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swapfile.c
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/*
* linux/mm/swapfile.c
*
* Copyright (C) 1991, 1992, 1993, 1994 Linus Torvalds
* Swap reorganised 29.12.95, Stephen Tweedie
*/
#include <linux/config.h>
#include <linux/mm.h>
#include <linux/hugetlb.h>
#include <linux/mman.h>
#include <linux/slab.h>
#include <linux/kernel_stat.h>
#include <linux/swap.h>
#include <linux/vmalloc.h>
#include <linux/pagemap.h>
#include <linux/namei.h>
#include <linux/shm.h>
#include <linux/blkdev.h>
#include <linux/writeback.h>
#include <linux/proc_fs.h>
#include <linux/seq_file.h>
#include <linux/init.h>
#include <linux/module.h>
#include <linux/rmap.h>
#include <linux/security.h>
#include <linux/backing-dev.h>
#include <linux/syscalls.h>
#include <asm/pgtable.h>
#include <asm/tlbflush.h>
#include <linux/swapops.h>
DEFINE_SPINLOCK(swaplock);
unsigned int nr_swapfiles;
long total_swap_pages;
static int swap_overflow;
EXPORT_SYMBOL(total_swap_pages);
static const char Bad_file[] = "Bad swap file entry ";
static const char Unused_file[] = "Unused swap file entry ";
static const char Bad_offset[] = "Bad swap offset entry ";
static const char Unused_offset[] = "Unused swap offset entry ";
struct swap_list_t swap_list = {-1, -1};
struct swap_info_struct swap_info[MAX_SWAPFILES];
static DECLARE_MUTEX(swapon_sem);
/*
* We need this because the bdev->unplug_fn can sleep and we cannot
* hold swap_list_lock while calling the unplug_fn. And swap_list_lock
* cannot be turned into a semaphore.
*/
static DECLARE_RWSEM(swap_unplug_sem);
#define SWAPFILE_CLUSTER 256
void swap_unplug_io_fn(struct backing_dev_info *unused_bdi, struct page *page)
{
swp_entry_t entry;
down_read(&swap_unplug_sem);
entry.val = page->private;
if (PageSwapCache(page)) {
struct block_device *bdev = swap_info[swp_type(entry)].bdev;
struct backing_dev_info *bdi;
/*
* If the page is removed from swapcache from under us (with a
* racy try_to_unuse/swapoff) we need an additional reference
* count to avoid reading garbage from page->private above. If
* the WARN_ON triggers during a swapoff it maybe the race
* condition and it's harmless. However if it triggers without
* swapoff it signals a problem.
*/
WARN_ON(page_count(page) <= 1);
bdi = bdev->bd_inode->i_mapping->backing_dev_info;
blk_run_backing_dev(bdi, page);
}
up_read(&swap_unplug_sem);
}
static inline unsigned long scan_swap_map(struct swap_info_struct *si)
{
unsigned long offset, last_in_cluster;
/*
* We try to cluster swap pages by allocating them sequentially
* in swap. Once we've allocated SWAPFILE_CLUSTER pages this
* way, however, we resort to first-free allocation, starting
* a new cluster. This prevents us from scattering swap pages
* all over the entire swap partition, so that we reduce
* overall disk seek times between swap pages. -- sct
* But we do now try to find an empty cluster. -Andrea
*/
si->flags += SWP_SCANNING;
if (unlikely(!si->cluster_nr)) {
si->cluster_nr = SWAPFILE_CLUSTER - 1;
if (si->pages - si->inuse_pages < SWAPFILE_CLUSTER)
goto lowest;
swap_device_unlock(si);
offset = si->lowest_bit;
last_in_cluster = offset + SWAPFILE_CLUSTER - 1;
/* Locate the first empty (unaligned) cluster */
for (; last_in_cluster <= si->highest_bit; offset++) {
if (si->swap_map[offset])
last_in_cluster = offset + SWAPFILE_CLUSTER;
else if (offset == last_in_cluster) {
swap_device_lock(si);
si->cluster_next = offset-SWAPFILE_CLUSTER-1;
goto cluster;
}
}
swap_device_lock(si);
goto lowest;
}
si->cluster_nr--;
cluster:
offset = si->cluster_next;
if (offset > si->highest_bit)
lowest: offset = si->lowest_bit;
checks: if (!(si->flags & SWP_WRITEOK))
goto no_page;
if (!si->highest_bit)
goto no_page;
if (!si->swap_map[offset]) {
if (offset == si->lowest_bit)
si->lowest_bit++;
if (offset == si->highest_bit)
si->highest_bit--;
si->inuse_pages++;
if (si->inuse_pages == si->pages) {
si->lowest_bit = si->max;
si->highest_bit = 0;
}
si->swap_map[offset] = 1;
si->cluster_next = offset + 1;
si->flags -= SWP_SCANNING;
return offset;
}
swap_device_unlock(si);
while (++offset <= si->highest_bit) {
if (!si->swap_map[offset]) {
swap_device_lock(si);
goto checks;
}
}
swap_device_lock(si);
goto lowest;
no_page:
si->flags -= SWP_SCANNING;
return 0;
}
swp_entry_t get_swap_page(void)
{
struct swap_info_struct *si;
pgoff_t offset;
int type, next;
int wrapped = 0;
swap_list_lock();
if (nr_swap_pages <= 0)
goto noswap;
nr_swap_pages--;
for (type = swap_list.next; type >= 0 && wrapped < 2; type = next) {
si = swap_info + type;
next = si->next;
if (next < 0 ||
(!wrapped && si->prio != swap_info[next].prio)) {
next = swap_list.head;
wrapped++;
}
if (!si->highest_bit)
continue;
if (!(si->flags & SWP_WRITEOK))
continue;
swap_list.next = next;
swap_device_lock(si);
swap_list_unlock();
offset = scan_swap_map(si);
swap_device_unlock(si);
if (offset)
return swp_entry(type, offset);
swap_list_lock();
next = swap_list.next;
}
nr_swap_pages++;
noswap:
swap_list_unlock();
return (swp_entry_t) {0};
}
static struct swap_info_struct * swap_info_get(swp_entry_t entry)
{
struct swap_info_struct * p;
unsigned long offset, type;
if (!entry.val)
goto out;
type = swp_type(entry);
if (type >= nr_swapfiles)
goto bad_nofile;
p = & swap_info[type];
if (!(p->flags & SWP_USED))
goto bad_device;
offset = swp_offset(entry);
if (offset >= p->max)
goto bad_offset;
if (!p->swap_map[offset])
goto bad_free;
swap_list_lock();
swap_device_lock(p);
return p;
bad_free:
printk(KERN_ERR "swap_free: %s%08lx\n", Unused_offset, entry.val);
goto out;
bad_offset:
printk(KERN_ERR "swap_free: %s%08lx\n", Bad_offset, entry.val);
goto out;
bad_device:
printk(KERN_ERR "swap_free: %s%08lx\n", Unused_file, entry.val);
goto out;
bad_nofile:
printk(KERN_ERR "swap_free: %s%08lx\n", Bad_file, entry.val);
out:
return NULL;
}
static void swap_info_put(struct swap_info_struct * p)
{
swap_device_unlock(p);
swap_list_unlock();
}
static int swap_entry_free(struct swap_info_struct *p, unsigned long offset)
{
int count = p->swap_map[offset];
if (count < SWAP_MAP_MAX) {
count--;
p->swap_map[offset] = count;
if (!count) {
if (offset < p->lowest_bit)
p->lowest_bit = offset;
if (offset > p->highest_bit)
p->highest_bit = offset;
if (p->prio > swap_info[swap_list.next].prio)
swap_list.next = p - swap_info;
nr_swap_pages++;
p->inuse_pages--;
}
}
return count;
}
/*
* Caller has made sure that the swapdevice corresponding to entry
* is still around or has not been recycled.
*/
void swap_free(swp_entry_t entry)
{
struct swap_info_struct * p;
p = swap_info_get(entry);
if (p) {
swap_entry_free(p, swp_offset(entry));
swap_info_put(p);
}
}
/*
* How many references to page are currently swapped out?
*/
static inline int page_swapcount(struct page *page)
{
int count = 0;
struct swap_info_struct *p;
swp_entry_t entry;
entry.val = page->private;
p = swap_info_get(entry);
if (p) {
/* Subtract the 1 for the swap cache itself */
count = p->swap_map[swp_offset(entry)] - 1;
swap_info_put(p);
}
return count;
}
/*
* We can use this swap cache entry directly
* if there are no other references to it.
*/
int can_share_swap_page(struct page *page)
{
int count;
BUG_ON(!PageLocked(page));
count = page_mapcount(page);
if (count <= 1 && PageSwapCache(page))
count += page_swapcount(page);
return count == 1;
}
/*
* Work out if there are any other processes sharing this
* swap cache page. Free it if you can. Return success.
*/
int remove_exclusive_swap_page(struct page *page)
{
int retval;
struct swap_info_struct * p;
swp_entry_t entry;
BUG_ON(PagePrivate(page));
BUG_ON(!PageLocked(page));
if (!PageSwapCache(page))
return 0;
if (PageWriteback(page))
return 0;
if (page_count(page) != 2) /* 2: us + cache */
return 0;
entry.val = page->private;
p = swap_info_get(entry);
if (!p)
return 0;
/* Is the only swap cache user the cache itself? */
retval = 0;
if (p->swap_map[swp_offset(entry)] == 1) {
/* Recheck the page count with the swapcache lock held.. */
write_lock_irq(&swapper_space.tree_lock);
if ((page_count(page) == 2) && !PageWriteback(page)) {
__delete_from_swap_cache(page);
SetPageDirty(page);
retval = 1;
}
write_unlock_irq(&swapper_space.tree_lock);
}
swap_info_put(p);
if (retval) {
swap_free(entry);
page_cache_release(page);
}
return retval;
}
/*
* Free the swap entry like above, but also try to
* free the page cache entry if it is the last user.
*/
void free_swap_and_cache(swp_entry_t entry)
{
struct swap_info_struct * p;
struct page *page = NULL;
p = swap_info_get(entry);
if (p) {
if (swap_entry_free(p, swp_offset(entry)) == 1)
page = find_trylock_page(&swapper_space, entry.val);
swap_info_put(p);
}
if (page) {
int one_user;
BUG_ON(PagePrivate(page));
page_cache_get(page);
one_user = (page_count(page) == 2);
/* Only cache user (+us), or swap space full? Free it! */
if (!PageWriteback(page) && (one_user || vm_swap_full())) {
delete_from_swap_cache(page);
SetPageDirty(page);
}
unlock_page(page);
page_cache_release(page);
}
}
/*
* Always set the resulting pte to be nowrite (the same as COW pages
* after one process has exited). We don't know just how many PTEs will
* share this swap entry, so be cautious and let do_wp_page work out
* what to do if a write is requested later.
*
* vma->vm_mm->page_table_lock is held.
*/
static void unuse_pte(struct vm_area_struct *vma, pte_t *pte,
unsigned long addr, swp_entry_t entry, struct page *page)
{
inc_mm_counter(vma->vm_mm, rss);
get_page(page);
set_pte_at(vma->vm_mm, addr, pte,
pte_mkold(mk_pte(page, vma->vm_page_prot)));
page_add_anon_rmap(page, vma, addr);
swap_free(entry);
/*
* Move the page to the active list so it is not
* immediately swapped out again after swapon.
*/
activate_page(page);
}
static int unuse_pte_range(struct vm_area_struct *vma, pmd_t *pmd,
unsigned long addr, unsigned long end,
swp_entry_t entry, struct page *page)
{
pte_t *pte;
pte_t swp_pte = swp_entry_to_pte(entry);
pte = pte_offset_map(pmd, addr);
do {
/*
* swapoff spends a _lot_ of time in this loop!
* Test inline before going to call unuse_pte.
*/
if (unlikely(pte_same(*pte, swp_pte))) {
unuse_pte(vma, pte, addr, entry, page);
pte_unmap(pte);
return 1;
}
} while (pte++, addr += PAGE_SIZE, addr != end);
pte_unmap(pte - 1);
return 0;
}
static inline int unuse_pmd_range(struct vm_area_struct *vma, pud_t *pud,
unsigned long addr, unsigned long end,
swp_entry_t entry, struct page *page)
{
pmd_t *pmd;
unsigned long next;
pmd = pmd_offset(pud, addr);
do {
next = pmd_addr_end(addr, end);
if (pmd_none_or_clear_bad(pmd))
continue;
if (unuse_pte_range(vma, pmd, addr, next, entry, page))
return 1;
} while (pmd++, addr = next, addr != end);
return 0;
}
static inline int unuse_pud_range(struct vm_area_struct *vma, pgd_t *pgd,
unsigned long addr, unsigned long end,
swp_entry_t entry, struct page *page)
{
pud_t *pud;
unsigned long next;
pud = pud_offset(pgd, addr);
do {
next = pud_addr_end(addr, end);
if (pud_none_or_clear_bad(pud))
continue;
if (unuse_pmd_range(vma, pud, addr, next, entry, page))
return 1;
} while (pud++, addr = next, addr != end);
return 0;
}
static int unuse_vma(struct vm_area_struct *vma,
swp_entry_t entry, struct page *page)
{
pgd_t *pgd;
unsigned long addr, end, next;
if (page->mapping) {
addr = page_address_in_vma(page, vma);
if (addr == -EFAULT)
return 0;
else
end = addr + PAGE_SIZE;
} else {
addr = vma->vm_start;
end = vma->vm_end;
}
pgd = pgd_offset(vma->vm_mm, addr);
do {
next = pgd_addr_end(addr, end);
if (pgd_none_or_clear_bad(pgd))
continue;
if (unuse_pud_range(vma, pgd, addr, next, entry, page))
return 1;
} while (pgd++, addr = next, addr != end);
return 0;
}
static int unuse_mm(struct mm_struct *mm,
swp_entry_t entry, struct page *page)
{
struct vm_area_struct *vma;
if (!down_read_trylock(&mm->mmap_sem)) {
/*
* Activate page so shrink_cache is unlikely to unmap its
* ptes while lock is dropped, so swapoff can make progress.
*/
activate_page(page);
unlock_page(page);
down_read(&mm->mmap_sem);
lock_page(page);
}
spin_lock(&mm->page_table_lock);
for (vma = mm->mmap; vma; vma = vma->vm_next) {
if (vma->anon_vma && unuse_vma(vma, entry, page))
break;
}
spin_unlock(&mm->page_table_lock);
up_read(&mm->mmap_sem);
/*
* Currently unuse_mm cannot fail, but leave error handling
* at call sites for now, since we change it from time to time.
*/
return 0;
}
/*
* Scan swap_map from current position to next entry still in use.
* Recycle to start on reaching the end, returning 0 when empty.
*/
static unsigned int find_next_to_unuse(struct swap_info_struct *si,
unsigned int prev)
{
unsigned int max = si->max;
unsigned int i = prev;
int count;
/*
* No need for swap_device_lock(si) here: we're just looking
* for whether an entry is in use, not modifying it; false
* hits are okay, and sys_swapoff() has already prevented new
* allocations from this area (while holding swap_list_lock()).
*/
for (;;) {
if (++i >= max) {
if (!prev) {
i = 0;
break;
}
/*
* No entries in use at top of swap_map,
* loop back to start and recheck there.
*/
max = prev + 1;
prev = 0;
i = 1;
}
count = si->swap_map[i];
if (count && count != SWAP_MAP_BAD)
break;
}
return i;
}
/*
* We completely avoid races by reading each swap page in advance,
* and then search for the process using it. All the necessary
* page table adjustments can then be made atomically.
*/
static int try_to_unuse(unsigned int type)
{
struct swap_info_struct * si = &swap_info[type];
struct mm_struct *start_mm;
unsigned short *swap_map;
unsigned short swcount;
struct page *page;
swp_entry_t entry;
unsigned int i = 0;
int retval = 0;
int reset_overflow = 0;
int shmem;
/*
* When searching mms for an entry, a good strategy is to
* start at the first mm we freed the previous entry from
* (though actually we don't notice whether we or coincidence
* freed the entry). Initialize this start_mm with a hold.
*
* A simpler strategy would be to start at the last mm we
* freed the previous entry from; but that would take less
* advantage of mmlist ordering, which clusters forked mms
* together, child after parent. If we race with dup_mmap(), we
* prefer to resolve parent before child, lest we miss entries
* duplicated after we scanned child: using last mm would invert
* that. Though it's only a serious concern when an overflowed
* swap count is reset from SWAP_MAP_MAX, preventing a rescan.
*/
start_mm = &init_mm;
atomic_inc(&init_mm.mm_users);
/*
* Keep on scanning until all entries have gone. Usually,
* one pass through swap_map is enough, but not necessarily:
* there are races when an instance of an entry might be missed.
*/
while ((i = find_next_to_unuse(si, i)) != 0) {
if (signal_pending(current)) {
retval = -EINTR;
break;
}
/*
* Get a page for the entry, using the existing swap
* cache page if there is one. Otherwise, get a clean
* page and read the swap into it.
*/
swap_map = &si->swap_map[i];
entry = swp_entry(type, i);
page = read_swap_cache_async(entry, NULL, 0);
if (!page) {
/*
* Either swap_duplicate() failed because entry
* has been freed independently, and will not be
* reused since sys_swapoff() already disabled
* allocation from here, or alloc_page() failed.
*/
if (!*swap_map)
continue;
retval = -ENOMEM;
break;
}
/*
* Don't hold on to start_mm if it looks like exiting.
*/
if (atomic_read(&start_mm->mm_users) == 1) {
mmput(start_mm);
start_mm = &init_mm;
atomic_inc(&init_mm.mm_users);
}
/*
* Wait for and lock page. When do_swap_page races with
* try_to_unuse, do_swap_page can handle the fault much
* faster than try_to_unuse can locate the entry. This
* apparently redundant "wait_on_page_locked" lets try_to_unuse
* defer to do_swap_page in such a case - in some tests,
* do_swap_page and try_to_unuse repeatedly compete.
*/
wait_on_page_locked(page);
wait_on_page_writeback(page);
lock_page(page);
wait_on_page_writeback(page);
/*
* Remove all references to entry.
* Whenever we reach init_mm, there's no address space
* to search, but use it as a reminder to search shmem.
*/
shmem = 0;
swcount = *swap_map;
if (swcount > 1) {
if (start_mm == &init_mm)
shmem = shmem_unuse(entry, page);
else
retval = unuse_mm(start_mm, entry, page);
}
if (*swap_map > 1) {
int set_start_mm = (*swap_map >= swcount);
struct list_head *p = &start_mm->mmlist;
struct mm_struct *new_start_mm = start_mm;
struct mm_struct *prev_mm = start_mm;
struct mm_struct *mm;
atomic_inc(&new_start_mm->mm_users);
atomic_inc(&prev_mm->mm_users);
spin_lock(&mmlist_lock);
while (*swap_map > 1 && !retval &&
(p = p->next) != &start_mm->mmlist) {
mm = list_entry(p, struct mm_struct, mmlist);
if (atomic_inc_return(&mm->mm_users) == 1) {
atomic_dec(&mm->mm_users);
continue;
}
spin_unlock(&mmlist_lock);
mmput(prev_mm);
prev_mm = mm;
cond_resched();
swcount = *swap_map;
if (swcount <= 1)
;
else if (mm == &init_mm) {
set_start_mm = 1;
shmem = shmem_unuse(entry, page);
} else
retval = unuse_mm(mm, entry, page);
if (set_start_mm && *swap_map < swcount) {
mmput(new_start_mm);
atomic_inc(&mm->mm_users);
new_start_mm = mm;
set_start_mm = 0;
}
spin_lock(&mmlist_lock);
}
spin_unlock(&mmlist_lock);
mmput(prev_mm);
mmput(start_mm);
start_mm = new_start_mm;
}
if (retval) {
unlock_page(page);
page_cache_release(page);
break;
}
/*
* How could swap count reach 0x7fff when the maximum
* pid is 0x7fff, and there's no way to repeat a swap
* page within an mm (except in shmem, where it's the
* shared object which takes the reference count)?
* We believe SWAP_MAP_MAX cannot occur in Linux 2.4.
*
* If that's wrong, then we should worry more about
* exit_mmap() and do_munmap() cases described above:
* we might be resetting SWAP_MAP_MAX too early here.
* We know "Undead"s can happen, they're okay, so don't
* report them; but do report if we reset SWAP_MAP_MAX.
*/
if (*swap_map == SWAP_MAP_MAX) {
swap_device_lock(si);
*swap_map = 1;
swap_device_unlock(si);
reset_overflow = 1;
}
/*
* If a reference remains (rare), we would like to leave
* the page in the swap cache; but try_to_unmap could
* then re-duplicate the entry once we drop page lock,
* so we might loop indefinitely; also, that page could
* not be swapped out to other storage meanwhile. So:
* delete from cache even if there's another reference,
* after ensuring that the data has been saved to disk -
* since if the reference remains (rarer), it will be
* read from disk into another page. Splitting into two
* pages would be incorrect if swap supported "shared
* private" pages, but they are handled by tmpfs files.
*
* Note shmem_unuse already deleted a swappage from
* the swap cache, unless the move to filepage failed:
* in which case it left swappage in cache, lowered its
* swap count to pass quickly through the loops above,
* and now we must reincrement count to try again later.
*/
if ((*swap_map > 1) && PageDirty(page) && PageSwapCache(page)) {
struct writeback_control wbc = {
.sync_mode = WB_SYNC_NONE,
};
swap_writepage(page, &wbc);
lock_page(page);
wait_on_page_writeback(page);
}
if (PageSwapCache(page)) {
if (shmem)
swap_duplicate(entry);
else
delete_from_swap_cache(page);
}
/*
* So we could skip searching mms once swap count went
* to 1, we did not mark any present ptes as dirty: must
* mark page dirty so shrink_list will preserve it.
*/
SetPageDirty(page);
unlock_page(page);
page_cache_release(page);
/*
* Make sure that we aren't completely killing
* interactive performance.
*/
cond_resched();
}
mmput(start_mm);
if (reset_overflow) {
printk(KERN_WARNING "swapoff: cleared swap entry overflow\n");
swap_overflow = 0;
}
return retval;
}
/*
* After a successful try_to_unuse, if no swap is now in use, we know we
* can empty the mmlist. swap_list_lock must be held on entry and exit.
* Note that mmlist_lock nests inside swap_list_lock, and an mm must be
* added to the mmlist just after page_duplicate - before would be racy.
*/
static void drain_mmlist(void)
{
struct list_head *p, *next;
unsigned int i;
for (i = 0; i < nr_swapfiles; i++)
if (swap_info[i].inuse_pages)
return;
spin_lock(&mmlist_lock);
list_for_each_safe(p, next, &init_mm.mmlist)
list_del_init(p);
spin_unlock(&mmlist_lock);
}
/*
* Use this swapdev's extent info to locate the (PAGE_SIZE) block which
* corresponds to page offset `offset'.
*/
sector_t map_swap_page(struct swap_info_struct *sis, pgoff_t offset)
{
struct swap_extent *se = sis->curr_swap_extent;
struct swap_extent *start_se = se;
for ( ; ; ) {
struct list_head *lh;
if (se->start_page <= offset &&
offset < (se->start_page + se->nr_pages)) {
return se->start_block + (offset - se->start_page);
}
lh = se->list.next;
if (lh == &sis->extent_list)
lh = lh->next;
se = list_entry(lh, struct swap_extent, list);
sis->curr_swap_extent = se;
BUG_ON(se == start_se); /* It *must* be present */
}
}
/*
* Free all of a swapdev's extent information
*/
static void destroy_swap_extents(struct swap_info_struct *sis)
{
while (!list_empty(&sis->extent_list)) {
struct swap_extent *se;
se = list_entry(sis->extent_list.next,
struct swap_extent, list);
list_del(&se->list);
kfree(se);
}
}
/*
* Add a block range (and the corresponding page range) into this swapdev's
* extent list. The extent list is kept sorted in page order.
*
* This function rather assumes that it is called in ascending page order.
*/
static int
add_swap_extent(struct swap_info_struct *sis, unsigned long start_page,
unsigned long nr_pages, sector_t start_block)
{
struct swap_extent *se;
struct swap_extent *new_se;
struct list_head *lh;
lh = sis->extent_list.prev; /* The highest page extent */
if (lh != &sis->extent_list) {
se = list_entry(lh, struct swap_extent, list);
BUG_ON(se->start_page + se->nr_pages != start_page);
if (se->start_block + se->nr_pages == start_block) {
/* Merge it */
se->nr_pages += nr_pages;
return 0;
}
}
/*
* No merge. Insert a new extent, preserving ordering.
*/
new_se = kmalloc(sizeof(*se), GFP_KERNEL);
if (new_se == NULL)
return -ENOMEM;
new_se->start_page = start_page;
new_se->nr_pages = nr_pages;
new_se->start_block = start_block;
list_add_tail(&new_se->list, &sis->extent_list);
return 1;
}
/*
* A `swap extent' is a simple thing which maps a contiguous range of pages
* onto a contiguous range of disk blocks. An ordered list of swap extents
* is built at swapon time and is then used at swap_writepage/swap_readpage
* time for locating where on disk a page belongs.
*
* If the swapfile is an S_ISBLK block device, a single extent is installed.
* This is done so that the main operating code can treat S_ISBLK and S_ISREG
* swap files identically.
*
* Whether the swapdev is an S_ISREG file or an S_ISBLK blockdev, the swap
* extent list operates in PAGE_SIZE disk blocks. Both S_ISREG and S_ISBLK
* swapfiles are handled *identically* after swapon time.
*
* For S_ISREG swapfiles, setup_swap_extents() will walk all the file's blocks
* and will parse them into an ordered extent list, in PAGE_SIZE chunks. If
* some stray blocks are found which do not fall within the PAGE_SIZE alignment
* requirements, they are simply tossed out - we will never use those blocks
* for swapping.
*
* For S_ISREG swapfiles we set S_SWAPFILE across the life of the swapon. This
* prevents root from shooting her foot off by ftruncating an in-use swapfile,
* which will scribble on the fs.
*
* The amount of disk space which a single swap extent represents varies.
* Typically it is in the 1-4 megabyte range. So we can have hundreds of
* extents in the list. To avoid much list walking, we cache the previous
* search location in `curr_swap_extent', and start new searches from there.
* This is extremely effective. The average number of iterations in
* map_swap_page() has been measured at about 0.3 per page. - akpm.
*/
static int setup_swap_extents(struct swap_info_struct *sis, sector_t *span)
{
struct inode *inode;
unsigned blocks_per_page;
unsigned long page_no;
unsigned blkbits;
sector_t probe_block;
sector_t last_block;
sector_t lowest_block = -1;
sector_t highest_block = 0;
int nr_extents = 0;
int ret;
inode = sis->swap_file->f_mapping->host;
if (S_ISBLK(inode->i_mode)) {
ret = add_swap_extent(sis, 0, sis->max, 0);
*span = sis->pages;
goto done;
}
blkbits = inode->i_blkbits;
blocks_per_page = PAGE_SIZE >> blkbits;
/*
* Map all the blocks into the extent list. This code doesn't try
* to be very smart.
*/
probe_block = 0;
page_no = 0;
last_block = i_size_read(inode) >> blkbits;
while ((probe_block + blocks_per_page) <= last_block &&
page_no < sis->max) {
unsigned block_in_page;
sector_t first_block;
first_block = bmap(inode, probe_block);
if (first_block == 0)
goto bad_bmap;
/*
* It must be PAGE_SIZE aligned on-disk
*/
if (first_block & (blocks_per_page - 1)) {
probe_block++;
goto reprobe;
}
for (block_in_page = 1; block_in_page < blocks_per_page;
block_in_page++) {
sector_t block;
block = bmap(inode, probe_block + block_in_page);
if (block == 0)
goto bad_bmap;
if (block != first_block + block_in_page) {
/* Discontiguity */
probe_block++;
goto reprobe;
}
}
first_block >>= (PAGE_SHIFT - blkbits);