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Date:   Tue, 12 Jan 2021 21:47:59 +0000
From:   Will Deacon <will@kernel.org>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     "Kirill A. Shutemov" <kirill@shutemov.name>,
        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>
Subject: Re: [PATCH v2 0/3] Create 'old' ptes for faultaround mappings on
 arm64 with hardware access flag
Message-ID: <20210112214759.GC10434@willie-the-truck>
References: <20210108171517.5290-1-will@kernel.org>
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On Mon, Jan 11, 2021 at 11:25:37AM -0800, Linus Torvalds wrote:
> On Mon, Jan 11, 2021 at 6:24 AM Kirill A. Shutemov <kirill@shutemov.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).

Alright then, I'll take another crack at this for v3 and see how far I
get.

Will