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

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Message-Id: <0E3DDDE4-FD20-40ED-A418-222AAA546596@gmail.com>
Date:   Mon, 31 Oct 2022 08:43:37 -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 10:00 PM, Linus Torvalds <torvalds@...ux-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”.

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 != 0, but I need to further explore it.