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); - mmap_write_lock(mm); - mmap_write_unlock(mm); } + mmap_write_lock(mm); if (mm->locked_vm) unlock_range(mm->mmap, ULONG_MAX); arch_exit_mmap(mm); vma = mm->mmap; - if (!vma) /* Can happen if dup_mmap() received an OOM */ + if (!vma) { + /* Can happen if dup_mmap() received an OOM */ + mmap_write_unlock(mm); return; + } lru_add_drain(); flush_cache_mm(mm); @@ -3171,16 +3173,14 @@ void exit_mmap(struct mm_struct *mm) free_pgtables(&tlb, vma, FIRST_USER_ADDRESS, USER_PGTABLES_CEILING); tlb_finish_mmu(&tlb); - /* - * Walk the list again, actually closing and freeing it, - * with preemption enabled, without holding any MM locks. - */ + /* Walk the list again, actually closing and freeing it. */ while (vma) { if (vma->vm_flags & VM_ACCOUNT) nr_accounted += vma_pages(vma); vma = remove_vma(vma); cond_resched(); } + mmap_write_unlock(mm); vm_unacct_memory(nr_accounted); } -- 2.34.0.rc2.393.gf8c9666880-goog
