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

[Yang Shi]

On 9/28/18 3:36 PM, Dan Williams wrote:
> 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.

Thanks for confirming this. Actually, I had the same question before I 
came up with this patch. "all cases" sounds a little bit misleading.