linux-kernel - Re: page-flags behavior on compound pages: a worry

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Message-ID: <20150810110955.GA27046@node.dhcp.inet.fi>
Date:	Mon, 10 Aug 2015 14:09:55 +0300
From:	"Kirill A. Shutemov" <kirill@...temov.name>
To:	Christoph Lameter <cl@...ux.com>
Cc:	Hugh Dickins <hughd@...gle.com>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	David Rientjes <rientjes@...gle.com>,
	Dave Hansen <dave.hansen@...el.com>,
	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	Vlastimil Babka <vbabka@...e.cz>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	Steve Capper <steve.capper@...aro.org>,
	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Michal Hocko <mhocko@...e.cz>,
	Jerome Marchand <jmarchan@...hat.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: page-flags behavior on compound pages: a worry

On Fri, Aug 07, 2015 at 10:28:49AM -0500, Christoph Lameter wrote:
> On Fri, 7 Aug 2015, Kirill A. Shutemov wrote:
> 
> > On Thu, Aug 06, 2015 at 03:45:31PM -0500, Christoph Lameter wrote:
> > > On Thu, 6 Aug 2015, Hugh Dickins wrote:
> > >
> > > > > I know a patchset which solves this! ;)
> > > >
> > > > Oh, and I know a patchset which avoids these problems completely,
> > > > by not using compound pages at all ;)
> > >
> > > Another dumb idea: Stop the insanity of splitting pages on the fly?
> > > Splitting pages should work like page migration: Lock everything down and
> > > ensure no one is using the page and then do it. That way the compound pages
> > > and its metadata are as stable as a regular page.
> >
> > That's what I do in refcounting patchset.
> 
> Looks like you make refcounting easier and avoid splitting in some cases
> maybe only splitting the pmd. But the fundamental issue still remains.
> Complexity is high since individual pages of a compound can be mapped and
> unmapped in multiple processes.
> 
> The compound would need to be always treated as a single order N entity
> in order to really get things simplified and make code cleaner.
> 
> Either all pages are mapped or none. Otherwise you have to manage the
> a schizoprenic view of pages. Sometimes an order N size entity is
> managed and sometimes a base page size page which is a fraction of the
> whole. Such a view of a memory object is pretty difficult to manage.

I don't see anything actionable here. Your wish list doesn't cope with
reality. Compound pages are mapped with PTEs for almost ten years and I
don't see why we should stop the practice.

-- 
 Kirill A. Shutemov
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