Re: [PATCH v6 6/7] mm: fix MADV_[FREE|DONTNEED] TLB flush miss problem

From: Minchan Kim
Date: Sun Aug 13 2017 - 21:26:56 EST


Hi Peter,

On Fri, Aug 11, 2017 at 03:30:20PM +0200, Peter Zijlstra wrote:
> On Tue, Aug 01, 2017 at 05:08:17PM -0700, Nadav Amit wrote:
> > void tlb_finish_mmu(struct mmu_gather *tlb,
> > unsigned long start, unsigned long end)
> > {
> > - arch_tlb_finish_mmu(tlb, start, end);
> > + /*
> > + * 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.
> > + */
> > + bool force = mm_tlb_flush_nested(tlb->mm);
> > +
> > + arch_tlb_finish_mmu(tlb, start, end, force);
> > }
>
> I don't understand the comment nor the ordering. What guarantees we see
> the increment if we need to?

How about this about commenting part?