On Fri, Oct 22, 2021 at 10:03:29AM +0200, Michal Hocko wrote: > On Thu 21-10-21 18:46:58, Suren Baghdasaryan wrote: > > Race between process_mrelease and exit_mmap, where free_pgtables is > > called while __oom_reap_task_mm is in progress, leads to kernel crash > > during pte_offset_map_lock call. oom-reaper avoids this race by setting > > MMF_OOM_VICTIM flag and causing exit_mmap to take and release > > mmap_write_lock, blocking it until oom-reaper releases mmap_read_lock. > > Reusing MMF_OOM_VICTIM for process_mrelease would be the simplest way to > > fix this race, however that would be considered a hack. Fix this race > > by elevating mm->mm_users and preventing exit_mmap from executing until > > process_mrelease is finished. Patch slightly refactors the code to adapt > > for a possible mmget_not_zero failure. > > This fix has considerable negative impact on process_mrelease performance > > and will likely need later optimization. > > I am not sure there is any promise that process_mrelease will run in > parallel with the exiting process. In fact the primary purpose of this > syscall is to provide a reliable way to oom kill from user space. If you > want to optimize process exit resp. its exit_mmap part then you should > be using other means. So I would be careful calling this a regression. > > I do agree that taking the reference count is the right approach here. I > was wrong previously [1] when saying that pinning the mm struct is > sufficient. I have completely forgot about the subtle sync in exit_mmap. > One way we can approach that would be to take exclusive mmap_sem > throughout the exit_mmap unconditionally. There was a push back against > that though so arguments would have to be re-evaluated. I have another reason for wanting to take the mmap_sem throughout exit_mmap. Liam and I are working on using the Maple tree to replace the rbtree & vma linked list. It uses lockdep to check that you haven't forgotten to take a lock (as of two days ago, that mean the mmap_sem or the RCU read lock) when walking the tree. So I'd like to hold it over: - unlock_range() - unmap_vmas() - free_pgtables() - while (vma) remove_vma() Which is basically the whole of exit_mmap(). I'd like to know more about why there was pushback on holding the mmap_lock across this -- we're exiting, so nobody else should have a reference to the mm?
