linux-kernel - Re: [BUGFIX] NULL pointer dereference in __vm_enough

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Date:	Sun, 12 Aug 2007 16:23:20 +0400
From:	Cyrill Gorcunov <gorcunov@...il.com>
To:	Balbir Singh <balbir@...ux.vnet.ibm.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Fengguang Wu <fengguang.wu@...il.com>
Subject: Re: [BUGFIX] NULL pointer dereference in __vm_enough_memory()

[Balbir Singh - Sun, Aug 12, 2007 at 02:55:32PM +0530]
| WU Fengguang wrote:
| > On Sun, Aug 12, 2007 at 01:48:31PM +0800, WU Fengguang wrote:
| >> On Sat, Aug 11, 2007 at 11:31:09PM +0530, Balbir Singh wrote:
| >>> Andrew Morton wrote:
| >>>> On Sat, 11 Aug 2007 20:00:12 +0530 "Balbir Singh" <balbir@...ux.vnet.ibm.com> wrote:
| >>>>> Shouldn't we just not stop vm accounting for kernel threads?
| >>>>>
| >>>> Could be.  It'd help heaps if we knew which patch in -mm caused
| >>>> this, but from a quick peek it seems to me that mainline should be
| >>>> vulnerable as well.
| >>> Thats a valid point. It would be interesting to see what the overcommit
| >>> setting was, when the panic occurred.
| >> FYI, I do have nondefault overcommit settings:
| >>
| >> vm.overcommit_memory = 2
| >> vm.lowmem_reserve_ratio = 1 1
| > 
| > Yes, the bug disappears when changing to default overcommit_memory!
| > 
| 
| Great! So the problem might have existed for some time, but we never
| saw it due to default over commit values? Were you using these values
| for over commit even before?
| 
| -- 
| 	Warm Regards,
| 	Balbir Singh
| 	Linux Technology Center
| 	IBM, ISTL

So the problem is that __vm_enough_memory is just _not_ capable
to be called from a thread with OVERCOMMIT_NEVER and Fengguang's
patch does fix it. The scenario Fengguang show us is growing
from keventd that starts a new user task. So Fengguang's patch
seems right to me ;)

		Cyrill

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