linux-kernel - Re: [PATCH 0/2] revert changes to zcache_do

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Date:	Fri, 24 Aug 2012 08:28:45 +0900
From:	Minchan Kim <minchan@...nel.org>
To:	Seth Jennings <sjenning@...ux.vnet.ibm.com>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Nitin Gupta <ngupta@...are.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Dan Magenheimer <dan.magenheimer@...cle.com>,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	devel@...verdev.osuosl.org, xiaoguangrong@...ux.vnet.ibm.com
Subject: Re: [PATCH 0/2] revert changes to zcache_do_preload()

On Thu, Aug 23, 2012 at 05:10:00PM -0500, Seth Jennings wrote:
> On 08/23/2012 03:56 PM, Minchan Kim wrote:
> > Hi Seth,
> > 
> > On Thu, Aug 23, 2012 at 10:33:09AM -0500, Seth Jennings wrote:
> >> This patchset fixes a regression in 3.6 by reverting two dependent
> >> commits that made changes to zcache_do_preload().
> >>
> >> The commits undermine an assumption made by tmem_put() in
> >> the cleancache path that preemption is disabled.  This change
> >> introduces a race condition that can result in the wrong page
> >> being returned by tmem_get(), causing assorted errors (segfaults,
> >> apparent file corruption, etc) in userspace.
> >>
> >> The corruption was discussed in this thread:
> >> https://lkml.org/lkml/2012/8/17/494
> > 
> > I think changelog isn't enough to explain what's the race.
> > Could you write it down in detail?
> 
> I didn't come upon this solution via code inspection, but
> rather through discovering that the issue didn't exist in
> v3.5 and just looking at the changes since then.

Okay, then, why do you think the patchsets are culprit?
I didn't look the cleanup patch series of Xiao at that time
so I can be wrong but as I just look through patch of
"zcache: optimize zcache_do_preload", I can't find any fault
because zcache_put_page checks irq_disable so we don't need
to disable preemption so it seems that patch is correct to me.
If the race happens by preemption, BUG_ON in zcache_put_page
should catch it.

What do you mean? Do you have any clue in your mind?

        The commits undermine an assumption made by tmem_put() in
        the cleancache path that preemption is disabled.

> 
> > And you should Cc'ed Xiao who is author of reverted patch.
> 
> Thanks for adding Xiao.  I meant to do this. For some reason
> I thought that you submitted that patchset :-/

Even, I didn't notice that patchset at that time. :)

> My bad.
> 
> Seth
> 
> --
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-- 
Kind regards,
Minchan Kim
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