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

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Message-ID: <1464031607.16365.60.camel@redhat.com>
Date:	Mon, 23 May 2016 15:26:47 -0400
From:	Rik van Riel <riel@...hat.com>
To:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.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, 2016-05-23 at 22:01 +0300, Kirill A. Shutemov wrote:
> 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.

Good point.

I guess the alternative is to have the page_table_lock
taken by a helper function (everywhere) that can return
failure if the page table was changed while the caller
was waiting for the lock.

Doable, but a fair amount of code churn.

-- 
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