Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Content-Type: text/plain;
        charset=utf-8
Mime-Version: 1.0 (Mac OS X Mail 16.0 \(3696.120.41.1.1\))
Subject: Re: [PATCH 01/13] mm: Update ptep_get_lockless()s comment
From:   Nadav Amit <nadav.amit@gmail.com>
In-Reply-To: <CAHk-=wjZnVURfhWMmWiDX3D0kuqnJ0PLM_Na-U7ufzqPMxucjw@mail.gmail.com>
Date:   Mon, 31 Oct 2022 08:43:37 -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>,
        jroedel@suse.de, ubizjak@gmail.com,
        Alistair Popple <apopple@nvidia.com>
Content-Transfer-Encoding: quoted-printable
Message-Id: <0E3DDDE4-FD20-40ED-A418-222AAA546596@gmail.com>
References: <20221022111403.531902164@infradead.org>
 <20221022114424.515572025@infradead.org>
 <2c800ed1-d17a-def4-39e1-09281ee78d05@nvidia.com>
 <Y1ZGNwUEdm15W6Xz@hirez.programming.kicks-ass.net>
 <CAG48ez3fG=WnvbiE5mJaD66_gJj_hohnV8CqBG9eNdjd7pJW3A@mail.gmail.com>
 <Y1fsYwshJ93FT21r@hirez.programming.kicks-ass.net>
 <CAG48ez3VE+3dVdUMK+Pg_942gR+h_TCcSaFxGwCbNfh3W+mfOA@mail.gmail.com>
 <Y1f7YvKuwOl1XEwU@hirez.programming.kicks-ass.net>
 <CAG48ez05gBiB2w7bL_3O_OUoBWazmHnRwMiuni_wQyXBUcaxbQ@mail.gmail.com>
 <Y1oub9MvqwGBlHkq@hirez.programming.kicks-ass.net>
 <CAHk-=wihPdOtXgJ32pLB6Xd-UvnaZW9YvOOjM24JGYRjNHeykA@mail.gmail.com>
 <6C548A9A-3AF3-4EC1-B1E5-47A7FFBEB761@gmail.com>
 <CAHk-=wh8oi0qQtYDFTfm7d1s5C8mG7ig=NfzGWt4zbjXMzcdqQ@mail.gmail.com>
 <F9E42822-DA1D-4192-8410-3BAE42E9E4A9@gmail.com>
 <B88D3073-440A-41C7-95F4-895D3F657EF2@gmail.com>
 <CAHk-=wgzT1QsSCF-zN+eS06WGVTBg4sf=6oTMg95+AEq7QrSCQ@mail.gmail.com>
 <47678198-C502-47E1-B7C8-8A12352CDA95@gmail.com>
 <CAHk-=wjzngbbwHw4nAsqo_RpyOtUDk5G+Wus=O0w0A6goHvBWA@mail.gmail.com>
 <CAHk-=wijU_YHSZq5N7vYK+qHPX0aPkaePaGOyWk4aqMvvSXxJA@mail.gmail.com>
 <140B437E-B994-45B7-8DAC-E9B66885BEEF@gmail.com>
 <CAHk-=wjX_P78xoNcGDTjhkgffs-Bhzcwp-mdsE1maeF57Sh0MA@mail.gmail.com>
 <CAHk-=wio=UKK9fX4z+0CnyuZG7L+U9OB7t7Dcrg4FuFHpdSsfw@mail.gmail.com>
 <CAHk-=wgz0QQd6KaRYQ8viwkZBt4xDGuZTFiTB8ifg7E3F2FxHg@mail.gmail.com>
 <CAHk-=wiwt4LC-VmqvYrphraF0=yQV=CQimDCb0XhtXwk8oKCCA@mail.gmail.com>
 <A48A5CEB-2C02-4101-B315-6792D042C605@gmail.com>
 <CAHk-=wjZnVURfhWMmWiDX3D0kuqnJ0PLM_Na-U7ufzqPMxucjw@mail.gmail.com>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Precedence: bulk

On Oct 30, 2022, at 10:00 PM, Linus Torvalds =
<torvalds@linux-foundation.org> wrote:

> So the current ordering rules are basically that we need to do
> set_page_dirty() *and* we need to flush the TLB's before dropping the
> page table lock. That's what gets us serialized with "mkclean=E2=80=9D.

I understand. I am still not sure whether ordering the set_page_dirty() =
and
dropping the mapcount reference cannot suffice for the reclaim logic not =
to
free the buffers if the page is dirtied.

According to the code, shrink_page_list() first checks for =
folio_mapped()
and then for folio_test_dirty() to check whether pageout() is necessary.
IIUC, the buffers are not dropped up to this point and set_page_dirty()
would always set the page-struct dirty bit.

IOW: In shrink_page_list(), when we decide on whether to pageout(), we
should see whether the page is dirty (give or take smp_rmb()).

But this is an optimization and I do not know all the cases in which =
buffers
might be dropped. My intuition says that they cannot be dropped while
mapcount !=3D 0, but I need to further explore it.