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SubjectRe: [v2 PATCH] mm: mmu_gather: remove __tlb_reset_range() for force flush
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> On May 14, 2019, at 12:15 AM, Jan Stancek <jstancek@redhat.com> wrote:
>
>
> ----- Original Message -----
>> On May 13, 2019 4:01 PM, Yang Shi <yang.shi@linux.alibaba.com> wrote:
>>
>>
>> On 5/13/19 9:38 AM, Will Deacon wrote:
>>> On Fri, May 10, 2019 at 07:26:54AM +0800, Yang Shi wrote:
>>>> diff --git a/mm/mmu_gather.c b/mm/mmu_gather.c
>>>> index 99740e1..469492d 100644
>>>> --- a/mm/mmu_gather.c
>>>> +++ b/mm/mmu_gather.c
>>>> @@ -245,14 +245,39 @@ void tlb_finish_mmu(struct mmu_gather *tlb,
>>>> {
>>>> /*
>>>> * If there are parallel threads are doing PTE changes on same range
>>>> - * under non-exclusive lock(e.g., mmap_sem read-side) but defer TLB
>>>> - * flush by batching, a thread has stable TLB entry can fail to flush
>>>> - * the TLB by observing pte_none|!pte_dirty, for example so flush TLB
>>>> - * forcefully if we detect parallel PTE batching threads.
>>>> + * under non-exclusive lock (e.g., mmap_sem read-side) but defer TLB
>>>> + * flush by batching, one thread may end up seeing inconsistent PTEs
>>>> + * and result in having stale TLB entries. So flush TLB forcefully
>>>> + * if we detect parallel PTE batching threads.
>>>> + *
>>>> + * However, some syscalls, e.g. munmap(), may free page tables, this
>>>> + * needs force flush everything in the given range. Otherwise this
>>>> + * may result in having stale TLB entries for some architectures,
>>>> + * e.g. aarch64, that could specify flush what level TLB.
>>>> */
>>>> - if (mm_tlb_flush_nested(tlb->mm)) {
>>>> - __tlb_reset_range(tlb);
>>>> - __tlb_adjust_range(tlb, start, end - start);
>>>> + if (mm_tlb_flush_nested(tlb->mm) && !tlb->fullmm) {
>>>> + /*
>>>> + * Since we can't tell what we actually should have
>>>> + * flushed, flush everything in the given range.
>>>> + */
>>>> + tlb->freed_tables = 1;
>>>> + tlb->cleared_ptes = 1;
>>>> + tlb->cleared_pmds = 1;
>>>> + tlb->cleared_puds = 1;
>>>> + tlb->cleared_p4ds = 1;
>>>> +
>>>> + /*
>>>> + * Some architectures, e.g. ARM, that have range invalidation
>>>> + * and care about VM_EXEC for I-Cache invalidation, need
>>>> force
>>>> + * vma_exec set.
>>>> + */
>>>> + tlb->vma_exec = 1;
>>>> +
>>>> + /* Force vma_huge clear to guarantee safer flush */
>>>> + tlb->vma_huge = 0;
>>>> +
>>>> + tlb->start = start;
>>>> + tlb->end = end;
>>>> }
>>> Whilst I think this is correct, it would be interesting to see whether
>>> or not it's actually faster than just nuking the whole mm, as I mentioned
>>> before.
>>>
>>> At least in terms of getting a short-term fix, I'd prefer the diff below
>>> if it's not measurably worse.
>>
>> I did a quick test with ebizzy (96 threads with 5 iterations) on my x86
>> VM, it shows slightly slowdown on records/s but much more sys time spent
>> with fullmm flush, the below is the data.
>>
>> nofullmm fullmm
>> ops (records/s) 225606 225119
>> sys (s) 0.69 1.14
>>
>> It looks the slight reduction of records/s is caused by the increase of
>> sys time.
>>
>>> Will
>>>
>>> --->8
>>>
>>> diff --git a/mm/mmu_gather.c b/mm/mmu_gather.c
>>> index 99740e1dd273..cc251422d307 100644
>>> --- a/mm/mmu_gather.c
>>> +++ b/mm/mmu_gather.c
>>> @@ -251,8 +251,9 @@ void tlb_finish_mmu(struct mmu_gather *tlb,
>>> * forcefully if we detect parallel PTE batching threads.
>>> */
>>> if (mm_tlb_flush_nested(tlb->mm)) {
>>> + tlb->fullmm = 1;
>>> __tlb_reset_range(tlb);
>>> - __tlb_adjust_range(tlb, start, end - start);
>>> + tlb->freed_tables = 1;
>>> }
>>>
>>> tlb_flush_mmu(tlb);
>>
>>
>> I think that this should have set need_flush_all and not fullmm.
>
> Wouldn't that skip the flush?
>
> If fulmm == 0, then __tlb_reset_range() sets tlb->end = 0.
> tlb_flush_mmu
> tlb_flush_mmu_tlbonly
> if (!tlb->end)
> return
>
> Replacing fullmm with need_flush_all, brings the problem back / reproducer hangs.

Maybe setting need_flush_all does not have the right effect, but setting
fullmm and then calling __tlb_reset_range() when the PTEs were already
zapped seems strange.

fullmm is described as:

/*
* we are in the middle of an operation to clear
* a full mm and can make some optimizations
*/

And this not the case.

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