Re: [RFC PATCH v3 1/4] mm/page_alloc: fix incorrect isolation behavior by rechecking migratetype

From: Vlastimil Babka
Date: Wed Sep 24 2014 - 09:30:40 EST

On 09/15/2014 04:31 AM, Joonsoo Kim wrote:
On Mon, Sep 08, 2014 at 10:31:29AM +0200, Vlastimil Babka wrote:
On 08/26/2014 10:08 AM, Joonsoo Kim wrote:

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index f86023b..51e0d13 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -740,9 +740,15 @@ static void free_one_page(struct zone *zone,
if (nr_scanned)
__mod_zone_page_state(zone, NR_PAGES_SCANNED, -nr_scanned);

+ if (unlikely(has_isolate_pageblock(zone))) {
+ migratetype = get_pfnblock_migratetype(page, pfn);
+ if (is_migrate_isolate(migratetype))
+ goto skip_counting;
+ }
+ __mod_zone_freepage_state(zone, 1 << order, migratetype);

Here, wouldn't a simple 'else __mod_zone_freepage_state...' look
better than goto + label? (same for the following 2 patches). Or
does that generate worse code?

To remove goto label, we need two __mod_zone_freepage_state() like
as below. On my system, it doesn't generate worse code, but, I am not
sure that this is true if more logic would be added. I think that
goto + label is better.

Oh right, I missed that. It's a bit subtle, but I don't see a nicer solution right now.

+ if (unlikely(has_isolate_pageblock(zone))) {
+ migratetype = get_pfnblock_migratetype(page, pfn);
+ if (!is_migrate_isolate(migratetype))
+ __mod_zone_freepage_state(zone, 1 << order, migratetype);
+ } else {
+ __mod_zone_freepage_state(zone, 1 << order, migratetype);

Yeah that would be uglier I guess.

Anyway, What do you think which one is better, either v2 or v3? Still, v3? :)

Yeah v3 is much better than v1 was, and better for backporting than v2. The changelogs also look quite clear. The overhead shouldn't be bad with the per-zone flag guarding get_pfnblock_migratetype.

I'm just not sure about patch 4 and potentially leaving unmerged budies behind. How would it look if instead we made sure isolation works on whole MAX_ORDER blocks instead?



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