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

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Date:   Thu, 27 Sep 2018 15:51:17 +0000
From:   Christopher Lameter <cl@...ux.com>
To:     Dmitry Vyukov <dvyukov@...il.com>
cc:     penberg@...nel.org, akpm@...ux-foundation.org, rientjes@...gle.com,
        iamjoonsoo.kim@....com, Dmitry Vyukov <dvyukov@...gle.com>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: don't warn about large allocations for slab

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.