On Wed 24-11-21 15:59:05, Suren Baghdasaryan wrote: > oom-reaper and process_mrelease system call should protect against > races with exit_mmap which can destroy page tables while they > walk the VMA tree. oom-reaper protects from that race by setting > MMF_OOM_VICTIM and by relying on exit_mmap to set MMF_OOM_SKIP > before taking and releasing mmap_write_lock. process_mrelease has > to elevate mm->mm_users to prevent such race. Both oom-reaper and > process_mrelease hold mmap_read_lock when walking the VMA tree. > The locking rules and mechanisms could be simpler if exit_mmap takes > mmap_write_lock while executing destructive operations such as > free_pgtables. > Change exit_mmap to hold the mmap_write_lock when calling > free_pgtables. Operations like unmap_vmas() and unlock_range() are not > destructive and could run under mmap_read_lock but for simplicity we > take one mmap_write_lock during almost the entire operation. Note > also that because oom-reaper checks VM_LOCKED flag, unlock_range() > should not be allowed to race with it. > In most cases this lock should be uncontended. Previously, Kirill > reported ~4% regression caused by a similar change [1]. We reran the > same test and although the individual results are quite noisy, the > percentiles show lower regression with 1.6% being the worst case [2]. > The change allows oom-reaper and process_mrelease to execute safely > under mmap_read_lock without worries that exit_mmap might destroy page > tables from under them. > > [1] https://lore.kernel.org/all/20170725141723.ivukwhddk2voyhuc@xxxxxxxxxxxxxxxxxx/ > [2] https://lore.kernel.org/all/CAJuCfpGC9-c9P40x7oy=jy5SphMcd0o0G_6U1-+JAziGKG6dGA@xxxxxxxxxxxxxx/ > > Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx> > --- > changes in v2 > - Moved mmap_write_unlock to cover remove_vma loop as well, per Matthew Wilcox > > mm/mmap.c | 16 ++++++++-------- > 1 file changed, 8 insertions(+), 8 deletions(-) > > diff --git a/mm/mmap.c b/mm/mmap.c > index bfb0ea164a90..f4e09d390a07 100644 > --- a/mm/mmap.c > +++ b/mm/mmap.c > @@ -3142,25 +3142,27 @@ void exit_mmap(struct mm_struct *mm) > * to mmu_notifier_release(mm) ensures mmu notifier callbacks in > * __oom_reap_task_mm() will not block. > * > - * This needs to be done before calling munlock_vma_pages_all(), > + * This needs to be done before calling unlock_range(), > * which clears VM_LOCKED, otherwise the oom reaper cannot > * reliably test it. > */ > (void)__oom_reap_task_mm(mm); > > set_bit(MMF_OOM_SKIP, &mm->flags); Why do you keep this in place? Other than that looks OK to me. Maybe we want to add an explicit note that vm_ops::close cannot take mmap_sem in any form. The changelog should also mention that you have considered remove_vma and its previous no MM locking assumption. You can argue that fput is async and close callback shouldn't really need mmap_sem. -- Michal Hocko SUSE Labs
