Re: [patch] __block_write_full_page bug

[Andrew Morton <akpm@osdl.org>]

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.