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

[Yang Shi <yang.shi@linux.alibaba.com>]

On 9/28/18 3:36 PM, Dan Williams wrote:
> On Fri, Sep 28, 2018 at 3:34 PM <yang.shi@linux.alibaba.com> 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@intel.com>
>> Cc: Jan Kara <jack@suse.cz>
>> Cc: Ross Zwisler <ross.zwisler@linux.intel.com>
>> Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
>> Signed-off-by: Yang Shi <yang.shi@linux.alibaba.com>
>> ---
>> 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.