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From: "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>
To: "Kirill A. Shutemov" <kirill@shutemov.name>,
        Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Andrew Morton <akpm@linux-foundation.org>,
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Subject: Re: [PATCH 06/19] mm: store mapcount for compound page separate
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Date: Fri, 21 Nov 2014 12:11:34 +0530
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"Kirill A. Shutemov" <kirill@shutemov.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