"Kirill A. Shutemov" <kirill@xxxxxxxxxxxxx> 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 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>