linux-kernel - Re: [PATCH] slab: prevent warnings when allocating with __GFP

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Date:	Tue, 11 Jun 2013 09:16:19 -0400
From:	Sasha Levin <sasha.levin@...cle.com>
To:	Pekka Enberg <penberg@...nel.org>
CC:	Christoph Lameter <cl@...two.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] slab: prevent warnings when allocating with __GFP_NOWARN

On 06/11/2013 02:28 AM, Pekka Enberg wrote:
> Hi Sasha,
>
> On Tue, Jun 11, 2013 at 3:54 AM, Sasha Levin <sasha.levin@...cle.com> wrote:
>> On 06/10/2013 07:40 PM, Christoph Lameter wrote:
>>>
>>> On Mon, 10 Jun 2013, Sasha Levin wrote:
>>>
>>>> [ 1691.807621] Call Trace:
>>>> [ 1691.809473]  [<ffffffff83ff4041>] dump_stack+0x4e/0x82
>>>> [ 1691.812783]  [<ffffffff8111fe12>] warn_slowpath_common+0x82/0xb0
>>>> [ 1691.817011]  [<ffffffff8111fe55>] warn_slowpath_null+0x15/0x20
>>>> [ 1691.819936]  [<ffffffff81243dcf>] kmalloc_slab+0x2f/0xb0
>>>> [ 1691.824942]  [<ffffffff81278d54>] __kmalloc+0x24/0x4b0
>>>> [ 1691.827285]  [<ffffffff8196ffe3>] ? security_capable+0x13/0x20
>>>> [ 1691.829405]  [<ffffffff812a26b7>] ? pipe_fcntl+0x107/0x210
>>>> [ 1691.831827]  [<ffffffff812a26b7>] pipe_fcntl+0x107/0x210
>>>> [ 1691.833651]  [<ffffffff812b7ea0>] ? fget_raw_light+0x130/0x3f0
>>>> [ 1691.835343]  [<ffffffff812aa5fb>] SyS_fcntl+0x60b/0x6a0
>>>> [ 1691.837008]  [<ffffffff8403ca98>] tracesys+0xe1/0xe6
>>>>
>>>> The caller specifically sets __GFP_NOWARN presumably to avoid this
>>>> warning on
>>>> slub but I'm not sure if there's any other reason.
>>>
>>>
>>> There must be another reason. Lets fix this.
>>
>> My, I feel silly now.
>>
>> I was the one who added __GFP_NOFAIL in the first place in
>> 2ccd4f4d ("pipe: fail cleanly when root tries F_SETPIPE_SZ
>> with big size").
>>
>> What happens is that root can go ahead and specify any size
>> it wants to be used as buffer size - and the kernel will
>> attempt to comply by allocation that buffer. Which fails
>> if the size is too big.
>>
>> Either way, even if we do end up doing something different,
>> shouldn't we prevent slab from spewing a warning if
>> __GFP_NOWARN is passed?
>
> Yeah, this is the size-from-userspace case I was thinking about. I think
> we have two options: either use your patch or drop the WARN_ON
> completely.
>
> Christoph, which one do you prefer?

I think that leaving the warning makes sense to catch similar
things which are actually bugs - we had a similar issue with
/dev/kmsg (if I remember correctly) which actually pointed to
a bug.


Thanks,
Sasha

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