On Tue, Nov 9, 2021 at 11:26 AM Michal Hocko <mhocko@xxxxxxxx> wrote: > > 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. 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@xxxxxxxxxx/ 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
