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SubjectRe: [PATCH 06/19] mm: store mapcount for compound page separate
Date
"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



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