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

[Matthew Wilcox <willy@infradead.org>]

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?