On May 14, 2019, at 12:15 AM, Jan Stancek jstancek@redhat.com wrote:

----- Original Message -----

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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:

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On Fri, May 10, 2019 at 07:26:54AM +0800, Yang Shi wrote:

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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.