On 2017/11/6 16:52, Michal Hocko wrote:
> On Mon 06-11-17 15:04:40, Bob Liu wrote:
>> On Mon, Nov 6, 2017 at 11:36 AM, Wang Nan <[email protected]> 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.
> No threads should be running in userspace by the time the reaper gets to
> unmap their address space. So the only potential case is they are
> accessing the user memory from the kernel when we should fault and we
> have MMF_UNSTABLE to cause a SIGBUS. So is the race you are describing
> real?
>
>>> 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.
> I do not have any objections to do per vma tlb flushing because it would
> free gathered pages sooner but I am not sure I see any real problem
> here. Have you seen any real issues or this is more of a review driven
> fix?
We saw the problem when we try to reuse oom reaper's code in
another situation. In our situation, we allow reaping a task
before all other tasks in its task group finish their exiting
procedure.
I'd like to know what ensures "No threads should be running in
userspace by the time the reaper"?
Thank you.
>>> Signed-off-by: Wang Nan <[email protected]>
>>> Cc: Bob Liu <[email protected]>
>>> Cc: Michal Hocko <[email protected]>
>>> Cc: Andrew Morton <[email protected]>
>>> Cc: Michal Hocko <[email protected]>
>>> Cc: David Rientjes <[email protected]>
>>> Cc: Ingo Molnar <[email protected]>
>>> Cc: Roman Gushchin <[email protected]>
>>> Cc: Konstantin Khlebnikov <[email protected]>
>>> Cc: Andrea Arcangeli <[email protected]>
>>> ---
>>> 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)),
>> --
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