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

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Date:	Thu, 17 May 2007 23:09:12 +0530
From:	Maneesh Soni <maneesh@...ibm.com>
To:	Greg KH <greg@...ah.com>
Cc:	Tejun Heo <htejun@...il.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

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.

> > 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. 

> 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.

Thanks
Maneesh



-- 
Maneesh Soni
Linux Technology Center,
IBM India Systems and Technology Lab, 
Bangalore, India
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