On Thu, Dec 30, 2021 at 12:24 AM Michal Hocko <mhocko@xxxxxxxx> wrote: > > On Wed 29-12-21 21:59:55, Suren Baghdasaryan wrote: > [...] > > After some more digging I think there are two acceptable options: > > > > 1. Call unlock_range() under mmap_write_lock and then downgrade it to > > read lock so that both exit_mmap() and __oom_reap_task_mm() can unmap > > vmas in parallel like this: > > > > if (mm->locked_vm) { > > mmap_write_lock(mm); > > unlock_range(mm->mmap, ULONG_MAX); > > mmap_write_downgrade(mm); > > } else > > mmap_read_lock(mm); > > ... > > unmap_vmas(&tlb, vma, 0, -1); > > mmap_read_unlock(mm); > > mmap_write_lock(mm); > > free_pgtables(&tlb, vma, FIRST_USER_ADDRESS, USER_PGTABLES_CEILING); > > ... > > mm->mmap = NULL; > > mmap_write_unlock(mm); > > > > This way exit_mmap() might block __oom_reap_task_mm() but for a much > > shorter time during unlock_range() call. > > IIRC unlock_range depends on page lock at some stage and that can mean > this will block for a long time or for ever when the holder of the lock > depends on a memory allocation. This was the primary problem why the oom > reaper skips over mlocked vmas. Oh, I missed that detail. I thought __oom_reap_task_mm() skips locked vmas only to avoid destroying pgds from under follow_page(). > > > 2. Introduce another vm_flag mask similar to VM_LOCKED which is set > > before munlock_vma_pages_range() clears VM_LOCKED so that > > __oom_reap_task_mm() can identify vmas being unlocked and skip them. > > > > Option 1 seems cleaner to me because it keeps the locking pattern > > around unlock_range() in exit_mmap() consistent with all other places > > it is used (in mremap() and munmap()) with mmap_write_lock taken. > > WDYT? > > It would be really great to make unlock_range oom reaper aware IMHO. What exactly do you envision? Say unlock_range() knows that it's racing with __oom_reap_task_mm() and that calling follow_page() is unsafe without locking, what should it do? > > You do not quote your change in the full length so it is not really > clear whether you are planning to drop __oom_reap_task_mm from exit_mmap > as well. Yes, that was the plan. > If yes then 1) could push oom reaper to timeout while the > unlock_range could be dropped on something so that wouldn't be an > improvement. 2) sounds like a workaround to me as it doesn't really > address the underlying problem. With (1) potentially blocking due to allocation I can see why this is a problem. Agree about (2). > > I have to say that I am not really a great fan of __oom_reap_task_mm in > exit_mmap but I would rather see it in place than making the surrounding > code more complex/tricky. Agree. So far I could not find a cleaner solution. I thought (1) would be a good one but the point you made renders it invalid. If you clarify your comment about making unlock_range oom reaper aware maybe that will open a new line of investigation? Thanks, Suren. > > -- > Michal Hocko > SUSE Labs