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

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Date:   Fri, 22 Oct 2021 14:04:24 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Suren Baghdasaryan <surenb@...gle.com>, akpm@...ux-foundation.org,
        rientjes@...gle.com, 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 Fri 22-10-21 12:32:08, Matthew Wilcox wrote:
> On Fri, Oct 22, 2021 at 10:03:29AM +0200, Michal Hocko wrote:
> > On Thu 21-10-21 18:46:58, Suren Baghdasaryan 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.
> > 
> > I am not sure there is any promise that process_mrelease will run in
> > parallel with the exiting process. In fact the primary purpose of this
> > syscall is to provide a reliable way to oom kill from user space. If you
> > want to optimize process exit resp. its exit_mmap part then you should
> > be using other means. So I would be careful calling this a regression.
> > 
> > I do agree that taking the reference count is the right approach here. I
> > was wrong previously [1] when saying that pinning the mm struct is
> > sufficient. I have completely forgot about the subtle sync in exit_mmap.
> > One way we can approach that would be to take exclusive mmap_sem
> > throughout the exit_mmap unconditionally. There was a push back against
> > that though so arguments would have to be re-evaluated.
> 
> I have another reason for wanting to take the mmap_sem throughout
> exit_mmap.  Liam and I are working on using the Maple tree to replace
> the rbtree & vma linked list.  It uses lockdep to check that you haven't
> forgotten to take a lock (as of two days ago, that mean the mmap_sem
> or the RCU read lock) when walking the tree.
> 
> So I'd like to hold it over:
> 
>  - unlock_range()
>  - unmap_vmas()
>  - free_pgtables()
>  - while (vma) remove_vma()
> 
> Which is basically the whole of exit_mmap().  I'd like to know more
> about why there was pushback on holding the mmap_lock across this
> -- we're exiting, so nobody else should have a reference to the mm?

https://lore.kernel.org/all/20170724072332.31903-1-mhocko@kernel.org/
-- 
Michal Hocko
SUSE Labs