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SubjectRe: [patch] __block_write_full_page bug
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.

<looks>

Were you using nobh? I guess not. What's to stop the new
mpage_writepage() from trying to write a page which is already under
PageWriteback()?

I don't think we understand this bug yet.
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