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[PATCH] can_share_swap_page: use page_mapcount

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Remember that ironic get_user_pages race?  when the raised page_count on a
page swapped out led do_wp_page to decide that it had to copy on write, so
substituted a different page into userspace.  2.6.7 onwards have Andrea's
solution, where try_to_unmap_one backs out if it finds page_count raised.

Which works, but is unsatisfying (rmap.c has no other page_count heuristics),
and was found a few months ago to hang an intensive page migration test.  A
year ago I was hesitant to engage page_mapcount, now it seems the right fix.

So remove the page_count hack from try_to_unmap_one; and use activate_page in
unuse_mm when dropping lock, to replace its secondary effect of helping
swapoff to make progress in that case.

Simplify can_share_swap_page (now called only on anonymous pages) to check
page_mapcount + page_swapcount == 1: still needs the page lock to stabilize
their (pessimistic) sum, but does not need swapper_space.tree_lock for that.

In do_swap_page, move swap_free and unlock_page below page_add_anon_rmap, to
keep sum on the high side, and correct when can_share_swap_page called.

Signed-off-by: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
This commit is contained in:
Hugh Dickins 2005-06-21 17:15:12 -07:00 committed by Linus Torvalds
parent d296e9cd02
commit c475a8ab62
3 changed files with 21 additions and 65 deletions
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10
mm/memory.c
View File

@ -1686,10 +1686,6 @@ static int do_swap_page(struct mm_struct * mm,
}
/* The page isn't present yet, go ahead with the fault. */
swap_free(entry);
if (vm_swap_full())
remove_exclusive_swap_page(page);
inc_mm_counter(mm, rss);
pte = mk_pte(page, vma->vm_page_prot);
@ -1697,12 +1693,16 @@ static int do_swap_page(struct mm_struct * mm,
pte = maybe_mkwrite(pte_mkdirty(pte), vma);
write_access = 0;
}
unlock_page(page);
flush_icache_page(vma, page);
set_pte_at(mm, address, page_table, pte);
page_add_anon_rmap(page, vma, address);
swap_free(entry);
if (vm_swap_full())
remove_exclusive_swap_page(page);
unlock_page(page);
if (write_access) {
if (do_wp_page(mm, vma, address,
page_table, pmd, pte) == VM_FAULT_OOM)

21
mm/rmap.c
View File

@ -539,27 +539,6 @@ static int try_to_unmap_one(struct page *page, struct vm_area_struct *vma)
goto out_unmap;
}
/*
* Don't pull an anonymous page out from under get_user_pages.
* GUP carefully breaks COW and raises page count (while holding
* page_table_lock, as we have here) to make sure that the page
* cannot be freed. If we unmap that page here, a user write
* access to the virtual address will bring back the page, but
* its raised count will (ironically) be taken to mean it's not
* an exclusive swap page, do_wp_page will replace it by a copy
* page, and the user never get to see the data GUP was holding
* the original page for.
*
* This test is also useful for when swapoff (unuse_process) has
* to drop page lock: its reference to the page stops existing
* ptes from being unmapped, so swapoff can make progress.
*/
if (PageSwapCache(page) &&
page_count(page) != page_mapcount(page) + 2) {
ret = SWAP_FAIL;
goto out_unmap;
}
/* Nuke the page table entry. */
flush_cache_page(vma, address, page_to_pfn(page));
pteval = ptep_clear_flush(vma, address, pte);

55
mm/swapfile.c
View File

@ -276,61 +276,37 @@ void swap_free(swp_entry_t entry)
}
/*
* Check if we're the only user of a swap page,
* when the page is locked.
* How many references to page are currently swapped out?
*/
static int exclusive_swap_page(struct page *page)
static inline int page_swapcount(struct page *page)
{
int retval = 0;
struct swap_info_struct * p;
int count = 0;
struct swap_info_struct *p;
swp_entry_t entry;
entry.val = page->private;
p = swap_info_get(entry);
if (p) {
/* Is the only swap cache user the cache itself? */
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)
retval = 1;
write_unlock_irq(&swapper_space.tree_lock);
}
/* Subtract the 1 for the swap cache itself */
count = p->swap_map[swp_offset(entry)] - 1;
swap_info_put(p);
}
return retval;
return count;
}
/*
* We can use this swap cache entry directly
* if there are no other references to it.
*
* Here "exclusive_swap_page()" does the real
* work, but we opportunistically check whether
* we need to get all the locks first..
*/
int can_share_swap_page(struct page *page)
{
int retval = 0;
int count;
if (!PageLocked(page))
BUG();
switch (page_count(page)) {
case 3:
if (!PagePrivate(page))
break;
/* Fallthrough */
case 2:
if (!PageSwapCache(page))
break;
retval = exclusive_swap_page(page);
break;
case 1:
if (PageReserved(page))
break;
retval = 1;
}
return retval;
BUG_ON(!PageLocked(page));
count = page_mapcount(page);
if (count <= 1 && PageSwapCache(page))
count += page_swapcount(page);
return count == 1;
}
/*
@ -529,9 +505,10 @@ static int unuse_mm(struct mm_struct *mm,
if (!down_read_trylock(&mm->mmap_sem)) {
/*
* Our reference to the page stops try_to_unmap_one from
* unmapping its ptes, so swapoff can make progress.
* 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);