On Tue, Nov 9, 2021 at 11:26 AM Michal Hocko [off-list ref] wrote:

On Tue 09-11-21 11:01:02, Suren Baghdasaryan wrote:
[...]

quoted

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@dhcp22.suse.cz (local),
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.

I see. So, it's the combination of MMF_OOM_SKIP and mmap_lock working
as a barrier which prevent them from racing with each other...
I wasn't sure how
https://lore.kernel.org/all/20170724072332.31903-1-mhocko@kernel.org/ (local)
was implementing this synchronization because it would take mmap_sem
write side after unmap_vmas() and IIUC there was no
"mmap_lock_write(); mmap_unlock_write();" sequence in exit_mmap at
that time. I'll need to checkout the old sources to figure this out.
Thanks,
Suren.

--
Michal Hocko
SUSE Labs