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Resurrect 'try_to_free_buffers()' VM hackery

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It's not pretty, but it appears that ext3 with data=journal will clean
pages without ever actually telling the VM that they are clean.  This,
in turn, will result in the VM (and balance_dirty_pages() in particular)
to never realize that the pages got cleaned, and wait forever for an
event that already happened.

Technically, this seems to be a problem with ext3 itself, but it used to
be hidden by 'try_to_free_buffers()' noticing this situation on its own,
and just working around the filesystem problem.

This commit re-instates that hack, in order to avoid a regression for
the 2.6.20 release. This fixes bugzilla 7844:

	http://bugzilla.kernel.org/show_bug.cgi?id=7844

Peter Zijlstra points out that we should probably retain the debugging
code that this removes from cancel_dirty_page(), and I agree, but for
the imminent release we might as well just silence the warning too
(since it's not a new bug: anything that triggers that warning has been
around forever).

Acked-by: Randy Dunlap <rdunlap@xenotime.net>
Acked-by: Jens Axboe <jens.axboe@oracle.com>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Linus Torvalds 2007-01-26 12:47:06 -08:00
commit ecdfc9787f
2 changed files with 28 additions and 8 deletions
Download patch file Download diff file Expand all files Collapse all files

15
fs/buffer.c
View file

@ -2834,7 +2834,7 @@ int try_to_free_buffers(struct page *page)
int ret = 0;
BUG_ON(!PageLocked(page));
if (PageDirty(page) || PageWriteback(page))
if (PageWriteback(page))
return 0;
if (mapping == NULL) { /* can this still happen? */
@ -2845,6 +2845,19 @@ int try_to_free_buffers(struct page *page)
spin_lock(&mapping->private_lock);
ret = drop_buffers(page, &buffers_to_free);
spin_unlock(&mapping->private_lock);
/*
* If the filesystem writes its buffers by hand (eg ext3)
* then we can have clean buffers against a dirty page. We
* clean the page here; otherwise the VM will never notice
* that the filesystem did any IO at all.
*
* Also, during truncate, discard_buffer will have marked all
* the page's buffers clean. We discover that here and clean
* the page also.
*/
if (ret)
cancel_dirty_page(page, PAGE_CACHE_SIZE);
out:
if (buffers_to_free) {
struct buffer_head *bh = buffers_to_free;

21
mm/truncate.c
View file

@ -51,15 +51,22 @@ static inline void truncate_partial_page(struct page *page, unsigned partial)
do_invalidatepage(page, partial);
}
/*
* This cancels just the dirty bit on the kernel page itself, it
* does NOT actually remove dirty bits on any mmap's that may be
* around. It also leaves the page tagged dirty, so any sync
* activity will still find it on the dirty lists, and in particular,
* clear_page_dirty_for_io() will still look at the dirty bits in
* the VM.
*
* Doing this should *normally* only ever be done when a page
* is truncated, and is not actually mapped anywhere at all. However,
* fs/buffer.c does this when it notices that somebody has cleaned
* out all the buffers on a page without actually doing it through
* the VM. Can you say "ext3 is horribly ugly"? Tought you could.
*/
void cancel_dirty_page(struct page *page, unsigned int account_size)
{
/* If we're cancelling the page, it had better not be mapped any more */
if (page_mapped(page)) {
static unsigned int warncount;
WARN_ON(++warncount < 5);
}
if (TestClearPageDirty(page)) {
struct address_space *mapping = page->mapping;
if (mapping && mapping_cap_account_dirty(mapping)) {