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Subject: Re: [PATCH 01/13] mm: Update ptep_get_lockless()s comment
From:   Nadav Amit <nadav.amit@gmail.com>
In-Reply-To: <B39368D9-7E5D-4864-819C-CDEEAB4BD92D@gmail.com>
Date:   Sat, 29 Oct 2022 14:03:46 -0700
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To:     Linus Torvalds <torvalds@linux-foundation.org>,
        John Hubbard <jhubbard@nvidia.com>
Precedence: bulk

On Oct 29, 2022, at 1:56 PM, Nadav Amit <nadav.amit@gmail.com> wrote:

> On Oct 29, 2022, at 1:15 PM, Linus Torvalds =
<torvalds@linux-foundation.org> wrote:
>=20
>> (b) we could move the "page_remove_rmap()" into the "flush-and-free" =
path too
>>=20
>> And (b) would be fairly easy - same model as that dirty bit patch,
>> just a 'do page_remove_rmap too' - except page_remove_rmap() wants =
the
>> vma as well (and we delay the TLB flush over multiple vma's, so it's
>> not just a "save vma in mmu_gather=E2=80=9D).
>=20
> (b) sounds reasonable and may potentially allow future performance
> improvements (batching, doing stuff without locks).
>=20
> It does appear to break a potential hidden assumption that rmap is =
removed
> while the ptl is acquired (at least in the several instances I =
samples).
> Yet, anyhow page_vma_mapped_walk() checks the PTE before calling the
> function, so it should be fine.
>=20
> I=E2=80=99ll give it a try.

I have just seen John=E2=80=99s and your emails. It seems (b) fell off. =
(a) is out
of my =E2=80=9Czone=E2=80=9D, and anyhow assuming it would not be solved =
soon, deferring
page_remove_rmap() might cause regressions.

(c) might be more intrusive and potentially induce overheads. If we need =
a
small backportable solution, I think the approach that I proposed =
(marking
the page dirty after the invalidation, before the PTL is released) is =
the
simplest one.

Please advise how to proceed.