Date: Mon, 10 Aug 2015 14:09:55 +0300
From: "Kirill A. Shutemov" <kirill@shutemov.name>
To: Christoph Lameter <cl@linux.com>
Cc: Hugh Dickins <hughd@google.com>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Andrea Arcangeli <aarcange@redhat.com>,
        David Rientjes <rientjes@google.com>,
        Dave Hansen <dave.hansen@intel.com>, Mel Gorman <mgorman@suse.de>,
        Rik van Riel <riel@redhat.com>, Vlastimil Babka <vbabka@suse.cz>,
        Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>,
        Steve Capper <steve.capper@linaro.org>,
        "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>,
        Johannes Weiner <hannes@cmpxchg.org>, Michal Hocko <mhocko@suse.cz>,
        Jerome Marchand <jmarchan@redhat.com>, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org
Subject: Re: page-flags behavior on compound pages: a worry
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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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