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

From: "Kirill A. Shutemov" <kirill@shutemov.name>
To: Oleg Nesterov <oleg@redhat.com>
Cc: Song Liu <songliubraving@fb.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Linux MM <linux-mm@kvack.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Matthew Wilcox <matthew.wilcox@oracle.com>,
	"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
	Kernel Team <Kernel-team@fb.com>,
	William Kucharski <william.kucharski@oracle.com>,
	"srikar@linux.vnet.ibm.com" <srikar@linux.vnet.ibm.com>
Subject: Re: [PATCH v12 5/6] khugepaged: enable collapse pmd for pte-mapped THP
Date: Mon, 12 Aug 2019 17:40:45 +0300	[thread overview]
Message-ID: <20190812144045.tkvipsyit3nccvuk@box> (raw)
In-Reply-To: <20190812132257.GB31560@redhat.com>

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