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

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Message-ID: <20190514114343.GN2589@hirez.programming.kicks-ass.net>
Date:   Tue, 14 May 2019 13:43:43 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Nadav Amit <namit@...are.com>
Cc:     Yang Shi <yang.shi@...ux.alibaba.com>,
        Will Deacon <will.deacon@....com>,
        "jstancek@...hat.com" <jstancek@...hat.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 02:01:34AM +0000, Nadav Amit wrote:
> > diff --git a/mm/mmu_gather.c b/mm/mmu_gather.c
> > index 99740e1dd273..cc251422d307 100644
> > --- a/mm/mmu_gather.c
> > +++ b/mm/mmu_gather.c
> > @@ -251,8 +251,9 @@ void tlb_finish_mmu(struct mmu_gather *tlb,
> >         * forcefully if we detect parallel PTE batching threads.
> >         */
> >        if (mm_tlb_flush_nested(tlb->mm)) {
> > +             tlb->fullmm = 1;
> >                __tlb_reset_range(tlb);
> > -             __tlb_adjust_range(tlb, start, end - start);
> > +             tlb->freed_tables = 1;
> >        }
> >
> >        tlb_flush_mmu(tlb);
> 
> 
> I think that this should have set need_flush_all and not fullmm.

Difficult, mmu_gather::need_flush_all is arch specific and not everybody
implements it.

And while mmu_gather::fullmm isn't strictly correct either; we can
(ab)use it here, because at tlb_finish_mmu() time the differences don't
matter anymore.