Re: [PATCH] mm: enforce THP for VM_NOHUGEPAGE dax mappings

[Dan Williams]

On Fri, Sep 28, 2018 at 3:34 PM <yang.shi@xxxxxxxxxxxxxxxxx> wrote:
>
> commit baabda261424517110ea98c6651f632ebf2561e3 ("mm: always enable thp
> for dax mappings") says madvise hguepage policy makes less sense for
> dax, and force enabling thp for dax mappings in all cases, even though
> THP is set to "never".
>
> However, transparent_hugepage_enabled() may return false if
> VM_NOHUGEPAGE is set even though the mapping is dax.
>
> So, move is_vma_dax() check to the very beginning to enforce THP for dax
> mappings in all cases.
>
> Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
> Cc: Jan Kara <jack@xxxxxxx>
> Cc: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx>
> Cc: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
> Signed-off-by: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>
> ---
> I didn't find anyone mention the check should be before VM_NOHUGEPAGE in
> the review for Dan's original patch. And, that patch commit log states
> clearly that THP for dax mapping for all cases even though THP is never.
> So, I'm supposed it should behave in this way.

No, if someone explicitly does MADV_NOHUGEPAGE then the kernel should
honor that, even if the mapping is DAX.