linux-kernel - Re: [PATCH -stable] sysfs: disable reclamation by default

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Date:	Thu, 17 May 2007 19:49:31 +0200
From:	Tejun Heo <htejun@...il.com>
To:	maneesh@...ibm.com
CC:	Greg KH <greg@...ah.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Clemens Schwaighofer <cs@...uila.co.jp>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Dipankar Sarma <dipankar@...ibm.com>,
	Chuck Ebbert <cebbert@...hat.com>
Subject: Re: [PATCH -stable] sysfs: disable reclamation by default

Maneesh Soni wrote:
> On Thu, May 17, 2007 at 05:04:23AM -0700, Greg KH wrote:
>> On Wed, May 16, 2007 at 08:31:00PM +0200, Tejun Heo wrote:
>>> sd->s_dentry updates made by dentry/inode reclamation are racy and can
>>> lead to BUG() or oops.  This is already fixed in -mm and the fix is
>>> scheduled to be merged into upstream for 2.6.23 but the fix
>>> reimplements sysfs dentry dropping and is too risky for -stable
>>> kernels.
>>>
> 
> But was the synchronization fix tested by people facing the race? I still
> don't understand the racy code path. The last google problem I saw had
> s_dentry field as NULL.

Please take a look at the following message.

  http://article.gmane.org/gmane.linux.kernel/521729

I could reproduce both races on my test machine fairly reliable with
parallel find, cat, mount/mount while repeatedly ins/rmmoding a libata
driver.

>>> This is an interim solution for -stable kernels.  sysfs reclamation is
>>> disabled by default and can be enabled by using sysfs.enable_reclaim
>>> kernel parameter.  Note that dentries are still created on demand, so
>>> attribute and symlinks nodes aren't allocated on creation.  They're
>>> allocated on first lookup and deallocated when the sysfs node is
>>> removed.
>> Ick, this is going to kill memory on big boxes (s390 and others) and I
>> don't really want to apply this it if at all possible.
>>
> At least not make it default. This might create boot issues with these
> boxes. 

Which makes oopsing the default.  Fun!  :-)

>> Maneesh, any other thoughts?
>>
> I actually wanted to investigate this oops but left it considering the
> rework being done by Tejun. If this still make sense we can have some
> more debug code stuffed there or get a crashdump (kdump) to get better
> understanding of the race.

The above message contains analysis of both races.  I just ported the
fixes.  I have a different test machine now and can't reproduce the
races with this one yet so I couldn't verify whether the patches
actually fix the problem.  I'll post the patches anyway.  If anyone can
reproduce these races, please verify the posted patches fix the problem.

Thanks.

-- 
tejun
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