linux-kernel - Re: [PATCH 01/13] mm: Update ptep_get

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Date:   Sat, 29 Oct 2022 12:28:33 -0700
From:   Nadav Amit <nadav.amit@...il.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Jann Horn <jannh@...gle.com>,
        John Hubbard <jhubbard@...dia.com>, X86 ML <x86@...nel.org>,
        Matthew Wilcox <willy@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        kernel list <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        jroedel@...e.de, ubizjak@...il.com,
        Alistair Popple <apopple@...dia.com>
Subject: Re: [PATCH 01/13] mm: Update ptep_get_lockless()s comment

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

> On Sat, Oct 29, 2022 at 11:58 AM Linus Torvalds
> <torvalds@...ux-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 ;^)
> 
> Well, it boots, and I see no difference with your PoC code.
> 
> It didn't fail for me before, it doesn't fail for me with those patches.
> 
> 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/