linux-kernel - Re: VM: Fix nasty and subtle race in shared mmap'ed page writeback

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Date:	Wed, 3 Jan 2007 23:06:29 -0800
From:	Andrew Morton <akpm@...l.org>
To:	David Miller <davem@...emloft.net>
Cc:	nickpiggin@...oo.com.au, torvalds@...l.org, gelma@...ma.net,
	linux-kernel@...r.kernel.org
Subject: Re: VM: Fix nasty and subtle race in shared mmap'ed page writeback

On Wed, 03 Jan 2007 22:56:07 -0800 (PST)
David Miller <davem@...emloft.net> wrote:

> From: Andrew Morton <akpm@...l.org>
> Date: Wed, 3 Jan 2007 22:12:20 -0800
> 
> > On Thu, 04 Jan 2007 17:03:43 +1100
> > Nick Piggin <nickpiggin@...oo.com.au> wrote:
> > 
> > > > That bug was introduced in 2.6.19, with the dirty page tracking patches.
> > > > 
> > > > 2.6.18 and earlier used ->private_lock coverage in try_to_free_buffers() to
> > > > prevent it.
> > > 
> > > Ohh, right you are, I was looking at 2.6.19 sources. The comments above
> > > ttfb match that as well. Curious that the dirty page patches were allowed
> > > to mess with this...
> > 
> > Frankly, those patches scared the crap out of me, specifically because of
> > the delicacy and complexity of the various dirtiness state coherencies. 
> > But I just didn't have the bandwidth to go through them with a sufficiently
> > fine toothcomb, sorry.
> > 
> > > Anyway that leaves us with the question of why Andrea's database is getting
> > > corrupted. Hopefully he can give us a minimal test-case.
> > 
> > It'd odd that stories of pre-2.6.19 BerkeleyDB corruption are now coming
> > out of the woodwork.  It's the first I've ever heard of them.
> 
> Note that the original rtorrent debian bug report was against 2.6.18

I think that was 2.6.18+debian-added-dirty-page-tracking-patches.

If that memory is correct, I'll assert (and emphasise) that the cause of the
alleged BerkeleyDB corruption is not known at this time.

The post-2.6.19 "fix" might make it go away.  But if it does, we do not know
why, and it might still be there, only harder to hit.

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