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mm: fix clear_page_dirty_for_io vs fault race

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Fix msync data loss and (less importantly) dirty page accounting
inaccuracies due to the race remaining in clear_page_dirty_for_io().

The deleted comment explains what the race was, and the added comments
explain how it is fixed.

Signed-off-by: Nick Piggin <npiggin@suse.de>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Miklos Szeredi <miklos@szeredi.hu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Nick Piggin 2007-07-19 01:47:22 -07:00 committed by Linus Torvalds
parent 83c54070ee
commit 79352894b2
2 changed files with 21 additions and 5 deletions
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9
mm/memory.c
View file

@ -1765,6 +1765,15 @@ static int do_wp_page(struct mm_struct *mm, struct vm_area_struct *vma,
unlock:
pte_unmap_unlock(page_table, ptl);
if (dirty_page) {
/*
* Yes, Virginia, this is actually required to prevent a race
* with clear_page_dirty_for_io() from clearing the page dirty
* bit after it clear all dirty ptes, but before a racing
* do_wp_page installs a dirty pte.
*
* do_no_page is protected similarly.
*/
wait_on_page_locked(dirty_page);
set_page_dirty_balance(dirty_page);
put_page(dirty_page);
}

17
mm/page-writeback.c
View file

@ -918,6 +918,8 @@ int clear_page_dirty_for_io(struct page *page)
{
struct address_space *mapping = page_mapping(page);
BUG_ON(!PageLocked(page));
if (mapping && mapping_cap_account_dirty(mapping)) {
/*
* Yes, Virginia, this is indeed insane.
@ -943,14 +945,19 @@ int clear_page_dirty_for_io(struct page *page)
* We basically use the page "master dirty bit"
* as a serialization point for all the different
* threads doing their things.
*
* FIXME! We still have a race here: if somebody
* adds the page back to the page tables in
* between the "page_mkclean()" and the "TestClearPageDirty()",
* we might have it mapped without the dirty bit set.
*/
if (page_mkclean(page))
set_page_dirty(page);
/*
* We carefully synchronise fault handlers against
* installing a dirty pte and marking the page dirty
* at this point. We do this by having them hold the
* page lock at some point after installing their
* pte, but before marking the page dirty.
* Pages are always locked coming in here, so we get
* the desired exclusion. See mm/memory.c:do_wp_page()
* for more comments.
*/
if (TestClearPageDirty(page)) {
dec_zone_page_state(page, NR_FILE_DIRTY);
return 1;