linux-kernel - Re: [PATCHv2 15/28] thp: handle file COW faults

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Message-ID: <20160216100838.GD46557@black.fi.intel.com>
Date:	Tue, 16 Feb 2016 13:08:38 +0300
From:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To:	Dave Hansen <dave.hansen@...el.com>
Cc:	Hugh Dickins <hughd@...gle.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Vlastimil Babka <vbabka@...e.cz>,
	Christoph Lameter <cl@...two.org>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	Jerome Marchand <jmarchan@...hat.com>,
	Yang Shi <yang.shi@...aro.org>,
	Sasha Levin <sasha.levin@...cle.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCHv2 15/28] thp: handle file COW faults

On Fri, Feb 12, 2016 at 10:36:25AM -0800, Dave Hansen wrote:
> On 02/11/2016 06:21 AM, Kirill A. Shutemov wrote:
> > File COW for THP is handled on pte level: just split the pmd.
> 
> More changelog.  More comments, please.

Okay, I'll add more.

> We don't want to COW THP's because we'll waste memory?  A COW that we
> could handle with 4k, we would have to handle with 2M, and that's
> inefficient and high-latency?

All of above.i

It's not clear how benefitial THP file COW mappings. And it would require
some code to make them work.

I think at some point we can consider teaching khugepaged to collapse such
pages, but allocating huge on fault is probably overkill.

> Seems like a good idea to me.  It would just be nice to ensure every
> reviewer doesn't have to think their way through it.

-- 
 Kirill A. Shutemov