linux-kernel - Re: [PATCH v2 0/3] Create 'old' ptes for faultaround mappings on arm64 with hardware access flag

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Date:   Fri, 8 Jan 2021 11:42:39 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Will Deacon <will@...nel.org>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Jan Kara <jack@...e.cz>, Minchan Kim <minchan@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Vinayak Menon <vinmenon@...eaurora.org>,
        Hugh Dickins <hughd@...gle.com>,
        Android Kernel Team <kernel-team@...roid.com>
Subject: Re: [PATCH v2 0/3] Create 'old' ptes for faultaround mappings on
 arm64 with hardware access flag

On Fri, Jan 8, 2021 at 11:34 AM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> Yeah, I think that's a side effect of "now the code really makes a lot
> more sense". Your subsequent patches 2-3 certainly are much simpler
> now

On that note - they could be simpler still if this was just done
entirely unconditionally..

I'm taking your word for "it makes sense", but when you say

  On CPUs with hardware AF/DBM, initialising prefaulted PTEs as 'old'
  improves vmscan behaviour and does not appear to introduce any overhead.

in the description for patch 3, it makes me wonder how noticeable the
overhead is on the hardware that _does_ take a fault on old pte's..

IOW, it would be lovely to see numbers if you have any like that..

Both ways, actually. Because I also wonder how noticeable the vmscan
improvement is. You say there's no measurable overhead for platforms
with hardware dirty/accessed bits, but maybe there's not a lot of
measurable improvements from a more exact accessed bit either?

             Linus