linux-kernel - Re: [PATCH] aoe: adjust ref of head for compound page tails

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Message-ID: <782613DB-4C38-450C-BBC2-E14FA64D44AC@coraid.com>
Date:	Wed, 7 Aug 2013 19:27:40 -0400
From:	Ed Cashin <ecashin@...aid.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] aoe: adjust ref of head for compound page tails

On Aug 7, 2013, at 5:26 PM, Andrew Morton wrote:

> On Wed, 7 Aug 2013 17:21:32 -0400 Ed Cashin <ecashin@...aid.com> wrote:
...
>> Sorry.  I tried to cover that but maybe should have separated it more clearly from the running text.
>> 
>> The patch changes the current behavior encountered with direct I/O and aoe:
>> 
>>  aoe can BUG when direct I/O used
>> 
>> ... to a new behavior:
>> 
>>  aoe does not BUG when direct I/O is used
>> 
>> And the user will never know that instead aoe is manipulating the _count of the head page, even though that's what's happening with this patch applied.
>> 
>> (The user-visible effects that motivated the older patch that added the BUG to aoe when page tails are used for block I/O was a VM_BUG_ON that occurred when the aoe driver manipulated the _count of a compound page tail.)
> 
> OK, I added "Fix a BUG which can trigger when direct-IO is used with
> AOE".  Which kernel versions are affected?


That BUG was added to the aoe driver in a commit that first appears in v3.7.  It persists until this patch adds the code that handles the situation that the BUG was intended to detect.

[ecashin@...ino linux]$ git describe --contains --match 'v*' 69cf2d85de773d998798e47e3335b85e5645d157
v3.7-rc1~110^2~22
[ecashin@...ino linux]$ 

-- 
  Ed Cashin
  ecashin@...aid.com


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