link.prev, link) {
                if (&rg->link == head)
                        break;
                if (rg->from > t)
                        break;

                /* If this area reaches higher then extend our area to
                 * include it completely.  If this is not the first area
                 * which we intend to reuse, free it. */
                if (rg->to > t)
                        t = rg->to;
                if (rg != nrg) {
                        list_del(&rg->link);
                        kfree(rg);
                }
        }
        nrg->from = f;
        nrg->to = t;
        return 0;
}

static long region_chg(struct list_head *head, long f, long t)
{
        struct file_region *rg, *nrg;
        long chg = 0;

        /* Locate the region we are before or in. */
        list_for_each_entry(rg, head, link)
                if (f <= rg->to)
                        break;

        /* If we are below the current region then a new region is required.
         * Subtle, allocate a new region at the position but make it zero
         * size such that we can guarantee to record the reservation. */
        if (&rg->link == head || t < rg->from) {
                nrg = kmalloc(sizeof(*nrg), GFP_KERNEL);
                if (!nrg)
                        return -ENOMEM;
                nrg->from = f;
                nrg->to   = f;
                INIT_LIST_HEAD(&nrg->link);
                list_add(&nrg->link, rg->link.prev);

                return t - f;
        }

        /* Round our left edge to the current segment if it encloses us. */
        if (f > rg->from)
                f = rg->from;
        chg = t - f;

        /* Check for and consume any regions we now overlap with. */
        list_for_each_entry(rg, rg->link.prev, link) {
                if (&rg->link == head)
                        break;
                if (rg->from > t)
                        return chg;

                /* We overlap with this area, if it extends futher than
                 * us then we must extend ourselves.  Account for its
                 * existing reservation. */
                if (rg->to > t) {
                        chg += rg->to - t;
                        t = rg->to;
                }
                chg -= rg->to - rg->from;
        }
        return chg;
}

static long region_truncate(struct list_head *head, long end)
{
        struct file_region *rg, *trg;
        long chg = 0;

        /* Locate the region we are either in or before. */
        list_for_each_entry(rg, head, link)
                if (end <= rg->to)
                        break;
        if (&rg->link == head)
                return 0;

        /* If we are in the middle of a region then adjust it. */
        if (end > rg->from) {
                chg = rg->to - end;
                rg->to = end;
                rg = list_entry(rg->link.next, typeof(*rg), link);
        }

        /* Drop any remaining regions. */
        list_for_each_entry_safe(rg, trg, rg->link.prev, link) {
                if (&rg->link == head)
                        break;
                chg += rg->to - rg->from;
                list_del(&rg->link);
                kfree(rg);
        }
        return chg;
}

static int hugetlb_acct_memory(long delta)
{
        int ret = -ENOMEM;

        spin_lock(&hugetlb_lock);
        /*
         * When cpuset is configured, it breaks the strict hugetlb page
         * reservation as the accounting is done on a global variable. Such
         * reservation is completely rubbish in the presence of cpuset because
         * the reservation is not checked against page availability for the
         * current cpuset. Application can still potentially OOM'ed by kernel
         * with lack of free htlb page in cpuset that the task is in.
         * Attempt to enforce strict accounting with cpuset is almost
         * impossible (or too ugly) because cpuset is too fluid that
         * task or memory node can be dynamically moved between cpusets.
         *
         * The change of semantics for shared hugetlb mapping with cpuset is
         * undesirable. However, in order to preserve some of the semantics,
         * we fall back to check against current free page availability as
         * a best attempt and hopefully to minimize the impact of changing
         * semantics that cpuset has.
         */
        if (delta > 0) {
                if (gather_surplus_pages(delta) < 0)
                        goto out;

                if (delta > cpuset_mems_nr(free_huge_pages_node))
                        goto out;
        }

        ret = 0;
        resv_huge_pages += delta;
        if (delta < 0)
                return_unused_surplus_pages((unsigned long) -delta);

out:
        spin_unlock(&hugetlb_lock);
        return ret;
}

int hugetlb_reserve_pages(struct inode *inode, long from, long to)
{
        long ret, chg;

        chg = region_chg(&inode->i_mapping->private_list, from, to);
        if (chg < 0)
                return chg;

        if (hugetlb_get_quota(inode->i_mapping, chg))
                return -ENOSPC;
        ret = hugetlb_acct_memory(chg);
        if (ret < 0)
                return ret;
        region_add(&inode->i_mapping->private_list, from, to);
        return 0;
}

void hugetlb_unreserve_pages(struct inode *inode, long offset, long freed)
{
        long chg = region_truncate(&inode->i_mapping->private_list, offset);

        spin_lock(&inode->i_lock);
        inode->i_blocks -= BLOCKS_PER_HUGEPAGE * freed;
        spin_unlock(&inode->i_lock);

        hugetlb_put_quota(inode->i_mapping, (chg - freed));
        hugetlb_acct_memory(-(chg - freed));
}