Re: [patch] __block_write_full_page bug

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Andrew Morton wrote:
Nick Piggin <[email protected]> wrote:

On Tue, 2005-04-26 at 04:50 -0700, Andrew Morton wrote:
> Nick Piggin <[email protected]> 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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