On Fri, Jul 31, 2015 at 09:25:13AM +0200, Vlastimil Babka wrote: > On 07/31/2015 09:11 AM, Mel Gorman wrote: > >On Fri, Jul 31, 2015 at 02:54:07PM +0900, Joonsoo Kim wrote: > >>Hello, Mel. > >> > >>On Mon, Jul 20, 2015 at 09:00:18AM +0100, Mel Gorman wrote: > >>>From: Mel Gorman <mgorman@xxxxxxx> > >>> > >>>High-order watermark checking exists for two reasons -- kswapd high-order > >>>awareness and protection for high-order atomic requests. Historically we > >>>depended on MIGRATE_RESERVE to preserve min_free_kbytes as high-order free > >>>pages for as long as possible. This patch introduces MIGRATE_HIGHATOMIC > >>>that reserves pageblocks for high-order atomic allocations. This is expected > >>>to be more reliable than MIGRATE_RESERVE was. > >> > >>I have some concerns on this patch. > >> > >>1) This patch breaks intention of __GFP_WAIT. > >>__GFP_WAIT is used when we want to succeed allocation even if we need > >>to do some reclaim/compaction work. That implies importance of > >>allocation success. But, reserved pageblock for MIGRATE_HIGHATOMIC makes > >>atomic allocation (~__GFP_WAIT) more successful than allocation with > >>__GFP_WAIT in many situation. It breaks basic assumption of gfp flags > >>and doesn't make any sense. > >> > > > >Currently allocation requests that do not specify __GFP_WAIT get the > >ALLOC_HARDER flag which allows them to dip further into watermark reserves. > >It already is the case that there are corner cases where a high atomic > >allocation can succeed when a non-atomic allocation would reclaim. > > I think (and said so before elsewhere) is that the problem is that > we don't currently distinguish allocations that can't wait (=are > really atomic and have no order-0 fallback) and allocations that > just don't want to wait (=they have fallbacks). The second ones > should obviously not access the current ALLOC_HARDER watermark-based > reserves nor the proposed highatomic reserves. Yes, I agree. If we distinguish such cases, I'm not sure that this kinds of reservation is needed. It is better that atomic high-order allocation in IRQ context should prepare their own fallback such as reserving memory or low-order allocation. > > Well we do look at __GFP_NO_KSWAPD flag to treat allocation as > non-atomic, so that covers THP allocations and two drivers. But the > recent networking commit fb05e7a89f50 didn't add the flag and nor > does Joonsoo's slub patch use it. Either we should rename the flag > and employ it where appropriate, or agree that access to reserves is > orthogonal concern to waking up kswapd, and distinguish non-atomic > non-__GFP_WAIT allocations differently. > > >>>A MIGRATE_HIGHORDER pageblock is created when an allocation request steals > >>>a pageblock but limits the total number to 10% of the zone. > >> > >>When steals happens, pageblock already can be fragmented and we can't > >>fully utilize this pageblock without allowing order-0 allocation. This > >>is very waste. > >> > > > >If the pageblock was stolen, it implies there was at least 1 usable page > >of the correct order. As the pageblock is then reserved, any pages that > >free in that block stay free for use by high-order atomic allocations. > >Else, the number of pageblocks will increase again until the 10% limit > >is hit. > > It's however true that many of the "any pages free in that block" > may be order-0, so they both won't be useful to high-order atomic > allocations, and won't be available to other allocations, so they > might remain unused. Agreed. Thanks. -- 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>