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

----- 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.
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