Re: [Bug 9182] Critical memory leak (dirty pages)

[Björn Steinbrink]

On 2007.12.20 08:25:56 -0800, Linus Torvalds wrote:
> 
> 
> On Thu, 20 Dec 2007, Bj?rn Steinbrink wrote:
> > 
> > OK, so I looked for PG_dirty anyway.
> > 
> > In 46d2277c796f9f4937bfa668c40b2e3f43e93dd0 you made try_to_free_buffers
> > bail out if the page is dirty.
> > 
> > Then in 3e67c0987d7567ad666641164a153dca9a43b11d, Andrew fixed
> > truncate_complete_page, because it called cancel_dirty_page (and thus
> > cleared PG_dirty) after try_to_free_buffers was called via
> > do_invalidatepage.
> > 
> > Now, if I'm not mistaken, we can end up as follows.
> > 
> > truncate_complete_page()
> >   cancel_dirty_page() // PG_dirty cleared, decr. dirty pages
> >   do_invalidatepage()
> >     ext3_invalidatepage()
> >       journal_invalidatepage()
> >         journal_unmap_buffer()
> >           __dispose_buffer()
> >             __journal_unfile_buffer()
> >               __journal_temp_unlink_buffer()
> >                 mark_buffer_dirty(); // PG_dirty set, incr. dirty pages
> 
> Good, this seems to be the exact path that actually triggers it. I got to 
> journal_unmap_buffer(), but was too lazy to actually then bother to follow 
> it all the way down - I decided that I didn't actually really even care 
> what the low-level FS layer did, I had already convinced myself that it 
> obviously must be dirtying the page some way, since that matched the 
> symptoms exactly (ie only the journaling case was impacted, and this was 
> all about the journal).
> 
> But perhaps more importantly: regardless of what the low-level filesystem 
> did at that point, the VM accounting shouldn't care, and should be robust 
> in the face of a low-level filesystem doing strange and wonderful things. 
> But thanks for bothering to go through the whole history and figure out 
> what exactly is up.

Oh well, after seeing the move of cancel_dirty_page, I just went
backwards from __set_page_dirty using cscope + some smart guessing and
quickly ended up at ext3_invalidatepage, so it wasn't that hard :-)

> > As try_to_free_buffers got its ext3 hack back in
> > ecdfc9787fe527491baefc22dce8b2dbd5b2908d, maybe
> > 3e67c0987d7567ad666641164a153dca9a43b11d should be reverted? (Except for
> > the accounting fix in cancel_dirty_page, of course).
> 
> Yes, I think we have room for cleanups now, and I agree: we ended up 
> reinstating some questionable code in the VM just because we didn't really 
> know or understand what was going on in the ext3 journal code. 

Hm, you attributed more to my mail than there was actually in it. I
didn't even start to think of cleanups (because I don't know jack about
the whole ext3/jdb stuff, so I simply cannot come up with any cleanups
(yet?)).What I meant is that we only did a half-revert of that hackery.

When try_to_free_buffers started to check for PG_dirty, the
cancel_dirty_page call had to be called before do_invalidatepage, to
"fix" a _huge_ leak.  But that caused the accouting breakage we're now
seeing, because we never account for the pages that got redirtied during
do_invalidatepage.

Then the change to try_to_free_buffers got reverted, so we no longer
need to call cancel_dirty_page before do_invalidatepage, but still we
do. Thus the accounting bug remains. So what I meant to suggest was
simply to actually "finish" the revert we started.

Or expressed as a patch:

diff --git a/mm/truncate.c b/mm/truncate.c
index cadc156..2974903 100644
--- a/mm/truncate.c
+++ b/mm/truncate.c
@@ -98,11 +98,11 @@ truncate_complete_page(struct address_space *mapping, struct page *page)
 	if (page->mapping != mapping)
 		return;
 
-	cancel_dirty_page(page, PAGE_CACHE_SIZE);
-
 	if (PagePrivate(page))
 		do_invalidatepage(page, 0);
 
+	cancel_dirty_page(page, PAGE_CACHE_SIZE);
+
 	remove_from_page_cache(page);
 	ClearPageUptodate(page);
 	ClearPageMappedToDisk(page);

I'll be the last one to comment on whether or not that causes inaccurate
accouting, so I'll just watch you and Jan battle that out until someone
comes up with a post-.24 patch to provide a clean fix for the issue.

Krzysztof, could you give this patch a test run?

If that "fixes" the problem for now, I'll try to come up with some
usable commit message, or if somehow wants to beat me to it, you can
already have my

Signed-off-by: Björn Steinbrink <B.Steinbrink@xxxxxx>

> > On a side note, before 8368e328dfe1c534957051333a87b3210a12743b the task
> > io accounting for cancelled writes happened always happened if the page
> > was dirty, regardless of page->mapping. This was also already true for
> > the old test_clear_page_dirty code, and the commit log for
> > 8368e328dfe1c534957051333a87b3210a12743b doesn't mention that semantic
> > change either, so maybe the "if (account_size)" block should be moved
> > out of the if "(mapping && ...)" block?
> 
> I think the "if (account_size)" thing was *purely* for me being worried 
> about hugetlb entries, and I think that's the only thing that passes in a 
> zero account size.
> 
> But hugetlbfs already has BDI_CAP_NO_ACCT_DIRTY set (exactly because we 
> cannot account for those pages *anyway*), so I think we could go further 
> than move the account_size outside of the test, I think we could probably 
> remove that test entirely and drop the whole thing.
> 
> The thing is, task_io_account_cancelled_write() doesn't make sense on 
> mappings that don't do dirty accounting, since those mappings are all 
> special anyway: they don't actually do any real IO, they are all in-ram 
> things. So I think it should stay inside the
> 
> 	if (mapping && mapping_cap_account_dirty(mapping))
> 		..
> 
> test, simply because I don't think it makes any conceptual sense outside 
> of it.
> 
> Hmm?

Yeah, you're right. It's symmetric with __set_page_dirty, which only
calls task_io_account_write when there's a mapping.

Björn
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