Re: [PATCH v2 02/10] mm/thp: make is_huge_zero_pmd() safe and quicker

[Kirill A. Shutemov]

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@xxxxxxxxxx>
> Cc: <stable@xxxxxxxxxxxxxxx>

Acked-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>

-- 
 Kirill A. Shutemov