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SubjectRe: [v2 PATCH] mm: mmu_gather: remove __tlb_reset_range() for force flush
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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);

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