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
