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

Peter Zijlstra
2017-08-11T13:31:08Z

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?