linux-kernel - Re: [PATCH v2 3/5] generic/pgtable: Introduce set_pte

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Message-ID: <CAPcyv4gLA_UryDZzdtAi-T=2x8io4V_2BwX_qTF7KS8Ff1Kb3A@mail.gmail.com>
Date:   Mon, 3 Dec 2018 09:57:09 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Borislav Petkov <bp@...en8.de>, X86 ML <x86@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 3/5] generic/pgtable: Introduce set_pte_safe()

On Mon, Dec 3, 2018 at 9:53 AM Dave Hansen <dave.hansen@...el.com> wrote:
>
> On 11/30/18 4:35 PM, Dan Williams wrote:
> > +/*
> > + * The _safe versions of set_{pte,pmd,pud,p4d,pgd} validate that the
> > + * entry was not populated previously. I.e. for cases where a flush-tlb
> > + * is elided, double-check that there is no stale mapping to shoot down.
> > + */
>
> Functionally these look great to me.
>
> The only thing I'd suggest is to make the comment more about when to use
> these, instead of what they do:
>
>         Use the set_p*_safe() version when confident that *no*
>         TLB flush will be required as a result of the "set", such
>         as setting non-present entries or when possibly superfluously
>         re-setting an entry.

The second sentence was meant to be a "why", but yes, it's entirely too subtle.