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

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Message-ID: <20170811133020.zozuuhbw72lzolj5@hirez.programming.kicks-ass.net>
Date:   Fri, 11 Aug 2017 15:30:20 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Nadav Amit <namit@...are.com>
Cc:     linux-mm@...ck.org, nadav.amit@...il.com,
        linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
        Minchan Kim <minchan@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Russell King <linux@...linux.org.uk>,
        Tony Luck <tony.luck@...el.com>,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        "David S. Miller" <davem@...emloft.net>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>,
        Jeff Dike <jdike@...toit.com>, linux-arch@...r.kernel.org
Subject: Re: [PATCH v6 6/7] mm: fix MADV_[FREE|DONTNEED] TLB flush miss
 problem

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?