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

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Date:   Mon, 11 Jan 2021 13:30:08 +0000
From:   Will Deacon <will@...nel.org>
To:     Linus Torvalds <torvalds@...ux-foundation.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 08, 2021 at 11:34:08AM -0800, Linus Torvalds wrote:
> On Fri, Jan 8, 2021 at 9:15 AM Will Deacon <will@...nel.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.

I picked the version from here:

  https://lore.kernel.org/r/20201229132819.najtavneutnf7ajp@box

and actually, I just noticed that willy spotted a typo in a comment, so
I'll fix that locally as well as adding the above to a 'Link:' tag for
reference.

> 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 played with a few different ways of doing this, but I can't say I prefer
them over what I ended up posting. Having a bunch of 'bool' arguments makes
the callers hard to read and brings into question what exactly vmf->flags is
for. I also tried adding a separate 'address' parameter so that vmf->address
is always the real faulting address, but 'address' is the thing to use for
the pte (i.e. prefault is when 'address != vmf->address'). That wasn't too
bad, but it made the normal finish_fault() case look weird.

So I think I'll leave it as-is and see if anybody wants to change it later
on.

Will