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Date:   Thu, 21 Oct 2021 19:24:54 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Suren Baghdasaryan <surenb@google.com>
Cc:     mhocko@kernel.org, mhocko@suse.com, rientjes@google.com,
        willy@infradead.org, hannes@cmpxchg.org, guro@fb.com,
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        linux-api@vger.kernel.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, kernel-team@android.com
Subject: Re: [PATCH 1/1] mm: prevent a race between process_mrelease and
 exit_mmap
Message-Id: <20211021192454.72ada9b3d59460cc3c037d2c@linux-foundation.org>
In-Reply-To: <20211022014658.263508-1-surenb@google.com>
References: <20211022014658.263508-1-surenb@google.com>
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On Thu, 21 Oct 2021 18:46:58 -0700 Suren Baghdasaryan <surenb@google.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?