Re: [RFC PATCH] mm, oom_reaper: gather each vma to prevent leaking TLB entry

[Bob Liu]

On Mon, Nov 6, 2017 at 11:36 AM, Wang Nan <wangnan0@xxxxxxxxxx> wrote:
> tlb_gather_mmu(&tlb, mm, 0, -1) means gathering all virtual memory space.
> In this case, tlb->fullmm is true. Some archs like arm64 doesn't flush
> TLB when tlb->fullmm is true:
>
>   commit 5a7862e83000 ("arm64: tlbflush: avoid flushing when fullmm == 1").
>

CC'ed Will Deacon.

> Which makes leaking of tlb entries. For example, when oom_reaper
> selects a task and reaps its virtual memory space, another thread
> in this task group may still running on another core and access
> these already freed memory through tlb entries.
>
> This patch gather each vma instead of gathering full vm space,
> tlb->fullmm is not true. The behavior of oom reaper become similar
> to munmapping before do_exit, which should be safe for all archs.
>
> Signed-off-by: Wang Nan <wangnan0@xxxxxxxxxx>
> Cc: Bob Liu <liubo95@xxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxx>
> Cc: David Rientjes <rientjes@xxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Roman Gushchin <guro@xxxxxx>
> Cc: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx>
> Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> ---
>  mm/oom_kill.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
>
> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> index dee0f75..18c5b35 100644
> --- a/mm/oom_kill.c
> +++ b/mm/oom_kill.c
> @@ -532,7 +532,6 @@ static bool __oom_reap_task_mm(struct task_struct *tsk, struct mm_struct *mm)
>          */
>         set_bit(MMF_UNSTABLE, &mm->flags);
>
> -       tlb_gather_mmu(&tlb, mm, 0, -1);
>         for (vma = mm->mmap ; vma; vma = vma->vm_next) {
>                 if (!can_madv_dontneed_vma(vma))
>                         continue;
> @@ -547,11 +546,13 @@ static bool __oom_reap_task_mm(struct task_struct *tsk, struct mm_struct *mm)
>                  * we do not want to block exit_mmap by keeping mm ref
>                  * count elevated without a good reason.
>                  */
> -               if (vma_is_anonymous(vma) || !(vma->vm_flags & VM_SHARED))
> +               if (vma_is_anonymous(vma) || !(vma->vm_flags & VM_SHARED)) {
> +                       tlb_gather_mmu(&tlb, mm, vma->vm_start, vma->vm_end);
>                         unmap_page_range(&tlb, vma, vma->vm_start, vma->vm_end,
>                                          NULL);
> +                       tlb_finish_mmu(&tlb, vma->vm_start, vma->vm_end);
> +               }
>         }
> -       tlb_finish_mmu(&tlb, 0, -1);
>         pr_info("oom_reaper: reaped process %d (%s), now anon-rss:%lukB, file-rss:%lukB, shmem-rss:%lukB\n",
>                         task_pid_nr(tsk), tsk->comm,
>                         K(get_mm_counter(mm, MM_ANONPAGES)),