linux-kernel - Re: [PATCH 06/19] mm: store mapcount for compound page separate

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Date:	Fri, 21 Nov 2014 12:11:34 +0530
From:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
To:	"Kirill A. Shutemov" <kirill@...temov.name>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
Cc:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Dave Hansen <dave.hansen@...el.com>,
	Hugh Dickins <hughd@...gle.com>, Mel Gorman <mgorman@...e.de>,
	Rik van Riel <riel@...hat.com>,
	Vlastimil Babka <vbabka@...e.cz>,
	Christoph Lameter <cl@...two.org>,
	Steve Capper <steve.capper@...aro.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Michal Hocko <mhocko@...e.cz>,
	"linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm\@kvack.org" <linux-mm@...ck.org>
Subject: Re: [PATCH 06/19] mm: store mapcount for compound page separate

"Kirill A. Shutemov" <kirill@...temov.name> writes:

> On Tue, Nov 18, 2014 at 08:43:00AM +0000, Naoya Horiguchi wrote:
>> > @@ -1837,6 +1839,9 @@ static void __split_huge_page_refcount(struct page *page,
>> >  	atomic_sub(tail_count, &page->_count);
>> >  	BUG_ON(atomic_read(&page->_count) <= 0);
>> >  
>> > +	page->_mapcount = *compound_mapcount_ptr(page);
>> 
>> Is atomic_set() necessary?
>
> Do you mean
> 	atomic_set(&page->_mapcount, atomic_read(compound_mapcount_ptr(page)));
> ?
>
> I don't see why we would need this. Simple assignment should work just
> fine. Or we have archs which will break?

Are you looking at architecture related atomic_set issues, or the fact
that we cannot have parallel _mapcount update and hence the above
assignment should be ok ? If the former, current thp code
use atomic_add instead of even using atomic_set when
updatinge page_tail->_count.  

		 * 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().
		 */
		atomic_add(page_mapcount(page) + page_mapcount(page_tail) + 1,
			   &page_tail->_count);



-aneesh

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