Re: [patch] __block_write_full_page bug

[Nick Piggin]

Andrew Morton wrote:
> Nick Piggin <nickpiggin@xxxxxxxxxxxx> wrote:
>
> > On Tue, 2005-04-26 at 04:50 -0700, Andrew Morton wrote:
> > > Nick Piggin <nickpiggin@xxxxxxxxxxxx> 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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