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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Fri, 8 Jan 2021 11:34:08 -0800
Message-ID: <CAHk-=wg3UkUdiTbqWFx3zBLXv9VJHuNZAa5QyDvXiSmD4gX94A@mail.gmail.com>
Subject: Re: [PATCH v2 0/3] Create 'old' ptes for faultaround mappings on
 arm64 with hardware access flag
To:     Will Deacon <will@kernel.org>
Cc:     Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux-MM <linux-mm@kvack.org>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Jan Kara <jack@suse.cz>, Minchan Kim <minchan@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
        Vinayak Menon <vinmenon@codeaurora.org>,
        Hugh Dickins <hughd@google.com>,
        Android Kernel Team <kernel-team@android.com>
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On Fri, Jan 8, 2021 at 9:15 AM Will Deacon <will@kernel.org> wrote:
>
> The big difference in this version is that I have reworked it based on
> Kirill's patch which he posted as a follow-up to the original. However,
> I can't tell where we've landed on that -- Linus seemed to like it, but
> Hugh was less enthusiastic.

Yeah, I like it, but I have to admit that it had a disturbingly high
number of small details wrong for several versions. I hope you picked
up the final version of the code.

At the same time, I do think that the "disturbingly high number of
issues" was primarily exactly _because_ the old code was so
incomprehensible, and I think the end result is much cleaner, so I
still like it.

>I think that my subsequent patches are an
> awful lot cleaner after the rework

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, although I'd be inclined to add an argument to "do_set_pte()"
that has the "write" and "pretault" bits in it, instead of having to
modify the 'vmf' structure.

I still dislike how we basically randomly modify the information in
that 'vmf' thing.

That said, now it's just a small detail - not really objectionable,
just a "this could be cleaner, I think".

I think it was Kirill who pointed out that we sadly cannot make 'vmf'
read-only anyway, because it does also contain those pre-allocation
details etc (vmf->pte etc) that are very much about what the current
"state" of the fault is. So while I would hope it could be more
read-only than it is, my wish that it could _actually_ be 'const' is
clearly just an irrelevant dream.

                   Linus