Messages in this thread | | | Subject | Re: Suspicious error for CMA stress test | From | Vlastimil Babka <> | Date | Fri, 18 Mar 2016 21:58:35 +0100 |
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On 03/18/2016 03:42 PM, Lucas Stach wrote: > Am Freitag, den 18.03.2016, 15:10 +0100 schrieb Vlastimil Babka: >> On 03/17/2016 04:52 PM, Joonsoo Kim wrote: >> > 2016-03-18 0:43 GMT+09:00 Vlastimil Babka <vbabka@suse.cz>: >> >> OK, here it is. Hanjun can you please retest this, as I'm not sure if you had >> the same code due to the followup one-liner patches in the thread. Lucas, see if >> it helps with your issue as well. Laura and Joonsoo, please also test and review >> and check changelog if my perception of the problem is accurate :) >> > > This doesn't help for my case, as it is still trying to merge pages in > isolated ranges. It even tries extra hard at doing so. > > With concurrent isolation and frees going on this may lead to the start > page of the range to be isolated merging into an higher order buddy page > if it isn't already pageblock aligned, leading both test_pages_isolated > and isolate_freepages to fail on an otherwise perfectly fine range. > > What I am arguing is that if a page is freed into an isolated range we > should not try merge it with it's buddies at all, by setting max_order = > order. If the range is isolated because want to isolate freepages from > it, the work to do the merging is wasted, as isolate_freepages will > split higher order pages into order-0 pages again. > > If we already finished isolating freepages and are in the process of > undoing the isolation, we don't strictly need to do the merging in > __free_one_page, but can defer it to unset_migratetype_isolate, allowing > to simplify those code paths by disallowing any merging of isolated > pages at all.
Oh, I think understand now. Yeah, skipping merging for pages in isolated pageblocks might be a rather elegant solution. But still, we would have to check buddy's migratetype at order >= pageblock_order like my patch does, which is annoying. Because even without isolated merging, the buddy might have already had order>=pageblock_order when it was isolated.
So what if isolation also split existing buddies in the pageblock immediately when it sets the MIGRATETYPE_ISOLATE on the pageblock? Then we would have it guaranteed that there's no isolated buddy - a buddy candidate at order >= pageblock_order either has a smaller order (so it's not a buddy) or is not MIGRATE_ISOLATE so it's safe to merge with.
Does that make sense?
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