linux-kernel - RE: [PATCH 13/36] mm: drop tail page refcounting

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Message-Id: <20150714140135.81C368B@black.fi.intel.com>
Date:	Tue, 14 Jul 2015 17:01:35 +0300 (EEST)
From:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:	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>,
	Jerome Marchand <jmarchan@...hat.com>,
	Sasha Levin <sasha.levin@...cle.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: RE: [PATCH 13/36] mm: drop tail page refcounting

Kirill A. Shutemov wrote:
> Tail page refcounting is utterly complicated and painful to support.
> 
> It uses ->_mapcount on tail pages to store how many times this page is
> pinned. get_page() bumps ->_mapcount on tail page in addition to
> ->_count on head. This information is required by split_huge_page() to
> be able to distribute pins from head of compound page to tails during
> the split.
> 
> We will need ->_mapcount to account PTE mappings of subpages of the
> compound page. We eliminate need in current meaning of ->_mapcount in
> tail pages by forbidding split entirely if the page is pinned.
> 
> The only user of tail page refcounting is THP which is marked BROKEN for
> now.
> 
> Let's drop all this mess. It makes get_page() and put_page() much
> simpler.
> 
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> Tested-by: Sasha Levin <sasha.levin@...cle.com>
> Acked-by: Vlastimil Babka <vbabka@...e.cz>

checkpatch fixlet:

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 57fcb78a3cef..681997bccc52 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -493,7 +493,7 @@ static inline void init_page_count(struct page *page)
 	atomic_set(&page->_count, 1);
 }
 
-void __put_page(struct page* page);
+void __put_page(struct page *page);
 
 static inline void put_page(struct page *page)
 {
--
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