linux-kernel - Re: [PATCH 4/4] mm: Add PGREUSE counter

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Date:   Sat, 22 Aug 2020 09:14:53 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Peter Xu <peterx@...hat.com>
Cc:     Linux-MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "Maya B . Gokhale" <gokhale2@...l.gov>,
        Yang Shi <yang.shi@...ux.alibaba.com>,
        Marty Mcfadden <mcfadden8@...l.gov>,
        Kirill Shutemov <kirill@...temov.name>,
        Oleg Nesterov <oleg@...hat.com>, Jann Horn <jannh@...gle.com>,
        Jan Kara <jack@...e.cz>, Kirill Tkhai <ktkhai@...tuozzo.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Christoph Hellwig <hch@....de>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 4/4] mm: Add PGREUSE counter

On Fri, Aug 21, 2020 at 4:50 PM Peter Xu <peterx@...hat.com> wrote:
>
> This accounts for wp_page_reuse() case, where we reused a page for COW.

If we do this, wouldn't it make more sense to also count the COW case
to see how they match up?

Right now we count faults and major faults. So as a result you can can
calculate minor faults trivially.

But if you count page reuse, you can't calculate any stats on it,
because most of the minor faults will presumably be for new pages
(either zero or cached file mappings).

So the "pgreuse" seems to be a counter without any context to it.

IOW, I get the feeling that either we should do this properly (and
maybe count "dirty faults" and "access" faults, at which point the
reuse case becomes a subcase of the dirty ones) or we shouldn't do it
at all. Creating a counter without any way to sanely compare it to
anything seems a bit pointless.

                Linus