On Tue, Jun 19, 2012 at 01:28:50PM +0200, Bartlomiej Zolnierkiewicz wrote: > On Tuesday 19 June 2012 11:00:15 Mel Gorman wrote: > > On Wed, May 23, 2012 at 09:22:00AM +0200, Bartlomiej Zolnierkiewicz wrote: > > > From: Bartlomiej Zolnierkiewicz <b.zolnierkie@xxxxxxxxxxx> > > > Subject: [PATCH] cma: cached pageblock type fixup > > > > > > CMA pages added to per-cpu pages lists in free_hot_cold_page() > > > have private field set to MIGRATE_CMA pageblock type . If this > > > happes just before start_isolate_page_range() in alloc_contig_range() > > > changes pageblock type of the page to MIGRATE_ISOLATE it may result > > > in the cached pageblock type being stale in free_pcppages_bulk() > > > (which may be triggered by drain_all_pages() in alloc_contig_range()), > > > > So what? > > "page being added to MIGRATE_CMA free list instead of MIGRATE_ISOLATE > one in __free_one_page() and (if the page is reused just before > test_pages_isolated() check) causing alloc_contig_range() failure." > And.... so what? If it's on the wrong free list then it can still be isolated from that list if it has not been allocated. If it has been allocated then it must be a MIGRATE_CMA-compatible allocation so the page can be migrated and the operation retried. > > The pages get freed to the MIGRATE_CMA region. At worst they will be > > used for an allocation request that is compatible with being migrated by > > CMA. This will delay the allocation time of alloc_contig_range() but is > > hardly critical. > > There is no waiting on MIGRATE_CMA pages in alloc_contig_range() to > become free again. If the conversion to MIGRATE_ISOLATE fails then > the whole alloc_contig_range() allocation fails on test_pages_isolated() > check. > Then retry the operation if alloc_contig_range() finds it raced. > > Your fix on the other hand adds another call to get_pageblock_type() to > > free_pcppages_bulk which is expensive. The change made to > > buffered_rmqueue() is horrific. It takes the per-cpu page allocation > > path and adds a spin lock to it which completely defeats the purpose of > > having the per-cpu allocation avoid taking locks. This will have a very > > heavy impact on performance, particularly on parallel workloads. > > > > As the impact of the race should be marginal and the cost of the fix is > > so unbelivably high I'm nacking this patch. If this race is a problem then > > it should be handled in alloc_contig_range() not in the allocator fast paths. > > Do you have any idea how this race can be handled in alloc_contig_range()? > If the page is on the wrong free list, just isolate it or move it to the MIGRATE_ISOLATE free list at that point. If it has been allocated then migrate it and move the resulting free page to the MIGRATE_ISOLATE list. -- 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/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>