linux-kernel - Re: [PATCH v2] mm/huge_memory.c: reorder operations in __split_huge_page

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Message-ID: <CALYGNiMjuW4BM2tSUOUmJUu7kX0HEL1EN-YFi=cvtstQi3YeHQ@mail.gmail.com>
Date:   Sun, 11 Feb 2018 18:32:10 +0300
From:   Konstantin Khlebnikov <koct9i@...il.com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     Konstantin Khlebnikov <khlebnikov@...dex-team.ru>,
        linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Michal Hocko <mhocko@...e.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Nicholas Piggin <npiggin@...il.com>
Subject: Re: [PATCH v2] mm/huge_memory.c: reorder operations in __split_huge_page_tail()

On Sun, Feb 11, 2018 at 6:14 PM, Kirill A. Shutemov
<kirill@...temov.name> wrote:
> On Sun, Feb 11, 2018 at 05:29:37PM +0300, Konstantin Khlebnikov wrote:
>> And replace page_ref_inc()/page_ref_add() with page_ref_unfreeze() which
>> is made especially for that and has semantic of smp_store_release().
>
> Nak on this part.
>
> page_ref_unfreeze() uses atomic_set() which neglects the situation in the
> comment you're removing.

Why? look into x86 smp_store_release
for PPro it use same sequence smp_wb + WRITE_ONCE
As I see spin_unlock uses exactly this macro.

Anyway if page_ref_unfreeze cannot handle races with
get_page_unless_zero() then it completely useless,

>
> You need at least explain why it's safe now.
>
> I would rather leave page_ref_inc()/page_ref_add() + explcit
> smp_mb__before_atomic().
>
> --
>  Kirill A. Shutemov
>
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