linux-kernel - Re: [v2 PATCH] mm: mmu_gather: remove __tlb_reset

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Message-ID: <20190514114919.GO2589@hirez.programming.kicks-ass.net>
Date:   Tue, 14 May 2019 13:49:19 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Nadav Amit <namit@...are.com>
Cc:     Jan Stancek <jstancek@...hat.com>,
        Yang Shi <yang.shi@...ux.alibaba.com>,
        Will Deacon <will.deacon@....com>,
        "minchan@...nel.org" <minchan@...nel.org>,
        "mgorman@...e.de" <mgorman@...e.de>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [v2 PATCH] mm: mmu_gather: remove __tlb_reset_range() for force
 flush

On Tue, May 14, 2019 at 07:21:33AM +0000, Nadav Amit wrote:
> > On May 14, 2019, at 12:15 AM, Jan Stancek <jstancek@...hat.com> wrote:

> > Replacing fullmm with need_flush_all, brings the problem back / reproducer hangs.
> 
> Maybe setting need_flush_all does not have the right effect, but setting
> fullmm and then calling __tlb_reset_range() when the PTEs were already
> zapped seems strange.
> 
> fullmm is described as:
> 
>         /*
>          * we are in the middle of an operation to clear
>          * a full mm and can make some optimizations
>          */
> 
> And this not the case.

Correct; starting with fullmm would be wrong. For instance
tlb_start_vma() would do the wrong thing because it assumes the whole mm
is going away. But we're at tlb_finish_mmu() time and there the
difference doesn't matter anymore.

But yes, that's a wee abuse.