linux-kernel - Re: [PATCH 3/3] mm, thp: make swapin readahead under down_read of mmap

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Message-ID: <20160523190154.GA79357@black.fi.intel.com>
Date:	Mon, 23 May 2016 22:01:54 +0300
From:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To:	Rik van Riel <riel@...hat.com>
Cc:	Michal Hocko <mhocko@...nel.org>,
	Ebru Akagunduz <ebru.akagunduz@...il.com>, linux-mm@...ck.org,
	hughd@...gle.com, akpm@...ux-foundation.org,
	n-horiguchi@...jp.nec.com, aarcange@...hat.com,
	iamjoonsoo.kim@....com, gorcunov@...nvz.org,
	linux-kernel@...r.kernel.org, mgorman@...e.de, rientjes@...gle.com,
	vbabka@...e.cz, aneesh.kumar@...ux.vnet.ibm.com,
	hannes@...xchg.org, boaz@...xistor.com
Subject: Re: [PATCH 3/3] mm, thp: make swapin readahead under down_read of
 mmap_sem

On Mon, May 23, 2016 at 02:49:09PM -0400, Rik van Riel wrote:
> On Mon, 2016-05-23 at 20:42 +0200, Michal Hocko wrote:
> > On Mon 23-05-16 20:14:11, Ebru Akagunduz wrote:
> > > 
> > > Currently khugepaged makes swapin readahead under
> > > down_write. This patch supplies to make swapin
> > > readahead under down_read instead of down_write.
> > You are still keeping down_write. Can we do without it altogether?
> > Blocking mmap_sem of a remote proces for write is certainly not nice.
> 
> Maybe Andrea can explain why khugepaged requires
> a down_write of mmap_sem?
> 
> If it were possible to have just down_read that
> would make the code a lot simpler.

You need a down_write() to retract page table. We need to make sure that
nobody sees the page table before we can replace it with huge pmd.

-- 
 Kirill A. Shutemov