2010/10/16 Christoph Lameter <cl@xxxxxxxxx>: > On Fri, 15 Oct 2010, KAMEZAWA Hiroyuki wrote: > >> But above is called all under pte_offset_map_lock(). >> get_page_unless_zero() #1 is not necessary because we do all under a >> pte_offset_map_lock(). > > The two (ptl and refcount) are entirely different. The ptl is for > protecting the page table. The refcount handles only the page. > > However, if the entry in the page table is pointing to the page then there > must have been a refcount taken on the page. So if you know that the page > is in the page table and you took the ptl then you can be sure that the > page refcount will not become zero. Therefore get_page_unless_zero() will > never fail and there is no need to take additional refcounts as long as > the page table lock is held and the page is not removed from the page > table. > Ok, thank you for explanation. I can make this function faster. >> Index: mmotm-1013/mm/vmscan.c >> =================================================================== >> --- mmotm-1013.orig/mm/vmscan.c >> +++ mmotm-1013/mm/vmscan.c >> @@ -1166,7 +1166,8 @@ static unsigned long clear_active_flags( >> * found will be decremented. >> * >> * Restrictions: >> - * (1) Must be called with an elevated refcount on the page. This is a >> + * (1) Must be called with an elevated refcount on the page, IOW, the >> + * caller must guarantee that there is a stable reference. This is a >> * fundamentnal difference from isolate_lru_pages (which is called >> * without a stable reference). >> * (2) the lru_lock must not be held. > > There is no need for this change since you have an elevated refcount. > IMH The words "stable reference" may be confusing since the refcount may > change. The elevated refcount protects against the freeing of the page. > Sure, drop change this in v2. I misunderstand "elevated refcount" means "extra get_page()". Thanks, -Kame -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxxx For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href