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

[Peter Zijlstra]

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?