[PATCH] unpaged: ZERO_PAGE in VM_UNPAGED

[linux-2.6] / kernel / kexec.c

/*
 * kexec.c - kexec system call
 * Copyright (C) 2002-2004 Eric Biederman  <ebiederm@xmission.com>
 *
 * This source code is licensed under the GNU General Public License,
 * Version 2.  See the file COPYING for more details.
 */

#include <linux/mm.h>
#include <linux/file.h>
#include <linux/slab.h>
#include <linux/fs.h>
#include <linux/kexec.h>
#include <linux/spinlock.h>
#include <linux/list.h>
#include <linux/highmem.h>
#include <linux/syscalls.h>
#include <linux/reboot.h>
#include <linux/syscalls.h>
#include <linux/ioport.h>
#include <linux/hardirq.h>

#include <asm/page.h>
#include <asm/uaccess.h>
#include <asm/io.h>
#include <asm/system.h>
#include <asm/semaphore.h>

/* Location of the reserved area for the crash kernel */
struct resource crashk_res = {
        .name  = "Crash kernel",
        .start = 0,
        .end   = 0,
        .flags = IORESOURCE_BUSY | IORESOURCE_MEM
};

int kexec_should_crash(struct task_struct *p)
{
        if (in_interrupt() || !p->pid || p->pid == 1 || panic_on_oops)
                return 1;
        return 0;
}

/*
 * When kexec transitions to the new kernel there is a one-to-one
 * mapping between physical and virtual addresses.  On processors
 * where you can disable the MMU this is trivial, and easy.  For
 * others it is still a simple predictable page table to setup.
 *
 * In that environment kexec copies the new kernel to its final
 * resting place.  This means I can only support memory whose
 * physical address can fit in an unsigned long.  In particular
 * addresses where (pfn << PAGE_SHIFT) > ULONG_MAX cannot be handled.
 * If the assembly stub has more restrictive requirements
 * KEXEC_SOURCE_MEMORY_LIMIT and KEXEC_DEST_MEMORY_LIMIT can be
 * defined more restrictively in <asm/kexec.h>.
 *
 * The code for the transition from the current kernel to the
 * the new kernel is placed in the control_code_buffer, whose size
 * is given by KEXEC_CONTROL_CODE_SIZE.  In the best case only a single
 * page of memory is necessary, but some architectures require more.
 * Because this memory must be identity mapped in the transition from
 * virtual to physical addresses it must live in the range
 * 0 - TASK_SIZE, as only the user space mappings are arbitrarily
 * modifiable.
 *
 * The assembly stub in the control code buffer is passed a linked list
 * of descriptor pages detailing the source pages of the new kernel,
 * and the destination addresses of those source pages.  As this data
 * structure is not used in the context of the current OS, it must
 * be self-contained.
 *
 * The code has been made to work with highmem pages and will use a
 * destination page in its final resting place (if it happens
 * to allocate it).  The end product of this is that most of the
 * physical address space, and most of RAM can be used.
 *
 * Future directions include:
 *  - allocating a page table with the control code buffer identity
 *    mapped, to simplify machine_kexec and make kexec_on_panic more
 *    reliable.
 */

/*
 * KIMAGE_NO_DEST is an impossible destination address..., for
 * allocating pages whose destination address we do not care about.
 */
#define KIMAGE_NO_DEST (-1UL)

static int kimage_is_destination_range(struct kimage *image,
                                       unsigned long start, unsigned long end);
static struct page *kimage_alloc_page(struct kimage *image,
                                       gfp_t gfp_mask,
                                       unsigned long dest);

static int do_kimage_alloc(struct kimage **rimage, unsigned long entry,
                            unsigned long nr_segments,
                            struct kexec_segment __user *segments)
{
        size_t segment_bytes;
        struct kimage *image;
        unsigned long i;
        int result;

        /* Allocate a controlling structure */
        result = -ENOMEM;
        image = kmalloc(sizeof(*image), GFP_KERNEL);
        if (!image)
                goto out;

        memset(image, 0, sizeof(*image));
        image->head = 0;
        image->entry = &image->head;
        image->last_entry = &image->head;
        image->control_page = ~0; /* By default this does not apply */
        image->start = entry;
        image->type = KEXEC_TYPE_DEFAULT;

        /* Initialize the list of control pages */
        INIT_LIST_HEAD(&image->control_pages);

        /* Initialize the list of destination pages */
        INIT_LIST_HEAD(&image->dest_pages);

        /* Initialize the list of unuseable pages */
        INIT_LIST_HEAD(&image->unuseable_pages);

        /* Read in the segments */
        image->nr_segments = nr_segments;
        segment_bytes = nr_segments * sizeof(*segments);
        result = copy_from_user(image->segment, segments, segment_bytes);
        if (result)
                goto out;

        /*
         * Verify we have good destination addresses.  The caller is
         * responsible for making certain we don't attempt to load
         * the new image into invalid or reserved areas of RAM.  This
         * just verifies it is an address we can use.
         *
         * Since the kernel does everything in page size chunks ensure
         * the destination addreses are page aligned.  Too many
         * special cases crop of when we don't do this.  The most
         * insidious is getting overlapping destination addresses
         * simply because addresses are changed to page size
         * granularity.
         */
        result = -EADDRNOTAVAIL;
        for (i = 0; i < nr_segments; i++) {
                unsigned long mstart, mend;

                mstart = image->segment[i].mem;
                mend   = mstart + image->segment[i].memsz;
                if ((mstart & ~PAGE_MASK) || (mend & ~PAGE_MASK))
                        goto out;
                if (mend >= KEXEC_DESTINATION_MEMORY_LIMIT)
                        goto out;
        }

        /* Verify our destination addresses do not overlap.
         * If we alloed overlapping destination addresses
         * through very weird things can happen with no
         * easy explanation as one segment stops on another.
         */
        result = -EINVAL;
        for (i = 0; i < nr_segments; i++) {
                unsigned long mstart, mend;
                unsigned long j;

                mstart = image->segment[i].mem;
                mend   = mstart + image->segment[i].memsz;
                for (j = 0; j < i; j++) {
                        unsigned long pstart, pend;
                        pstart = image->segment[j].mem;
                        pend   = pstart + image->segment[j].memsz;
                        /* Do the segments overlap ? */
                        if ((mend > pstart) && (mstart < pend))
                                goto out;
                }
        }

        /* Ensure our buffer sizes are strictly less than
         * our memory sizes.  This should always be the case,
         * and it is easier to check up front than to be surprised
         * later on.
         */
        result = -EINVAL;
        for (i = 0; i < nr_segments; i++) {
                if (image->segment[i].bufsz > image->segment[i].memsz)
                        goto out;
        }

        result = 0;
out:
        if (result == 0)
                *rimage = image;
        else
                kfree(image);

        return result;

}

static int kimage_normal_alloc(struct kimage **rimage, unsigned long entry,
                                unsigned long nr_segments,
                                struct kexec_segment __user *segments)
{
        int result;
        struct kimage *image;

        /* Allocate and initialize a controlling structure */
        image = NULL;
        result = do_kimage_alloc(&image, entry, nr_segments, segments);
        if (result)
                goto out;

        *rimage = image;

        /*
         * Find a location for the control code buffer, and add it
         * the vector of segments so that it's pages will also be
         * counted as destination pages.
         */
        result = -ENOMEM;
        image->control_code_page = kimage_alloc_control_pages(image,
                                           get_order(KEXEC_CONTROL_CODE_SIZE));
        if (!image->control_code_page) {
                printk(KERN_ERR "Could not allocate control_code_buffer\n");
                goto out;
        }

        result = 0;
 out:
        if (result == 0)
                *rimage = image;
        else
                kfree(image);

        return result;
}

static int kimage_crash_alloc(struct kimage **rimage, unsigned long entry,
                                unsigned long nr_segments,
                                struct kexec_segment __user *segments)
{
        int result;
        struct kimage *image;
        unsigned long i;

        image = NULL;
        /* Verify we have a valid entry point */
        if ((entry < crashk_res.start) || (entry > crashk_res.end)) {
                result = -EADDRNOTAVAIL;
                goto out;
        }

        /* Allocate and initialize a controlling structure */
        result = do_kimage_alloc(&image, entry, nr_segments, segments);
        if (result)
                goto out;

        /* Enable the special crash kernel control page
         * allocation policy.
         */
        image->control_page = crashk_res.start;
        image->type = KEXEC_TYPE_CRASH;

        /*
         * Verify we have good destination addresses.  Normally
         * the caller is responsible for making certain we don't
         * attempt to load the new image into invalid or reserved
         * areas of RAM.  But crash kernels are preloaded into a
         * reserved area of ram.  We must ensure the addresses
         * are in the reserved area otherwise preloading the
         * kernel could corrupt things.
         */
        result = -EADDRNOTAVAIL;
        for (i = 0; i < nr_segments; i++) {
                unsigned long mstart, mend;

                mstart = image->segment[i].mem;
                mend = mstart + image->segment[i].memsz - 1;
                /* Ensure we are within the crash kernel limits */
                if ((mstart < crashk_res.start) || (mend > crashk_res.end))
                        goto out;
        }

        /*
         * Find a location for the control code buffer, and add
         * the vector of segments so that it's pages will also be
         * counted as destination pages.
         */
        result = -ENOMEM;
        image->control_code_page = kimage_alloc_control_pages(image,
                                           get_order(KEXEC_CONTROL_CODE_SIZE));
        if (!image->control_code_page) {
                printk(KERN_ERR "Could not allocate control_code_buffer\n");
                goto out;
        }

        result = 0;
out:
        if (result == 0)
                *rimage = image;
        else
                kfree(image);

        return result;
}

static int kimage_is_destination_range(struct kimage *image,
                                        unsigned long start,
                                        unsigned long end)
{
        unsigned long i;

        for (i = 0; i < image->nr_segments; i++) {
                unsigned long mstart, mend;

                mstart = image->segment[i].mem;
                mend = mstart + image->segment[i].memsz;
                if ((end > mstart) && (start < mend))
                        return 1;
        }

        return 0;
}

static struct page *kimage_alloc_pages(gfp_t gfp_mask, unsigned int order)
{
        struct page *pages;

        pages = alloc_pages(gfp_mask, order);
        if (pages) {
                unsigned int count, i;
                pages->mapping = NULL;
                set_page_private(pages, order);
                count = 1 << order;
                for (i = 0; i < count; i++)
                        SetPageReserved(pages + i);
        }

        return pages;
}

static void kimage_free_pages(struct page *page)
{
        unsigned int order, count, i;

        order = page_private(page);
        count = 1 << order;
        for (i = 0; i < count; i++)
                ClearPageReserved(page + i);
        __free_pages(page, order);
}

static void kimage_free_page_list(struct list_head *list)
{
        struct list_head *pos, *next;

        list_for_each_safe(pos, next, list) {
                struct page *page;

                page = list_entry(pos, struct page, lru);
                list_del(&page->lru);
                kimage_free_pages(page);
        }
}

static struct page *kimage_alloc_normal_control_pages(struct kimage *image,
                                                        unsigned int order)
{
        /* Control pages are special, they are the intermediaries
         * that are needed while we copy the rest of the pages
         * to their final resting place.  As such they must
         * not conflict with either the destination addresses
         * or memory the kernel is already using.
         *
         * The only case where we really need more than one of
         * these are for architectures where we cannot disable
         * the MMU and must instead generate an identity mapped
         * page table for all of the memory.
         *
         * At worst this runs in O(N) of the image size.
         */
        struct list_head extra_pages;
        struct page *pages;
        unsigned int count;

        count = 1 << order;
        INIT_LIST_HEAD(&extra_pages);

        /* Loop while I can allocate a page and the page allocated
         * is a destination page.
         */
        do {
                unsigned long pfn, epfn, addr, eaddr;

                pages = kimage_alloc_pages(GFP_KERNEL, order);
                if (!pages)
                        break;
                pfn   = page_to_pfn(pages);
                epfn  = pfn + count;
                addr  = pfn << PAGE_SHIFT;
                eaddr = epfn << PAGE_SHIFT;
                if ((epfn >= (KEXEC_CONTROL_MEMORY_LIMIT >> PAGE_SHIFT)) ||
                              kimage_is_destination_range(image, addr, eaddr)) {
                        list_add(&pages->lru, &extra_pages);
                        pages = NULL;
                }
        } while (!pages);

        if (pages) {
                /* Remember the allocated page... */
                list_add(&pages->lru, &image->control_pages);

                /* Because the page is already in it's destination
                 * location we will never allocate another page at
                 * that address.  Therefore kimage_alloc_pages
                 * will not return it (again) and we don't need
                 * to give it an entry in image->segment[].
                 */
        }
        /* Deal with the destination pages I have inadvertently allocated.
         *
         * Ideally I would convert multi-page allocations into single
         * page allocations, and add everyting to image->dest_pages.
         *
         * For now it is simpler to just free the pages.
         */
        kimage_free_page_list(&extra_pages);

        return pages;
}

static struct page *kimage_alloc_crash_control_pages(struct kimage *image,
                                                      unsigned int order)
{
        /* Control pages are special, they are the intermediaries
         * that are needed while we copy the rest of the pages
         * to their final resting place.  As such they must
         * not conflict with either the destination addresses
         * or memory the kernel is already using.
         *
         * Control pages are also the only pags we must allocate
         * when loading a crash kernel.  All of the other pages
         * are specified by the segments and we just memcpy
         * into them directly.
         *
         * The only case where we really need more than one of
         * these are for architectures where we cannot disable
         * the MMU and must instead generate an identity mapped
         * page table for all of the memory.
         *
         * Given the low demand this implements a very simple
         * allocator that finds the first hole of the appropriate
         * size in the reserved memory region, and allocates all
         * of the memory up to and including the hole.
         */
        unsigned long hole_start, hole_end, size;
        struct page *pages;

        pages = NULL;
        size = (1 << order) << PAGE_SHIFT;
        hole_start = (image->control_page + (size - 1)) & ~(size - 1);
        hole_end   = hole_start + size - 1;
        while (hole_end <= crashk_res.end) {
                unsigned long i;

                if (hole_end > KEXEC_CONTROL_MEMORY_LIMIT)
                        break;
                if (hole_end > crashk_res.end)
                        break;
                /* See if I overlap any of the segments */
                for (i = 0; i < image->nr_segments; i++) {
                        unsigned long mstart, mend;

                        mstart = image->segment[i].mem;
                        mend   = mstart + image->segment[i].memsz - 1;
                        if ((hole_end >= mstart) && (hole_start <= mend)) {
                                /* Advance the hole to the end of the segment */
                                hole_start = (mend + (size - 1)) & ~(size - 1);
                                hole_end   = hole_start + size - 1;
                                break;
                        }
                }
                /* If I don't overlap any segments I have found my hole! */
                if (i == image->nr_segments) {
                        pages = pfn_to_page(hole_start >> PAGE_SHIFT);
                        break;
                }
        }
        if (pages)
                image->control_page = hole_end;

        return pages;
}


struct page *kimage_alloc_control_pages(struct kimage *image,
                                         unsigned int order)
{
        struct page *pages = NULL;

        switch (image->type) {
        case KEXEC_TYPE_DEFAULT:
                pages = kimage_alloc_normal_control_pages(image, order);
                break;
        case KEXEC_TYPE_CRASH:
                pages = kimage_alloc_crash_control_pages(image, order);
                break;
        }

        return pages;
}

static int kimage_add_entry(struct kimage *image, kimage_entry_t entry)
{
        if (*image->entry != 0)
                image->entry++;

        if (image->entry == image->last_entry) {
                kimage_entry_t *ind_page;
                struct page *page;

                page = kimage_alloc_page(image, GFP_KERNEL, KIMAGE_NO_DEST);
                if (!page)
                        return -ENOMEM;

                ind_page = page_address(page);
                *image->entry = virt_to_phys(ind_page) | IND_INDIRECTION;
                image->entry = ind_page;
                image->last_entry = ind_page +
                                      ((PAGE_SIZE/sizeof(kimage_entry_t)) - 1);
        }
        *image->entry = entry;
        image->entry++;
        *image->entry = 0;

        return 0;
}

static int kimage_set_destination(struct kimage *image,
                                   unsigned long destination)
{
        int result;

        destination &= PAGE_MASK;
        result = kimage_add_entry(image, destination | IND_DESTINATION);
        if (result == 0)
                image->destination = destination;

        return result;
}


static int kimage_add_page(struct kimage *image, unsigned long page)
{
        int result;

        page &= PAGE_MASK;
        result = kimage_add_entry(image, page | IND_SOURCE);
        if (result == 0)
                image->destination += PAGE_SIZE;

        return result;
}


static void kimage_free_extra_pages(struct kimage *image)
{
        /* Walk through and free any extra destination pages I may have */
        kimage_free_page_list(&image->dest_pages);

        /* Walk through and free any unuseable pages I have cached */
        kimage_free_page_list(&image->unuseable_pages);

}
static int kimage_terminate(struct kimage *image)
{
        if (*image->entry != 0)
                image->entry++;

        *image->entry = IND_DONE;

        return 0;
}

#define for_each_kimage_entry(image, ptr, entry) \
        for (ptr = &image->head; (entry = *ptr) && !(entry & IND_DONE); \
                ptr = (entry & IND_INDIRECTION)? \
                        phys_to_virt((entry & PAGE_MASK)): ptr +1)

static void kimage_free_entry(kimage_entry_t entry)
{
        struct page *page;

        page = pfn_to_page(entry >> PAGE_SHIFT);
        kimage_free_pages(page);
}

static void kimage_free(struct kimage *image)
{
        kimage_entry_t *ptr, entry;
        kimage_entry_t ind = 0;

        if (!image)
                return;

        kimage_free_extra_pages(image);
        for_each_kimage_entry(image, ptr, entry) {
                if (entry & IND_INDIRECTION) {
                        /* Free the previous indirection page */
                        if (ind & IND_INDIRECTION)
                                kimage_free_entry(ind);
                        /* Save this indirection page until we are
                         * done with it.
                         */
                        ind = entry;
                }
                else if (entry & IND_SOURCE)
                        kimage_free_entry(entry);
        }
        /* Free the final indirection page */
        if (ind & IND_INDIRECTION)
                kimage_free_entry(ind);

        /* Handle any machine specific cleanup */
        machine_kexec_cleanup(image);

        /* Free the kexec control pages... */
        kimage_free_page_list(&image->control_pages);
        kfree(image);
}

static kimage_entry_t *kimage_dst_used(struct kimage *image,
                                        unsigned long page)
{
        kimage_entry_t *ptr, entry;
        unsigned long destination = 0;

        for_each_kimage_entry(image, ptr, entry) {
                if (entry & IND_DESTINATION)
                        destination = entry & PAGE_MASK;
                else if (entry & IND_SOURCE) {
                        if (page == destination)
                                return ptr;
                        destination += PAGE_SIZE;
                }
        }

        return NULL;
}

static struct page *kimage_alloc_page(struct kimage *image,
                                        gfp_t gfp_mask,
                                        unsigned long destination)
{
        /*
         * Here we implement safeguards to ensure that a source page
         * is not copied to its destination page before the data on
         * the destination page is no longer useful.
         *
         * To do this we maintain the invariant that a source page is
         * either its own destination page, or it is not a
         * destination page at all.
         *
         * That is slightly stronger than required, but the proof
         * that no problems will not occur is trivial, and the
         * implementation is simply to verify.
         *
         * When allocating all pages normally this algorithm will run
         * in O(N) time, but in the worst case it will run in O(N^2)
         * time.   If the runtime is a problem the data structures can
         * be fixed.
         */
        struct page *page;
        unsigned long addr;

        /*
         * Walk through the list of destination pages, and see if I
         * have a match.
         */
        list_for_each_entry(page, &image->dest_pages, lru) {
                addr = page_to_pfn(page) << PAGE_SHIFT;
                if (addr == destination) {
                        list_del(&page->lru);
                        return page;
                }
        }
        page = NULL;
        while (1) {
                kimage_entry_t *old;

                /* Allocate a page, if we run out of memory give up */
                page = kimage_alloc_pages(gfp_mask, 0);
                if (!page)
                        return NULL;
                /* If the page cannot be used file it away */
                if (page_to_pfn(page) >
                                (KEXEC_SOURCE_MEMORY_LIMIT >> PAGE_SHIFT)) {
                        list_add(&page->lru, &image->unuseable_pages);
                        continue;
                }
                addr = page_to_pfn(page) << PAGE_SHIFT;

                /* If it is the destination page we want use it */
                if (addr == destination)
                        break;

                /* If the page is not a destination page use it */
                if (!kimage_is_destination_range(image, addr,
                                                  addr + PAGE_SIZE))
                        break;

                /*
                 * I know that the page is someones destination page.
                 * See if there is already a source page for this
                 * destination page.  And if so swap the source pages.
                 */
                old = kimage_dst_used(image, addr);
                if (old) {
                        /* If so move it */
                        unsigned long old_addr;
                        struct page *old_page;

                        old_addr = *old & PAGE_MASK;
                        old_page = pfn_to_page(old_addr >> PAGE_SHIFT);
                        copy_highpage(page, old_page);
                        *old = addr | (*old & ~PAGE_MASK);

                        /* The old page I have found cannot be a
                         * destination page, so return it.
                         */
                        addr = old_addr;
                        page = old_page;
                        break;
                }
                else {
                        /* Place the page on the destination list I
                         * will use it later.
                         */
                        list_add(&page->lru, &image->dest_pages);
                }
        }

        return page;
}

static int kimage_load_normal_segment(struct kimage *image,
                                         struct kexec_segment *segment)
{
        unsigned long maddr;
        unsigned long ubytes, mbytes;
        int result;
        unsigned char __user *buf;

        result = 0;
        buf = segment->buf;
        ubytes = segment->bufsz;
        mbytes = segment->memsz;
        maddr = segment->mem;

        result = kimage_set_destination(image, maddr);
        if (result < 0)
                goto out;

        while (mbytes) {
                struct page *page;
                char *ptr;
                size_t uchunk, mchunk;

                page = kimage_alloc_page(image, GFP_HIGHUSER, maddr);
                if (page == 0) {
                        result  = -ENOMEM;
                        goto out;
                }
                result = kimage_add_page(image, page_to_pfn(page)
                                                                << PAGE_SHIFT);
                if (result < 0)
                        goto out;

                ptr = kmap(page);
                /* Start with a clear page */
                memset(ptr, 0, PAGE_SIZE);
                ptr += maddr & ~PAGE_MASK;
                mchunk = PAGE_SIZE - (maddr & ~PAGE_MASK);
                if (mchunk > mbytes)
                        mchunk = mbytes;

                uchunk = mchunk;
                if (uchunk > ubytes)
                        uchunk = ubytes;

                result = copy_from_user(ptr, buf, uchunk);
                kunmap(page);
                if (result) {
                        result = (result < 0) ? result : -EIO;
                        goto out;
                }
                ubytes -= uchunk;
                maddr  += mchunk;
                buf    += mchunk;
                mbytes -= mchunk;
        }
out:
        return result;
}

static int kimage_load_crash_segment(struct kimage *image,
                                        struct kexec_segment *segment)
{
        /* For crash dumps kernels we simply copy the data from
         * user space to it's destination.
         * We do things a page at a time for the sake of kmap.
         */
        unsigned long maddr;
        unsigned long ubytes, mbytes;
        int result;
        unsigned char __user *buf;

        result = 0;
        buf = segment->buf;
        ubytes = segment->bufsz;
        mbytes = segment->memsz;
        maddr = segment->mem;
        while (mbytes) {
                struct page *page;
                char *ptr;
                size_t uchunk, mchunk;

                page = pfn_to_page(maddr >> PAGE_SHIFT);
                if (page == 0) {
                        result  = -ENOMEM;
                        goto out;
                }
                ptr = kmap(page);
                ptr += maddr & ~PAGE_MASK;
                mchunk = PAGE_SIZE - (maddr & ~PAGE_MASK);
                if (mchunk > mbytes)
                        mchunk = mbytes;

                uchunk = mchunk;
                if (uchunk > ubytes) {
                        uchunk = ubytes;
                        /* Zero the trailing part of the page */
                        memset(ptr + uchunk, 0, mchunk - uchunk);
                }
                result = copy_from_user(ptr, buf, uchunk);
                kunmap(page);
                if (result) {
                        result = (result < 0) ? result : -EIO;
                        goto out;
                }
                ubytes -= uchunk;
                maddr  += mchunk;
                buf    += mchunk;
                mbytes -= mchunk;
        }
out:
        return result;
}

static int kimage_load_segment(struct kimage *image,
                                struct kexec_segment *segment)
{
        int result = -ENOMEM;

        switch (image->type) {
        case KEXEC_TYPE_DEFAULT:
                result = kimage_load_normal_segment(image, segment);
                break;
        case KEXEC_TYPE_CRASH:
                result = kimage_load_crash_segment(image, segment);
                break;
        }

        return result;
}

/*
 * Exec Kernel system call: for obvious reasons only root may call it.
 *
 * This call breaks up into three pieces.
 * - A generic part which loads the new kernel from the current
 *   address space, and very carefully places the data in the
 *   allocated pages.
 *
 * - A generic part that interacts with the kernel and tells all of
 *   the devices to shut down.  Preventing on-going dmas, and placing
 *   the devices in a consistent state so a later kernel can
 *   reinitialize them.
 *
 * - A machine specific part that includes the syscall number
 *   and the copies the image to it's final destination.  And
 *   jumps into the image at entry.
 *
 * kexec does not sync, or unmount filesystems so if you need
 * that to happen you need to do that yourself.
 */
struct kimage *kexec_image = NULL;
static struct kimage *kexec_crash_image = NULL;
/*
 * A home grown binary mutex.
 * Nothing can wait so this mutex is safe to use
 * in interrupt context :)
 */
static int kexec_lock = 0;

asmlinkage long sys_kexec_load(unsigned long entry, unsigned long nr_segments,
                                struct kexec_segment __user *segments,
                                unsigned long flags)
{
        struct kimage **dest_image, *image;
        int locked;
        int result;

        /* We only trust the superuser with rebooting the system. */
        if (!capable(CAP_SYS_BOOT))
                return -EPERM;

        /*
         * Verify we have a legal set of flags
         * This leaves us room for future extensions.
         */
        if ((flags & KEXEC_FLAGS) != (flags & ~KEXEC_ARCH_MASK))
                return -EINVAL;

        /* Verify we are on the appropriate architecture */
        if (((flags & KEXEC_ARCH_MASK) != KEXEC_ARCH) &&
                ((flags & KEXEC_ARCH_MASK) != KEXEC_ARCH_DEFAULT))
                return -EINVAL;

        /* Put an artificial cap on the number
         * of segments passed to kexec_load.
         */
        if (nr_segments > KEXEC_SEGMENT_MAX)
                return -EINVAL;

        image = NULL;
        result = 0;

        /* Because we write directly to the reserved memory
         * region when loading crash kernels we need a mutex here to
         * prevent multiple crash  kernels from attempting to load
         * simultaneously, and to prevent a crash kernel from loading
         * over the top of a in use crash kernel.
         *
         * KISS: always take the mutex.
         */
        locked = xchg(&kexec_lock, 1);
        if (locked)
                return -EBUSY;

        dest_image = &kexec_image;
        if (flags & KEXEC_ON_CRASH)
                dest_image = &kexec_crash_image;
        if (nr_segments > 0) {
                unsigned long i;

                /* Loading another kernel to reboot into */
                if ((flags & KEXEC_ON_CRASH) == 0)
                        result = kimage_normal_alloc(&image, entry,
                                                        nr_segments, segments);
                /* Loading another kernel to switch to if this one crashes */
                else if (flags & KEXEC_ON_CRASH) {
                        /* Free any current crash dump kernel before
                         * we corrupt it.
                         */
                        kimage_free(xchg(&kexec_crash_image, NULL));
                        result = kimage_crash_alloc(&image, entry,
                                                     nr_segments, segments);
                }
                if (result)
                        goto out;

                result = machine_kexec_prepare(image);
                if (result)
                        goto out;

                for (i = 0; i < nr_segments; i++) {
                        result = kimage_load_segment(image, &image->segment[i]);
                        if (result)
                                goto out;
                }
                result = kimage_terminate(image);
                if (result)
                        goto out;
        }
        /* Install the new kernel, and  Uninstall the old */
        image = xchg(dest_image, image);

out:
        xchg(&kexec_lock, 0); /* Release the mutex */
        kimage_free(image);

        return result;
}

#ifdef CONFIG_COMPAT
asmlinkage long compat_sys_kexec_load(unsigned long entry,
                                unsigned long nr_segments,
                                struct compat_kexec_segment __user *segments,
                                unsigned long flags)
{
        struct compat_kexec_segment in;
        struct kexec_segment out, __user *ksegments;
        unsigned long i, result;

        /* Don't allow clients that don't understand the native
         * architecture to do anything.
         */
        if ((flags & KEXEC_ARCH_MASK) == KEXEC_ARCH_DEFAULT)
                return -EINVAL;

        if (nr_segments > KEXEC_SEGMENT_MAX)
                return -EINVAL;

        ksegments = compat_alloc_user_space(nr_segments * sizeof(out));
        for (i=0; i < nr_segments; i++) {
                result = copy_from_user(&in, &segments[i], sizeof(in));
                if (result)
                        return -EFAULT;

                out.buf   = compat_ptr(in.buf);
                out.bufsz = in.bufsz;
                out.mem   = in.mem;
                out.memsz = in.memsz;

                result = copy_to_user(&ksegments[i], &out, sizeof(out));
                if (result)
                        return -EFAULT;
        }

        return sys_kexec_load(entry, nr_segments, ksegments, flags);
}
#endif

void crash_kexec(struct pt_regs *regs)
{
        struct kimage *image;
        int locked;


        /* Take the kexec_lock here to prevent sys_kexec_load
         * running on one cpu from replacing the crash kernel
         * we are using after a panic on a different cpu.
         *
         * If the crash kernel was not located in a fixed area
         * of memory the xchg(&kexec_crash_image) would be
         * sufficient.  But since I reuse the memory...
         */
        locked = xchg(&kexec_lock, 1);
        if (!locked) {
                image = xchg(&kexec_crash_image, NULL);
                if (image) {
                        machine_crash_shutdown(regs);
                        machine_kexec(image);
                }
                xchg(&kexec_lock, 0);
        }
}