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

[Yang Shi]

On 6/22/18 12:59 AM, Kirill A. Shutemov wrote:
> On Thu, Jun 21, 2018 at 11:15:48PM +0000, yang.shi@xxxxxxxxxxxxxxxxx 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@xxxxxxxxxxxxxxxxx>
> > Cc: Hugh Dickins <hughd@xxxxxxxxxx>
> > Cc: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> > Cc: Vlastimil Babka <vbabka@xxxxxxx>
> > ---
> > 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

>