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

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Message-ID: <6e59a56d-7bb3-76d3-201a-8a4ad4638602@linux.alibaba.com>
Date:   Fri, 22 Jun 2018 09:44:27 -0700
From:   Yang Shi <yang.shi@...ux.alibaba.com>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:     hughd@...gle.com, vbabka@...e.cz, akpm@...ux-foundation.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [v2 PATCH 1/2] mm: thp: register mm for khugepaged when merging
 vma for shmem



On 6/22/18 9:19 AM, Kirill A. Shutemov wrote:
> On Fri, Jun 22, 2018 at 04:04:12PM +0000, yang.shi@...ux.alibaba.com wrote:
>>
>> On 6/22/18 12:59 AM, Kirill A. Shutemov wrote:
>>> On Thu, Jun 21, 2018 at 11:15:48PM +0000, yang.shi@...ux.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@...ux.alibaba.com>
>>>> Cc: Hugh Dickins <hughd@...gle.com>
>>>> Cc: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
>>>> Cc: Vlastimil Babka <vbabka@...e.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?
> No, if shmem_file(vma->vm_file) is true, vma->anon_vma doesn't matter.
> We don't care about anon_vma in such VMA as we don't touch file-private
> pages.

Thanks, I misunderstood that. hugepage_vma_check() sounds reasonable. 
Will fix in v3 soon.

Yang

>