Re: [patch] __block_write_full_page bug

From: Nick Piggin
Date: Tue Apr 26 2005 - 19:12:19 EST

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
then a bit later does

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.


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

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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