On 03/18/2016 03:42 PM, Lucas Stach wrote:

Am Freitag, den 18.03.2016, 15:10 +0100 schrieb Vlastimil Babka:

quoted

On 03/17/2016 04:52 PM, Joonsoo Kim wrote:

quoted

2016-03-18 0:43 GMT+09:00 Vlastimil Babka [off-list ref]:

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?