On Fri, Nov 21, 2014 at 12:11:34PM +0530, Aneesh Kumar K.V wrote: > "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); We don't have anything like get_page_unless_zero() for _mapcount as far as I can see. And we have similar assignment there now: page_tail->_mapcount = page->_mapcount; Anyway the assignment goes away by the end of patchset. -- Kirill A. Shutemov -- 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>