On Tue, Nov 9, 2021 at 11:50 AM Michal Hocko <mhocko@xxxxxxxx> wrote: > > On Tue 09-11-21 11:37:06, Suren Baghdasaryan wrote: > > 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. > > My memory is rather dimm but AFAIR the main problem was freeing page > tables and freeing vmas not unmap_vmas. That one was no modifying the > vma list. Essentially it was just a slightly modified madvise don't > need. So that part was allowed to race with oom_reaper. So, both unmap_vmas and __oom_reap_task_mm do not modify vma list and therefore can execute concurrently. That makes sense, thanks. Then I guess, if we want to be semantically correct in exit_mmap(), we would have to take mmap_read_lock before unmap_vmas, then drop it and take mmap_write_lock before free_pgtables. > -- > Michal Hocko > SUSE Labs
