linux-kernel - Re: [PATCH 1/2] mm, memcg: Fix corruption on 64-bit divisor in memory.high throttling

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Date:   Mon, 16 Mar 2020 12:19:07 -0400
From:   Johannes Weiner <hannes@...xchg.org>
To:     Chris Down <chris@...isdown.name>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Tejun Heo <tj@...nel.org>, linux-mm@...ck.org,
        cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...com
Subject: Re: [PATCH 1/2] mm, memcg: Fix corruption on 64-bit divisor in
 memory.high throttling

On Thu, Mar 12, 2020 at 06:02:54PM +0000, Chris Down wrote:
> 0e4b01df8659 had a bunch of fixups to use the right division method.
> However, it seems that after all that it still wasn't right -- div_u64
> takes a 32-bit divisor.
> 
> The headroom is still large (2^32 pages), so on mundane systems you
> won't hit this, but this should definitely be fixed.
> 
> Fixes: 0e4b01df8659 ("mm, memcg: throttle allocators when failing reclaim over memory.high")
> Reported-by: Johannes Weiner <hannes@...xchg.org>
> Signed-off-by: Chris Down <chris@...isdown.name>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Tejun Heo <tj@...nel.org>
> Cc: linux-mm@...ck.org
> Cc: cgroups@...r.kernel.org
> Cc: linux-kernel@...r.kernel.org
> Cc: kernel-team@...com
> Cc: stable@...r.kernel.org # 5.4.x

div_u64 versus div64_u64 is really a handgrenade. We just fixed a
bunch of those in psi as well.

Acked-by: Johannes Weiner <hannes@...xchg.org>