linux-kernel - Re: [PATCH v2 02/10] mm/thp: make is_huge_zero

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Date:   Wed, 9 Jun 2021 13:22:46 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Hugh Dickins <hughd@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Yang Shi <shy828301@...il.com>,
        Wang Yugui <wangyugui@...-tech.com>,
        Matthew Wilcox <willy@...radead.org>,
        Naoya Horiguchi <naoya.horiguchi@....com>,
        Alistair Popple <apopple@...dia.com>,
        Ralph Campbell <rcampbell@...dia.com>, Zi Yan <ziy@...dia.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Minchan Kim <minchan@...nel.org>, Jue Wang <juew@...gle.com>,
        Peter Xu <peterx@...hat.com>, Jan Kara <jack@...e.cz>,
        Shakeel Butt <shakeelb@...gle.com>,
        Oscar Salvador <osalvador@...e.de>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 02/10] mm/thp: make is_huge_zero_pmd() safe and quicker

On Tue, Jun 08, 2021 at 09:08:09PM -0700, Hugh Dickins wrote:
> Most callers of is_huge_zero_pmd() supply a pmd already verified present;
> but a few (notably zap_huge_pmd()) do not - it might be a pmd migration
> entry, in which the pfn is encoded differently from a present pmd: which
> might pass the is_huge_zero_pmd() test (though not on x86, since L1TF
> forced us to protect against that); or perhaps even crash in pmd_page()
> applied to a swap-like entry.
> 
> Make it safe by adding pmd_present() check into is_huge_zero_pmd() itself;
> and make it quicker by saving huge_zero_pfn, so that is_huge_zero_pmd()
> will not need to do that pmd_page() lookup each time.
> 
> __split_huge_pmd_locked() checked pmd_trans_huge() before: that worked,
> but is unnecessary now that is_huge_zero_pmd() checks present.
> 
> Fixes: e71769ae5260 ("mm: enable thp migration for shmem thp")
> Signed-off-by: Hugh Dickins <hughd@...gle.com>
> Cc: <stable@...r.kernel.org>

Acked-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>

-- 
 Kirill A. Shutemov