linux-kernel - Re: [PATCH] oom_reaper: close race without using oom

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Date:   Tue, 18 Jul 2017 10:17:54 -0400
From:   Johannes Weiner <hannes@...xchg.org>
To:     Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Cc:     linux-mm@...ck.org, mhocko@...nel.org, rientjes@...gle.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] oom_reaper: close race without using oom_lock

On Tue, Jul 18, 2017 at 11:06:50PM +0900, Tetsuo Handa wrote:
> Commit e2fe14564d3316d1 ("oom_reaper: close race with exiting task")
> guarded whole OOM reaping operations using oom_lock. But there was no
> need to guard whole operations. We needed to guard only setting of
> MMF_OOM_REAPED flag because get_page_from_freelist() in
> __alloc_pages_may_oom() is called with oom_lock held.
>
> If we change to guard only setting of MMF_OOM_SKIP flag, the OOM reaper
> can start reaping operations as soon as wake_oom_reaper() is called.
> But since setting of MMF_OOM_SKIP flag at __mmput() is not guarded with
> oom_lock, guarding only the OOM reaper side is not sufficient.
> 
> If we change the OOM killer side to ignore MMF_OOM_SKIP flag once,
> there is no need to guard setting of MMF_OOM_SKIP flag, and we can
> guarantee a chance to call get_page_from_freelist() in
> __alloc_pages_may_oom() without depending on oom_lock serialization.
> 
> This patch makes MMF_OOM_SKIP act as if MMF_OOM_REAPED, and adds a new
> flag which acts as if MMF_OOM_SKIP, in order to close both race window
> (the OOM reaper side and __mmput() side) without using oom_lock.

I have no idea what this is about - a race window fix? A performance
optimization? A code simplification?

Users and vendors are later going to read through these changelogs and
have to decide whether they want this patch or upgrade to a kernel
containing it. Please keep these people in mind when writing the
subject and first paragraph of the changelogs.