linux-kernel - Re: [PATCH] mm: don't warn about large allocations for slab

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Date:   Thu, 27 Sep 2018 19:17:01 +0200
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Christopher Lameter <cl@...ux.com>
Cc:     Dmitry Vyukov <dvyukov@...il.com>,
        Pekka Enberg <penberg@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Linux-MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: don't warn about large allocations for slab

On Thu, Sep 27, 2018 at 5:51 PM, Christopher Lameter <cl@...ux.com> wrote:
> On Thu, 27 Sep 2018, Dmitry Vyukov wrote:
>
>> From: Dmitry Vyukov <dvyukov@...gle.com>
>>
>> This warning does not seem to be useful. Most of the time it fires when
>> allocation size depends on syscall arguments. We could add __GFP_NOWARN
>> to these allocation sites, but having a warning only to suppress it
>> does not make lots of sense. Moreover, this warnings never fires for
>> constant-size allocations and never for slub, because there are
>> additional checks and fallback to kmalloc_large() for large allocations
>> and kmalloc_large() does not warn. So the warning only fires for
>> non-constant allocations and only with slab, which is odd to begin with.
>> The warning leads to episodic unuseful syzbot reports. Remote it.
>
> /Remove/
>
> If its only for slab then KMALLOC_MAX_CACHE_SIZE and KMALLOC_MAX_SIZE are
> the same value.
>
>> While we are here also fix the check. We should check against
>> KMALLOC_MAX_CACHE_SIZE rather than KMALLOC_MAX_SIZE. It all kinda
>> worked because for slab the constants are the same, and slub always
>> checks the size against KMALLOC_MAX_CACHE_SIZE before kmalloc_slab().
>> But if we get there with size > KMALLOC_MAX_CACHE_SIZE anyhow
>> bad things will happen.
>
> Then the WARN_ON is correct just change the constant used. Ensure that
> SLAB does the same checks as SLUB.

Mailed v2 which adds the checks to slab.

I think the warning is still slightly wrong. It means a bug in slab
code, it has nothing to do with user-passed flags.