Re: [PATCH 1/2] mm: protect free_pgtables with mmap_lock write lock in exit_mmap

[Suren Baghdasaryan]

On Tue, Nov 16, 2021 at 1:57 PM Suren Baghdasaryan <surenb@xxxxxxxxxx> 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/

Friendly nudge.
Michal, Matthew, from our discussion in
https://lore.kernel.org/all/YXKhOKIIngIuJaYi@xxxxxxxxxxxxxxxxxxxx I
was under the impression this change would be interesting for you. Any
feedback?

>
> Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>
> ---
>  mm/mmap.c | 11 +++++++----
>  1 file changed, 7 insertions(+), 4 deletions(-)
>
> diff --git a/mm/mmap.c b/mm/mmap.c
> index bfb0ea164a90..69b3036c6dee 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);
> @@ -3170,6 +3172,7 @@ void exit_mmap(struct mm_struct *mm)
>         unmap_vmas(&tlb, vma, 0, -1);
>         free_pgtables(&tlb, vma, FIRST_USER_ADDRESS, USER_PGTABLES_CEILING);
>         tlb_finish_mmu(&tlb);
> +       mmap_write_unlock(mm);
>
>         /*
>          * Walk the list again, actually closing and freeing it,
> --
> 2.34.0.rc1.387.gb447b232ab-goog
>