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

[Michal Hocko]

On Tue 09-11-21 11:01:02, Suren Baghdasaryan wrote:
[...]
> Discussing how the patch I want to post works for maple trees that
> Matthew is working on, I've got a question:
> 
> IIUC, according to Michal's post here:
> https://lore.kernel.org/all/20170725154514.GN26723@xxxxxxxxxxxxxx,
> unmap_vmas() can race with other mmap_lock read holders (including
> oom_reap_task_mm()) with no issues.
> Maple tree patchset requires rcu read lock or the mmap semaphore be
> held (read or write side) when walking the tree, including inside
> unmap_vmas(). When asked, he told me that he is not sure why it's
> currently "safe" to walk the vma->vm_next list in unmap_vmas() while
> another thread is reaping the mm.
> Michal (or maybe someone else), could you please clarify why
> unmap_vmas() can safely race with oom_reap_task_mm()? Or maybe my
> understanding was wrong?

I cannot really comment on the mapple tree part. But the existing
synchronization between oom reaper and exit_mmap is based on
- oom_reaper takes mmap_sem for reading
- exit_mmap sets MMF_OOM_SKIP and takes the exclusive mmap_sem before 
  unmap_vmas.

The oom_reaper therefore can either unmap the address space if the lock
is taken before exit_mmap or it would it would bale out on MMF_OOM_SKIP
if it takes the lock afterwards. So the reaper cannot race with
unmap_vmas.

-- 
Michal Hocko
SUSE Labs