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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: <CAHk-=wgiDa9Xy5rLMR_gvC-WVvdkxABgXXADbKhwswYof7ptdg@mail.gmail.com>
Date:   Sat, 29 Oct 2022 12:28:33 -0700
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Jann Horn <jannh@google.com>,
        John Hubbard <jhubbard@nvidia.com>, X86 ML <x86@kernel.org>,
        Matthew Wilcox <willy@infradead.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        kernel list <linux-kernel@vger.kernel.org>,
        Linux-MM <linux-mm@kvack.org>,
        Andrea Arcangeli <aarcange@redhat.com>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
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        Alistair Popple <apopple@nvidia.com>
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To:     Linus Torvalds <torvalds@linux-foundation.org>
Precedence: bulk

On Oct 29, 2022, at 12:14 PM, Linus Torvalds =
<torvalds@linux-foundation.org> wrote:

> On Sat, Oct 29, 2022 at 11:58 AM Linus Torvalds
> <torvalds@linux-foundation.org> wrote:
>> Still not tested in any way, shape, or form. I decided I wanted to
>> send this one before booting into this and possibly blowing up ;^)
>=20
> Well, it boots, and I see no difference with your PoC code.
>=20
> It didn't fail for me before, it doesn't fail for me with those =
patches.
>=20
> Again, the "it doesn't fail for me" is probably because I'm running it
> incorrectly, although for all I know there can also be hardware
> differences.

Please give me some time to test it. I presume you ran it with block ram
device (not tmpfs) and not on a virtual machine (which can affect =
besides
Intel/AMD implementation differences).

But even if your patches work and the tests pass, I am not sure it means
that everything is fine. I did not try to trigger a race with
shrink_page_list(), and doing that might be harder than the race I tried =
to
create before. I need to do some tracing to understand what I was =
missing in
my understanding of the shrink_page_list() - assuming that I am mistaken
about the buffers being potentially released.

I would note that my concern about releasing the buffers is partially =
driven
by to issues that were reported before [1]. I am actually not sure how =
they
were resolved.


[1] https://lore.kernel.org/all/20180103100430.GE4911@quack2.suse.cz/