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Date:   Tue, 24 Jul 2018 12:29:13 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Tejun Heo <tj@kernel.org>
Cc:     Balbir Singh <bsingharora@gmail.com>, linux-kernel@vger.kernel.org,
        kernel-team@fb.com
Subject: Re: [PATCH] delayacct: Fix crash in delayacct_blkio_end() after
 delayacct init failure
Message-Id: <20180724122913.af508cf58033e9b1147e7def@linux-foundation.org>
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On Tue, 24 Jul 2018 10:55:42 -0700 Tejun Heo <tj@kernel.org> wrote:

> While forking, if delayacct init fails due to memory shortage, it
> continues expecting all delayacct users to check task->delays pointer
> against NULL before dereferencing it, which all of them used to do.
> 
> c96f5471ce7d ("delayacct: Account blkio completion on the correct
> task"), while updating delayacct_blkio_end() to take the target task
> instead of always using %current, made the function test NULL on
> %current->delays and then continue to operated on @p->delays.  If
> %current succeeded init while @p didn't, it leads to the following
> crash.
> 

lgtm.

How did you make this happen, btw?  Fault injection, or did a small
GFP_KERNEL allocation fail?