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

[Joonsoo Kim]

On Wed, Sep 24, 2014 at 03:30:26PM +0200, Vlastimil Babka wrote:
> 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);
> >>>+
> >>>+skip_counting:
> >>
> >>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.

Okay. I will go this way. :)

> 
> 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?
> 

If alloc_contig_range() succeed, and later, free_contig_range() is
called for free, there would be no leaving unmerged buddies.
If we fail on alloc_contig_range(), we can get unmerged buddies, but,
that's rare case and it's not big matter because normally we don't
want to allocate page with MAXORDER-1. We mostly want to allocate page
with pageblock_order at maximum. After some split and merging of freepage,
freepage could be MAXORDER-1 page again so that's not real issue, IMO.

Thanks.
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