linux-kernel - Re: [PATCHv6 32/36] thp: reintroduce split_huge

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Date:	Mon, 22 Jun 2015 14:28:00 +0300
From:	"Kirill A. Shutemov" <kirill@...temov.name>
To:	Vlastimil Babka <vbabka@...e.cz>
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>,
	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
Subject: Re: [PATCHv6 32/36] thp: reintroduce split_huge_page()

On Wed, Jun 10, 2015 at 05:44:30PM +0200, Vlastimil Babka wrote:
> On 06/03/2015 07:06 PM, Kirill A. Shutemov wrote:
> >+static int __split_huge_page_tail(struct page *head, int tail,
> >+		struct lruvec *lruvec, struct list_head *list)
> >+{
> >+	int mapcount;
> >+	struct page *page_tail = head + tail;
> >+
> >+	mapcount = page_mapcount(page_tail);
> 
> Isn't page_mapcount() unnecessarily heavyweight here? When you are splitting
> a page, it already should have zero compound_mapcount() and shouldn't be
> PageDoubleMap(), no? So you should care about page->_mapcount only? Sure,
> splitting THP is not a hotpath, but when done 512 times per split, it could
> make some difference in the split's latency.

Okay, replaced with direct atomic_read().

> >+	VM_BUG_ON_PAGE(atomic_read(&page_tail->_count) != 0, page_tail);
> >+
> >+	/*
> >+	 * tail_page->_count is zero and not changing from under us. But
> >+	 * get_page_unless_zero() may be running from under us on the
> >+	 * tail_page. If we used atomic_set() below instead of atomic_add(), we
> >+	 * would then run atomic_set() concurrently with
> >+	 * get_page_unless_zero(), and atomic_set() is implemented in C not
> >+	 * using locked ops. spin_unlock on x86 sometime uses locked ops
> >+	 * because of PPro errata 66, 92, so unless somebody can guarantee
> >+	 * atomic_set() here would be safe on all archs (and not only on x86),
> >+	 * it's safer to use atomic_add().
> 
> I would be surprised if this was the first place to use atomic_set() with
> potential concurrent atomic_add(). Shouldn't atomic_*() API guarantee that
> this works?

I don't have much insight on the issue. This part is carried over from
pre-rework split_huge_page().

> 
> >+	 */
> >+	atomic_add(page_mapcount(page_tail) + 1, &page_tail->_count);
> 
> You already have the value in mapcount variable, so why read it again.

Fixed.

-- 
 Kirill A. Shutemov
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