linux-kernel - Re: [PATCH v5 2/2] mm/oom_kill: Have the OOM reaper and exit_mmap() traverse the maple tree in opposite order

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Message-ID: <bcabe7cb-d09c-4789-ae39-407b2e0653da@lucifer.local>
Date: Tue, 26 Aug 2025 14:50:00 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: "Liam R. Howlett" <Liam.Howlett@...cle.com>,
        zhongjinji <zhongjinji@...or.com>, mhocko@...e.com,
        rientjes@...gle.com, shakeel.butt@...ux.dev, akpm@...ux-foundation.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org, tglx@...utronix.de,
        liulu.liu@...or.com, feng.han@...or.com
Subject: Re: [PATCH v5 2/2] mm/oom_kill: Have the OOM reaper and exit_mmap()
 traverse the maple tree in opposite order

On Tue, Aug 26, 2025 at 09:37:22AM -0400, Liam R. Howlett wrote:
> I really don't think this is worth doing.  We're avoiding a race between
> oom and a task unmap - the MMF bits should be used to avoid this race -
> or at least mitigate it.

Yes for sure, as explored at length in previous discussions this feels like
we're papering over cracks here.

_However_, I'm sort of ok with a minimalistic fix that solves the proximate
issue even if it is that, as long as it doesn't cause issues in doing so.

So this is my take on the below and why I'm open to it!

>
> They are probably both under the read lock, but considering how rare it
> would be, would a racy flag check be enough - it is hardly critical to
> get right.  Either would reduce the probability.

Zongjinji - I'm stil not sure that you've really indicated _why_ you're
seeing such a tight and unusual race. Presumably some truly massive number
of tasks being OOM'd and unmapping but... yeah that seems odd anyway.

But again, if we can safely fix this in a way that doesn't hurt stuff too
much I'm ok with it (of course, these are famous last words in the kernel
often...!)

Liam - are you open to a solution on the basis above, or do you feel we
ought simply to fix the underlying issue here?

to me we're at a simple enough implementaiton of this (esp. utilising the
helper you mention) that probably kthis is fine (like the meme,
or... hopefully not :)

I will go with your judgment here!

Cheers, Lorenzo