linux-kernel - Re: [patch v2] mm, oom: fix concurrent munlock and oom reaper unmap

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Message-Id: <201804180355.w3I3tM6T001187@www262.sakura.ne.jp>
Date:   Wed, 18 Apr 2018 12:55:22 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
To:     David Rientjes <rientjes@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Roman Gushchin <guro@...com>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [patch v2] mm, oom: fix concurrent munlock and oom reaper unmap

David Rientjes wrote:
> Fix this by reusing MMF_UNSTABLE to specify that an mm should not be
> reaped.  This prevents the concurrent munlock_vma_pages_range() and
> unmap_page_range().  The oom reaper will simply not operate on an mm that
> has the bit set and leave the unmapping to exit_mmap().

This change assumes that munlock_vma_pages_all()/unmap_vmas()/free_pgtables()
are never blocked for memory allocation. Is that guaranteed? For example,
i_mmap_lock_write() from unmap_single_vma() from unmap_vmas() is never blocked
for memory allocation? Commit 97b1255cb27c551d ("mm,oom_reaper: check for
MMF_OOM_SKIP before complaining") was waiting for i_mmap_lock_write() from
unlink_file_vma() from free_pgtables(). Is it really guaranteed that somebody
else who is holding that lock is never waiting for memory allocation?