Re: [v2 PATCH 1/2] mm: thp: register mm for khugepaged when merging vma for shmem

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

On 6/22/18 12:59 AM, Kirill A. Shutemov wrote:
> On Thu, Jun 21, 2018 at 11:15:48PM +0000, yang.shi@linux.alibaba.com wrote:
>> When merging anonymous page vma, if the size of vma can fit in at least
>> one hugepage, the mm will be registered for khugepaged for collapsing
>> THP in the future.
>>
>> But, it skips shmem vma. Doing so for shmem too, but not file-private
>> mapping, when merging vma in order to increase the odd to collapse
>> hugepage by khugepaged.
>>
>> Signed-off-by: Yang Shi <yang.shi@linux.alibaba.com>
>> Cc: Hugh Dickins <hughd@google.com>
>> Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
>> Cc: Vlastimil Babka <vbabka@suse.cz>
>> ---
>> v1 --> 2:
>> * Exclude file-private mapping per Kirill's comment
>>
>>   mm/khugepaged.c | 8 ++++++--
>>   1 file changed, 6 insertions(+), 2 deletions(-)
>>
>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>> index d7b2a4b..9b0ec30 100644
>> --- a/mm/khugepaged.c
>> +++ b/mm/khugepaged.c
>> @@ -440,8 +440,12 @@ int khugepaged_enter_vma_merge(struct vm_area_struct *vma,
>>   		 * page fault if needed.
>>   		 */
>>   		return 0;
>> -	if (vma->vm_ops || (vm_flags & VM_NO_KHUGEPAGED))
>> -		/* khugepaged not yet working on file or special mappings */
>> +	if ((vma->vm_ops && (!shmem_file(vma->vm_file) || vma->anon_vma)) ||
>> +	    (vm_flags & VM_NO_KHUGEPAGED))
>> +		/*
>> +		 * khugepaged not yet working on non-shmem file or special
>> +		 * mappings. And, file-private shmem THP is not supported.
>> +		 */
>>   		return 0;
> My point was that vma->anon_vma check above this one should not prevent
> collapse for shmem.
>
> Looking into this more, I think we should just replace all these checks
> with hugepage_vma_check() call.

I got a little bit confused here. I thought the condition to *not* 
collapse file-private shmem mapping should be:

shmem_file(vma->vm_file) && vma->anon_vma

Is this right?

If it is right, it looks hugepage_vma_check() can't do this since it 
just returns true if shmem_file is true and it is page aligned without 
checking vma->anon_vma.

Thanks,
Yang

>