After several days of effort, Linus Torvalds tracked down and posted a patch for a low level data corruption bug [story [1]]. In a series of emails, Linus thoroughly explained the thought process involved in isolating the exact problem. He described the bug as being a difference of expectations between the filesystem and the VM:
"Both filesystem and VM actually _think_ they do the right thing, because they simply have totally different expectations. The filesystem thinks that it should care about dirty buffers (that got marked dirty _after_ they were dirtied), while the filesystem thinks that it cares about dirty _pages_ (that got dirted at any time _before_ 'writepage()' was called). Neither is really 'wrong', per se, it's just that the two parts have different expectations, and the _combination_ just doesn't work. 'set_page_dirty()' at some point meant 'the writes have been done', but these days it really means something else."
Linus posted the fix in a followup email, "putting on the thinking cap, there's actually a fairly simple an nonintrusive patch. It still has a tiny tiny race (see the comment), but I bet nobody can really hit it in real life anyway, and I know several ways to fix it, so I'm not really _that_ worried about it." A comment in the patch explains, "we use this sequence to make sure that (a) we account for dirty stats properly (b) we tell the low-level filesystem to mark the whole page dirty if it was dirty in a pagetable. Only to then (c) clean the page again and return 1 to cause the writeback. This way we avoid all nasty races with the dirty bit in multiple places and clearing them concurrently from different threads." Another comments explains the remaining race, "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."
From: Linus Torvalds [email blocked]
To: Segher Boessenkool [email blocked]
Subject: Re: [PATCH] mm: fix page_mkclean_one
Date: Thu, 28 Dec 2006 22:48:51 -0800 (PST)
On Fri, 29 Dec 2006, Segher Boessenkool wrote:
>
> > I think what might be happening is that pdflush writes them out fine,
> > however we don't trap writes by the application _during_ that writeout.
>
> Yeah. I believe that more exactly it happens if the very last
> write to the page causes a writeback (due to dirty balancing)
> while another writeback for the page is already happening.
>
> As usual in these cases, I have zero proof.
I actually have proof to the contrary, ie I have traces that say "the
write was started" after the last write.
And the VM layer in this area is actually fairly sane and civilized. It
has a bit that says "writeback in progress", and if that bit is set, it
simply _will_not_ start a new write. It even has various BUG_ON()'s to
that effect.
So everything I have ever seen says that the VM layer is actually doing
everything right.
> It's the do_wp_page -> balance_dirty_pages -> generic_writepages
> path for sure. Maybe it's enough to change
>
> if (wbc->sync_mode != WB_SYNC_NONE)
> wait_on_page_writeback(page);
>
> if (PageWriteback(page) ||
> !clear_page_dirty_for_io(page)) {
> unlock_page(page);
> continue;
> }
Notice how this one basically says:
- if it's under writeback, don't even clear the page dirty flag.
Your suggested change:
> if (wbc->sync_mode != WB_SYNC_NONE)
> wait_on_page_writeback(page);
>
> if (PageWriteback(page)) {
> redirty_page_for_writepage(wbc, page);
makes no sense, because we simply never _did_ the "clear_page_dirty()" if
the thing was under writeback in the first place. That's how C
conditionals work. So there's no reason to "redirty" it, because it
wasn't cleaned in the first place.
I've double- and triple-checked the dirty bits, including having traces
that actually say that the IO was started (from a VM perspective) _after_
the last write was done. The IO just didn't hit the disk.
I'm personally fairly convinced that it's not a VM issue, but a "IO
issue". Either in a low-level filesystem or in some of the fs/buffer.c
helper routines.
But I'd love to be proven wrong.
I do have a few interesting details from the trace I haven't really
analyzed yet. Here's the trace for events on one of the pages that was
corrupted. Note how the events are numbered (there were 171640 events
total), so the thing you see is just a small set of events from the whole
big trace, but it's the ones that talk about _that_ particular page.
I've grouped them so hat "consecutive" events group together. That just
means that no events on any other pages happened in between those events,
and it is usually a sign that it's really one single call-chain that
causes all the events.
For example, for the first group of three events (44366-44368), it's the
page fault that brings in the page, and since it's a write-fault, it will
not only map the page, it will mark the page itself dirty and then also
set the TAG_DIRTY on the mapping. So the "group" is just really a result
of one single event happening, which causes several things to happen to
that page. That's exactly what you'd expect.
Anyway, here is the list of events that page went through:
44366 PG 00000f6d: mm/memory.c:2254 mapping at b789fc54 (write)
44367 PG 00000f6d: mm/page-writeback.c:817 setting dirty
44368 PG 00000f6d: fs/buffer.c:738 setting TAG_DIRTY
64231 PG 00000f6d: mm/page-writeback.c:872 clean_for_io
64232 PG 00000f6d: mm/rmap.c:451 cleaning PTE b789f000
64233 PG 00000f6d: mm/page-writeback.c:914 set writeback
64234 PG 00000f6d: mm/page-writeback.c:916 setting TAG_WRITEBACK
64235 PG 00000f6d: mm/page-writeback.c:922 clearing TAG_DIRTY
67570 PG 00000f6d: mm/page-writeback.c:891 end writeback
67571 PG 00000f6d: mm/page-writeback.c:893 clearing TAG_WRITEBACK
76705 PG 00000f6d: mm/page-writeback.c:817 setting dirty
76706 PG 00000f6d: fs/buffer.c:725 dirtied buffers
76707 PG 00000f6d: fs/buffer.c:738 setting TAG_DIRTY
105267 PG 00000f6d: mm/page-writeback.c:872 clean_for_io
105268 PG 00000f6d: mm/rmap.c:451 cleaning PTE b789f000
105269 PG 00000f6d: mm/page-writeback.c:914 set writeback
105270 PG 00000f6d: mm/page-writeback.c:916 setting TAG_WRITEBACK
105271 PG 00000f6d: mm/page-writeback.c:922 clearing TAG_DIRTY
105272 PG 00000f6d: mm/page-writeback.c:891 end writeback
105273 PG 00000f6d: mm/page-writeback.c:893 clearing TAG_WRITEBACK
128032 PG 00000f6d: mm/memory.c:670 unmapped at b789f000
132662 PG 00000f6d: mm/filemap.c:119 Removing page cache
148278 PG 00000f6d: mm/memory.c:2254 mapping at b789f000 (read)
166326 PG 00000f6d: mm/memory.c:670 unmapped at b789f000
171958 PG 00000f6d: mm/filemap.c:119 Removing page cache
And notice that big grouping of seven events (105267-105273). The five
first events really _do_ make sense together: it's our page cleaning that
happens. But notice how the "end writeback" happens _immediately_.
Here's another page cleaning event for the page that preceded that page,
and did _not_ get corrupted:
105262 PG 00000f6c: mm/page-writeback.c:872 clean_for_io
105263 PG 00000f6c: mm/rmap.c:451 cleaning PTE b789e000
105264 PG 00000f6c: mm/page-writeback.c:914 set writeback
105265 PG 00000f6c: mm/page-writeback.c:916 setting TAG_WRITEBACK
105266 PG 00000f6c: mm/page-writeback.c:922 clearing TAG_DIRTY
108437 PG 00000f6c: mm/page-writeback.c:891 end writeback
108438 PG 00000f6c: mm/page-writeback.c:893 clearing TAG_WRITEBACK
and this looks a lot more like what you'd expect: other thngs happened in
between the "clear dirty, set writeback" stage and the "end writeback"
stage. That's what you'd expect to see if there was actually overlapping
IO and/or work.
(And notice that that _was_ what you saw even for the corrupted page for
the _first_ writeback: you saw the group-of-five that indicated a page
cleaning event had started, and then a group-of-two to indicate that the
writeback finished).
So I find this kind of pattern really suspicious. We have a missing
writeout, and my traces show (I didn't analyze this _particular_ one
closely, but I did the previous trace for another page that I posted) that
the writeback was actually started after the write that went missing was
done. AND I have this trace that seems to show that the writeback
basically completed immediately, with no other work in between.
That to me says: "somebody didn't actually write out out". The VM layer
asked the filesystem to do the write, but the filesystem just didn't do
it. I personally think it's because some buffer-head BH_dirty bit got
scrogged, but it could be some event that makes the filesystem simply not
do the IO because it thinks the "disk queues are too full", so it just
says "IO completed", without actually doing anything at all.
Now, the fact that it apparently happens for all of ext2, ext3
and reiserfs (but NOT apparently with "data=writeback"), makes me suspect
that there is some common interaction, and that it's somehow BH-related
(they all share much of the buffer head infrastructure). So it doesn't
look like it's just a bug in one random filesystem, I think it's a bug in
some buffer-head infrastructure/helper function.
So I don't think it's "core VM". I don't think it's the "page cache". I
think we handle the dirty state correctly at that level.
It looks more like "buffer cache" or "filesystem" to me by now.
(Btw, don't get me wrong - the above sequence numbers are in no way
*proof* of anything. You could get big groups for one page just because
something ended up being synchronous. I'll add some timestamps to my
traces to make it easier to see where there was real IO going on and where
there wasn't).
Linus
From: Linus Torvalds [email blocked]
Subject: Ok, explained.. (was Re: [PATCH] mm: fix page_mkclean_one)
Date: Fri, 29 Dec 2006 00:58:12 -0800 (PST)
On Thu, 28 Dec 2006, Linus Torvalds wrote:
>
> So everything I have ever seen says that the VM layer is actually doing
> everything right.
That was true, but at the same time, it's not. Let me explain.
> That to me says: "somebody didn't actually write out out". The VM layer
> asked the filesystem to do the write, but the filesystem just didn't do
> it. I personally think it's because some buffer-head BH_dirty bit got
> scrogged
Ok, I have proof of this now.
Here's a trace (with cycle counts), and with a new trace event added: this
is for another corrupted page. I have:
49105 PG 000015d8 (14800): mm/page-writeback.c:872 clean_for_io
49106 PG 000015d8 (6900): mm/rmap.c:451 cleaning PTE b7fa6000
49107 PG 000015d8 (9900): mm/page-writeback.c:914 set writeback
49108 PG 000015d8 (6480): mm/page-writeback.c:916 setting TAG_WRITEBACK
49109 PG 000015d8 (7110): mm/page-writeback.c:922 clearing TAG_DIRTY
49110 PG 000015d8 (7190): fs/buffer.c:1713 no IO underway
49111 PG 000015d8 (6180): mm/page-writeback.c:891 end writeback
49112 PG 000015d8 (6460): mm/page-writeback.c:893 clearing TAG_WRITEBACK
where that first column is the trace event number again, and the "PG
000015d8" is that corrupted page. The thing in the parenthesis is "CPU
cycles since last event), and the important part to note is that this is
indeed all one single thing with no actual IO anywhere (~7000 CPU cycles
may sound like a lot, but (a) it's not that many cache misses and (b) a
lot of it is the logging overhead - back-to-back log events will take
about 3500 cycles) just because it does the actual ASCII printk() etc.
Also, the new event is:
fs/buffer.c:1713 no IO underway
which is just the
if (nr_underway == 0)
case in fs/buffer.c
And I now finally really believe that I fully understand the corruption,
but I don't have a simple solution, much less a patch.
What the problem basically boils down to is that "set_page_dirty()" is
supposed to be a mark for dirtying THE WHOLE PAGE, but it really is not
"the whole page when the 'set_page_dirty()' itself happens", but more of a
"the next writepage() needs to write back the whole page" thing.
And that's not what "__set_page_dirty_buffers()" really does.
Because what "__set_page_dirty_buffers()" does is that AT THE TIME THE
"set_page_dirty()" IS CALLED, it will mark all the buffers on that page as
dirty. That may _sound_ like what we want, but it really isn't. Because by
the time "writepage()" is actually called (which can be MUCH MUCH later),
some internal filesystem activity may actually have cleaned one or more of
those buffers in the meantime, and now we call "writepage()" (which really
wants to write them _all_), and it will write only part of them, or none
at all.
So the VM thought that since it did a "writepage()", all the dirty state
at that point got written back. But it didn't - the filesystem could have
written back part or all of the page much earlier, and the writepage()
actually does nothing at all.
Both filesystem and VM actually _think_ they do the right thing, because
they simply have totally different expectations. The filesystem thinks
that it should care about dirty buffers (that got marked dirty _after_
they were dirtied), while the filesystem thinks that it cares about dirty
_pages_ (that got dirted at any time _before_ "writepage()" was called).
Neither is really "wrong", per se, it's just that the two parts have
different expectations, and the _combination_ just doesn't work.
"set_page_dirty()" at some point meant "the writes have been done", but
these days it really means something else.
Now, the reason there is no trivial patch is not that this is conceptually
really hard to fix. I can see several different approaches to fixing it,
but they all really boil down to two alternatives:
(a) splitting the one "PG_dirty" bit up into two bits: the
"PG_writescheduled" bit and the "PG_alldirty" bit.
The "PG_write_scheduled" bit would be the bit that the filesystem
would set when it has pending dirty data that it wrote itself (and
that may not cover the whole page), and is the part of PG_dirty that
sets the PAGECACHE_TAG_DIRTY. It's also what forces "writepage()" to
be called.
The "PG_alldirty" bit is just an additional "somebody else dirtied
random parts of this page, and we don't know what" flag, which is set
by "set_page_dirty()" in addition to doing the PG_write_scheduled
stuff. We would test-and-clear it at "writepage()" time, and pass it
in to "writepages()" to tell the writepage() function that it can't
just write out its own small limited notion of what is dirty.
(There are various variations on this whole theme: instead of having
a flag to "writepage()", we could split the "whole page" case out as
a separate callback or similar)
(b) making sure that all "set_page_dirty()" calls are _after_ the page
has been marked dirty (which in the case of memory mapped pages would
mean that we would _not_ call it when we mark the page writable at
all, we would call it when we _remove_ the dirty bit and mark it
unwritable). That would have the nice fearture that it wouldn't
require any FS-level changes, which would be a nice thing - it would
basically make the VM dirty accounting work the way the FS layer now
already expects it to.
I think (b) is conceptually simpler, and I think I'll try it tomorrow
after I've slept on it. The reason I mention (a) at all is that I like the
conceptual notion of telling he filesystem ahead of time that "you're
going to get a full dirty page", because what (b) will inevitably lead to
is that the filesystem will maintain its own partial state, and then
effectively just before it gets the writepage() notification, it will be
told it was all pointless, because we just dirtied the whole thing.
IOW, the advantage of (a) is also it's disadvantage: it tells the
filesystem more. The disadvantage is that it would require VFS interface
changes exactly to do that (ie the "mapping->set_page_dirty()" semantics
would also be split up into two, and it would now be a "prepare to write
the whole page during the next 'writepage()'" thing).
So to recap: the VM layer really expected "writepage()" to act as if it
wrote out the whole page. It doesn't. Not in the presense of the buffer
layer and the filesystem having written out some buffers independently of
the VM layer earlier.
I think this also explains why "data=ordered" broke, and "data=writeback"
didn't. When ext3 does "ordered" writebacks, it will do file data
writebacks on its own, in _its_ order. In contrast, when it does
"data=writeback", it will do the writebacks exactly as the VM presents
them, and won't write any buffers on its own - which makes the bug go
away, because now VM and FS end up agreeing about the semantics of
"writepage()".
Andrew, do you see anything wrong in my thinking?
Peter - on a VM level, the fix would be:
- remove the "set_page_dirty()" from the page fault path, and just set
the PAGECACHE_TAG_DIRTY instead.
- clear_page_dirty_for_io() would now need to check the mappings of the
page even if it wasn't marked PG_dirty (or we'd have another page flag
for the "page is dirty in page tables"), which is kind of a mixture of
(a) and (b) cases above, except we don't expose it to the FS.
- if it was dirty in the page tables, we do a "set_page_dirty()" after
cleaning the page tables, and then the rest of
"clear_page_dirty_for_io()" really boils down to a simple
"TestAndClearDirty(page)"
Hmm? I'd love it if somebody else wrote the patch and tested it, because
I'm getting sick and tired of this bug ;)
Linus
From: Linus Torvalds [email blocked]
Subject: Re: Ok, explained.. (was Re: [PATCH] mm: fix page_mkclean_one)
Date: Fri, 29 Dec 2006 02:48:35 -0800 (PST)
On Fri, 29 Dec 2006, Linus Torvalds wrote:
>
> Hmm? I'd love it if somebody else wrote the patch and tested it, because
> I'm getting sick and tired of this bug ;)
Who the hell am I kidding? I haven't been able to sleep right for the last
few days over this bug. It was really getting to me.
And putting on the thinking cap, there's actually a fairly simple an
nonintrusive patch. It still has a tiny tiny race (see the comment), but I
bet nobody can really hit it in real life anyway, and I know several ways
to fix it, so I'm not really _that_ worried about it.
The patch is mostly a comment. The "real" meat of it is actually just a
few lines.
Can anybody get corruption with this thing applied? It goes on top of
plain v2.6.20-rc2.
Linus
----
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index b3a198c..ec01da1 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -862,17 +862,46 @@ int clear_page_dirty_for_io(struct page *page)
{
struct address_space *mapping = page_mapping(page);
- if (!mapping)
- return TestClearPageDirty(page);
-
- if (TestClearPageDirty(page)) {
- if (mapping_cap_account_dirty(mapping)) {
- page_mkclean(page);
+ if (mapping && mapping_cap_account_dirty(mapping)) {
+ /*
+ * Yes, Virginia, this is indeed insane.
+ *
+ * We use this sequence to make sure that
+ * (a) we account for dirty stats properly
+ * (b) we tell the low-level filesystem to
+ * mark the whole page dirty if it was
+ * dirty in a pagetable. Only to then
+ * (c) clean the page again and return 1 to
+ * cause the writeback.
+ *
+ * This way we avoid all nasty races with the
+ * dirty bit in multiple places and clearing
+ * them concurrently from different threads.
+ *
+ * Note! Normally the "set_page_dirty(page)"
+ * has no effect on the actual dirty bit - since
+ * that will already usually be set. But we
+ * need the side effects, and it can help us
+ * avoid races.
+ *
+ * We basically use the page "master dirty bit"
+ * as a serialization point for all the different
+ * threds 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);
+ if (TestClearPageDirty(page)) {
dec_zone_page_state(page, NR_FILE_DIRTY);
+ return 1;
}
- return 1;
+ return 0;
}
- return 0;
+ return TestClearPageDirty(page);
}
EXPORT_SYMBOL(clear_page_dirty_for_io);
From: Andrei Popa <andrei.popa@i-neo.ro>
To: Linus Torvalds [email blocked]
Subject: Re: Ok, explained.. (was Re: [PATCH] mm: fix page_mkclean_one)
Date: Fri, 29 Dec 2006 13:16:46 +0200
On Fri, 2006-12-29 at 02:48 -0800, Linus Torvalds wrote:
>
> On Fri, 29 Dec 2006, Linus Torvalds wrote:
> >
> > Hmm? I'd love it if somebody else wrote the patch and tested it, because
> > I'm getting sick and tired of this bug ;)
>
> Who the hell am I kidding? I haven't been able to sleep right for the last
> few days over this bug. It was really getting to me.
>
> And putting on the thinking cap, there's actually a fairly simple an
> nonintrusive patch. It still has a tiny tiny race (see the comment), but I
> bet nobody can really hit it in real life anyway, and I know several ways
> to fix it, so I'm not really _that_ worried about it.
>
> The patch is mostly a comment. The "real" meat of it is actually just a
> few lines.
>
> Can anybody get corruption with this thing applied? It goes on top of
> plain v2.6.20-rc2.
Tested with rtorrent and there is no corruption.
Related Links:
- Archive of above thread [2]