Re: [PATCH] slab: prevent warnings when allocating with __GFP_NOWARN

Pekka Enberg
2013-06-11T06:28:51Z

Hi Sasha,

On Tue, Jun 11, 2013 at 3:54 AM, Sasha Levin <sasha.levin@oracle.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?

Pekka