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

[Jan Stancek]

----- Original Message -----
> 
> 
> On May 13, 2019 4:01 PM, Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx> 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.