linux-kernel - Re: [PATCHv6 00/36] THP refcounting redesign

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Date:	Mon, 22 Jun 2015 16:21:25 +0300
From:	"Kirill A. Shutemov" <kirill@...temov.name>
To:	Jerome Marchand <jmarchan@...hat.com>
Cc:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Hugh Dickins <hughd@...gle.com>,
	Dave Hansen <dave.hansen@...el.com>,
	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	Vlastimil Babka <vbabka@...e.cz>,
	Christoph Lameter <cl@...two.org>,
	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>,
	Sasha Levin <sasha.levin@...cle.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCHv6 00/36] THP refcounting redesign

On Tue, Jun 16, 2015 at 03:17:13PM +0200, Jerome Marchand wrote:
> On 06/03/2015 07:05 PM, Kirill A. Shutemov wrote:
> > Hello everybody,
> > 
> > Here's new revision of refcounting patchset. Please review and consider
> > applying.
> > 
> > The goal of patchset is to make refcounting on THP pages cheaper with
> > simpler semantics and allow the same THP compound page to be mapped with
> > PMD and PTEs. This is required to get reasonable THP-pagecache
> > implementation.
> > 
> > With the new refcounting design it's much easier to protect against
> > split_huge_page(): simple reference on a page will make you the deal.
> > It makes gup_fast() implementation simpler and doesn't require
> > special-case in futex code to handle tail THP pages.
> > 
> > It should improve THP utilization over the system since splitting THP in
> > one process doesn't necessary lead to splitting the page in all other
> > processes have the page mapped.
> > 
> > The patchset drastically lower complexity of get_page()/put_page()
> > codepaths. I encourage people look on this code before-and-after to
> > justify time budget on reviewing this patchset.
> > 
> > = Changelog =
> > 
> > v6:
> >   - rebase to since-4.0;
> >   - optimize mapcount handling: significantely reduce overhead for most
> >     common cases.
> >   - split pages on migrate_pages();
> >   - remove infrastructure for handling splitting PMDs on all architectures;
> >   - fix page_mapcount() for hugetlb pages;
> > 
> 
> Hi Kirill,
> 
> I ran some LTP mm tests and hugemmap tests trigger the following:
> 
> [  438.749457] page:ffffea0000df8000 count:2 mapcount:0 mapping:          (null) index:0x0 compound_mapcount: 0
> [  438.750089] flags: 0x3ffc0000004001(locked|head)
> [  438.750089] page dumped because: VM_BUG_ON_PAGE(page_mapped(page))

Did you run with original or updated version of patch 27/36?
In original post of v6 there was bug: page_mapped() always returned true.

-- 
 Kirill A. Shutemov
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