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 11:25:37 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     Will Deacon <will@...nel.org>,
        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 Mon, Jan 11, 2021 at 6:24 AM Kirill A. Shutemov <kirill@...temov.name> wrote:
>
> I wonder if it would be acceptable to pass down to faultaround a copy
> of vmf, so it mess with it without risking to corrupt the original one?

I'd almost prefer to split vmf into two parts: the 'this is the fault
info' part and the 'this is the fault handling state' part.

So the first one would be filled in by the actual page faulter (or
GUP) - and then be 'const' during the lookup, while the second one
would be set up by handle_mm_fault() and would contain that "this is
the current state of my fault state machine" and contain things like
that ->pte thing.

And then if somebody actually needs to pass in "modified fault state"
(ie that whole "I'm doing fault-around, so I'll use multiple
addresses") they'd never modify the address in the fault info, they'd
just pass the address as an explicit argument (like most cases already
do - the "change addr or flags in vmf" is actually already _fairly_
rare).

               Linus