Re: [PATCH 1/1] mm: prevent a race between process_mrelease and exit_mmap

[Andrew Morton]

On Thu, 21 Oct 2021 18:46:58 -0700 Suren Baghdasaryan <surenb@xxxxxxxxxx> 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?