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DateWed, 27 Apr 2005 10:10:19 +1000
FromNick Piggin <>
SubjectRe: [patch] __block_write_full_page bug
Andrew Morton wrote:
> Nick Piggin <nickpiggin@yahoo.com.au> wrote:
>
>>On Tue, 2005-04-26 at 04:50 -0700, Andrew Morton wrote:
>> > Nick Piggin <nickpiggin@yahoo.com.au> wrote:
>> > >
>> > > When running
>> > > fsstress -v -d $DIR/tmp -n 1000 -p 1000 -l 2
>> > > on an ext2 filesystem with 1024 byte block size, on SMP i386 with 4096 byte
>> > > page size over loopback to an image file on a tmpfs filesystem, I would
>> > > very quickly hit
>> > > BUG_ON(!buffer_async_write(bh));
>> > > in fs/buffer.c:end_buffer_async_write
>> > >
>> > > It seems that more than one request would be submitted for a given bh
>> > > at a time. __block_write_full_page looks like the culprit - with the
>> > > following patch things are very stable.
>> >
>> > What's the bug? I don't see it.
>> >
>>
>> Ah, the bug is that end_buffer_async_write first does
>> BUG_ON(!buffer_async_write(bh));
>> then a bit later does
>> clear_buffer_async_write(bh);
>>
>> That's where it was blowing up for me, because end_buffer_async_write
>> was being run twice for that buffer.
>>
>> Or did you mean *how* is it being run twice? I didn't exactly find
>> the stack traces involved, but I imagine that simply testing
>> buffer_async_write catches other requests in flight - ie. we've
>> lost track of exactly which ones we own.
>>
>
>
> How can such a thing come about? Both PageLocked() and PageWriteback() are
> supposed to stop new writeback being started against the page.
>

You have a point.

> <looks>
>
> Were you using nobh? I guess not. What's to stop the new

No

> mpage_writepage() from trying to write a page which is already under
> PageWriteback()?
>

Don't know... I didn't think mapping->writepage should be called
for a PageWriteback page?

> I don't think we understand this bug yet.
>

It appears not. I'll look into it further.

--
SUSE Labs, Novell Inc.


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