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

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Date:   Mon, 12 Aug 2019 17:40:45 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     Song Liu <songliubraving@...com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Matthew Wilcox <matthew.wilcox@...cle.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Kernel Team <Kernel-team@...com>,
        William Kucharski <william.kucharski@...cle.com>,
        "srikar@...ux.vnet.ibm.com" <srikar@...ux.vnet.ibm.com>
Subject: Re: [PATCH v12 5/6] khugepaged: enable collapse pmd for pte-mapped
 THP

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