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

[Yang Shi]

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.

ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ nofullmmÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ fullmm
ops (records/s) ÂÂÂÂÂÂÂÂÂÂÂÂ 225606ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ 225119
sys (s)ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ 0.69ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ 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);