Re: [PATCH 1/1] mm: prevent a race between process_mrelease and exit_mmap
From: Michal Hocko <mhocko@suse.com>
Date: 2021-11-09 19:26:59
Also in:
linux-mm, lkml
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@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. -- Michal Hocko SUSE Labs