linux-kernel - Re: [PATCH v4 2/9] mm: pagewalk: Take the pagetable lock in walk_pte

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Message-ID: <CAHk-=wh=zsXPmSTJJgEgVr4ebgwTQ-wx-P1c7+koJAK-h2f_vQ@mail.gmail.com>
Date:   Wed, 9 Oct 2019 09:07:00 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     Thomas Hellström (VMware) 
        <thomas_os@...pmail.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        Thomas Hellstrom <thellstrom@...are.com>,
        Matthew Wilcox <willy@...radead.org>,
        Will Deacon <will.deacon@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Rik van Riel <riel@...riel.com>,
        Minchan Kim <minchan@...nel.org>,
        Michal Hocko <mhocko@...e.com>,
        Huang Ying <ying.huang@...el.com>,
        Jérôme Glisse <jglisse@...hat.com>
Subject: Re: [PATCH v4 2/9] mm: pagewalk: Take the pagetable lock in walk_pte_range()

On Wed, Oct 9, 2019 at 8:14 AM Kirill A. Shutemov <kirill@...temov.name> wrote:
>
> If ->pte_entry() fails on the first entry of the page table, pte - 1 will
> point out side the page table.
>
> And the '- 1' is totally unnecessary as we break the loop before pte++ on
> the last iteration.

Good catch. Too much copying the wrong pattern from other sources.

I do wish we didn't have this pattern of "update pte, then do
pte_unmap as long as it's in the same page". Yeah, it avoids a
variable, but still... But it is what it is, and we just need to be
careful.

             Linus