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

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Date:   Sun, 30 Oct 2022 21:09:37 -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 30, 2022, at 6:47 PM, Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> The reason I haven't actually tested it is partly because I never
> recreated the original problem Navav reported, and partly because the
> meat of patch 4/4 is just the same "encode an extra flag bit in the
> low bit of the page pointer" that I _did_ test, just doing the "remove
> rmap" instead of "set dirty".
> 
> In other words, I *think* this should make Nadav's test-case happy,
> and avoid the warning he saw.

I am sorry for not managing to make it reproducible on your system. The fact
that you did not get the warning that I got means that it is not a
hardware-TLB differences issue (at least not only that), but the race does
not happen on your system (assuming you used ext4 on the BRD).

Anyhow, I ran the tests with the patches and there are no failures.
Thanks for addressing this issue.

I understand from the code that you decided to drop the deferring of
set_page_dirty(), which could - at least for the munmap case (where
mmap_lock is taken for write) - prevent the need for “force_flush” and
potentially save TLB flushes.

I was just wondering whether the reason for that is that you wanted
to have small backportable and conservative patches, or whether you
changed your mind about it.