On Thu, Feb 02, 2012 at 08:53:25PM +0100, Michal Nazarewicz wrote: > >On Tue, Jan 31, 2012 at 05:23:59PM +0100, Marek Szyprowski wrote: > >>Pages, which have incorrect migrate type on free finally > >>causes pageblock migration type change from MIGRATE_CMA to MIGRATE_MOVABLE. > > On Thu, 02 Feb 2012 13:47:29 +0100, Mel Gorman <mel@xxxxxxxxx> wrote: > >I'm not quite seeing this. In free_hot_cold_page(), the pageblock > >type is checked so the page private should be set to MIGRATE_CMA or > >MIGRATE_ISOLATE for the CMA area. It's not clear how this can change a > >pageblock to MIGRATE_MOVABLE in error. > > Here's what I think may happen: > > When drain_all_pages() is called, __free_one_page() is called for each page on > pcp list with migrate type deducted from page_private() which is MIGRATE_CMA. > This result in the page being put on MIGRATE_CMA freelist even though its > pageblock's migrate type is MIGRATE_ISOLATE. > Ok, although it will only be allocated for MIGRATE_CMA-compatible requests so it is not a disaster. > When allocation happens and pcp list is empty, rmqueue_bulk() will get executed > with migratetype argument set to MIGRATE_MOVABLE. It calls __rmqueue() to grab > some pages and because the page described above is on MIGRATE_CMA freelist it > may be returned back to rmqueue_bulk(). > This will allocate the page from a pageblock we are trying to isolate pages from, but only for a movable page that can still be migrated. It does mean that CMA is doing more work than it should of course and the problem also impacts memory hot-remove. It's worse for memory hot-remove because potentially an UNMOVABLE page was allocated from a MIGRATE_ISOLATE pageblock. > But, pageblock's migrate type is not MIGRATE_CMA but MIGRATE_ISOLATE, so the > following code: > > #ifdef CONFIG_CMA > if (is_pageblock_cma(page)) > set_page_private(page, MIGRATE_CMA); > else > #endif > set_page_private(page, migratetype); > > will set it's private to MIGRATE_MOVABLE and in the end the page lands back > on MIGRATE_MOVABLE pcp list but this time with page_private == MIGRATE_MOVABLE > and not MIGRATE_CMA. > > One more drain_all_pages() (which may happen since alloc_contig_range() calls > set_migratetype_isolate() for each block) and next __rmqueue_fallback() may > convert the whole pageblock to MIGRATE_MOVABLE. > > I know, this sounds crazy and improbable, but I couldn't find an easier path > to destruction. As you pointed, once the page is allocated, free_hot_cold_page() > will do the right thing by reading pageblock's migrate type. > Ok, it's crazy but the problem is there. > Marek is currently experimenting with various patches including the following > change: > > #ifdef CONFIG_CMA > int mt = get_pageblock_migratetype(page); > if (is_migrate_cma(mt) || mt == MIGRATE_ISOLATE) > set_page_private(page, mt); > else > #endif > set_page_private(page, migratetype); > > As a matter of fact, if __rmqueue() was changed to return migrate type of the > freelist it took page from, we could avoid this get_pageblock_migratetype() all > together. For now, however, I'd rather not go that way just yet -- I'll be happy > to dig into it once CMA gets merged. > Ok, thanks for persisting with this. -- Mel Gorman SUSE Labs -- 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/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>