linux-kernel - Re: [PATCH] mm: mempolicy: don't have to split pmd for huge zero page

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Message-ID: <YL+ttjqJ9lEMndiA@dhcp22.suse.cz>
Date:   Tue, 8 Jun 2021 19:49:42 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Yang Shi <shy828301@...il.com>
Cc:     Zi Yan <ziy@...dia.com>, nao.horiguchi@...il.com,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Hugh Dickins <hughd@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: mempolicy: don't have to split pmd for huge zero page

On Tue 08-06-21 10:15:36, Yang Shi wrote:
[...]
> I did some archeology, the findings are:
> 
> The zero page has PageReserved flag set, it was skipped by the
> explicit PageReserved check in mempolicy.c since commit f4598c8b3678
> ("[PATCH] migration: make sure there is no attempt to migrate reserved
> pages."). The zero page was not used anymore by do_anonymous_page()
> since 2.6.24 by commit 557ed1fa2620 ("remove ZERO_PAGE"), then
> reinstated by commit a13ea5b759645 ("mm: reinstate ZERO_PAGE") and
> this commit added zero page check in vm_normal_page(), so mempolicy
> doesn't depend on PageReserved check to skip zero page anymore since
> then.
> 
> So the zero page is skipped by mempolicy.c since 2.6.16.

Thanks a lot! This is really useful. Can you just add it to the
changelog so others do not have to go through the painful archeology.

With that, feel free to add
Acked-by: Michal Hocko <mhocko@...e.com>

Thanls!
-- 
Michal Hocko
SUSE Labs