linux-kernel - Re: [PATCH 1/1] mm: prevent a race between process_mrelease and exit

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Date:   Thu, 21 Oct 2021 19:24:54 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Suren Baghdasaryan <surenb@...gle.com>
Cc:     mhocko@...nel.org, mhocko@...e.com, rientjes@...gle.com,
        willy@...radead.org, hannes@...xchg.org, guro@...com,
        riel@...riel.com, minchan@...nel.org, christian@...uner.io,
        hch@...radead.org, oleg@...hat.com, david@...hat.com,
        jannh@...gle.com, shakeelb@...gle.com, luto@...nel.org,
        christian.brauner@...ntu.com, fweimer@...hat.com, jengelh@...i.de,
        linux-api@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, kernel-team@...roid.com
Subject: Re: [PATCH 1/1] mm: prevent a race between process_mrelease and
 exit_mmap

On Thu, 21 Oct 2021 18:46:58 -0700 Suren Baghdasaryan <surenb@...gle.com> wrote:

> Race between process_mrelease and exit_mmap, where free_pgtables is
> called while __oom_reap_task_mm is in progress, leads to kernel crash
> during pte_offset_map_lock call. oom-reaper avoids this race by setting
> MMF_OOM_VICTIM flag and causing exit_mmap to take and release
> mmap_write_lock, blocking it until oom-reaper releases mmap_read_lock.
> Reusing MMF_OOM_VICTIM for process_mrelease would be the simplest way to
> fix this race, however that would be considered a hack. Fix this race
> by elevating mm->mm_users and preventing exit_mmap from executing until
> process_mrelease is finished. Patch slightly refactors the code to adapt
> for a possible mmget_not_zero failure.
> This fix has considerable negative impact on process_mrelease performance
> and will likely need later optimization.

Has the impact been quantified?

And where's the added cost happening?  The changes all look quite
lightweight?