Re: [PATCH v12 5/6] khugepaged: enable collapse pmd for pte-mapped THP

[Kirill A. Shutemov]

On Mon, Aug 12, 2019 at 03:22:58PM +0200, Oleg Nesterov wrote:
> On 08/12, Kirill A. Shutemov wrote:
> >
> > On Fri, Aug 09, 2019 at 06:01:18PM +0000, Song Liu wrote:
> > > +		if (pte_none(*pte) || !pte_present(*pte))
> > > +			continue;
> >
> > You don't need to check both. Present is never none.
> 
> Agreed.
> 
> Kirill, while you are here, shouldn't retract_page_tables() check
> vma->anon_vma (and probably do mm_find_pmd) under vm_mm->mmap_sem?
> 
> Can't it race with, say, do_cow_fault?

vma->anon_vma can race, but it doesn't matter. False-negative is fine.
It's attempt to avoid taking mmap_sem where it can be not productive.

mm_find_pmd() cannot race with do_cow_fault() since the page is locked.
__do_fault() has to return locked page before we touch page tables.
It is somewhat subtle, but I wanted to avoid taking mmap_sem where it is
possible.

-- 
 Kirill A. Shutemov