Re: [PATCH v3 04/13] mm, compaction: move pageblock checks up from isolate_migratepages_range()

[Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>]

On Tue, Jun 24, 2014 at 05:34:32PM +0200, Vlastimil Babka wrote:
> On 06/24/2014 06:52 AM, Naoya Horiguchi wrote:
> >>-	low_pfn = isolate_migratepages_range(zone, cc, low_pfn, end_pfn, false);
> >>-	if (!low_pfn || cc->contended)
> >>-		return ISOLATE_ABORT;
> >>+		/* Do not scan within a memory hole */
> >>+		if (!pfn_valid(low_pfn))
> >>+			continue;
> >>+
> >>+		page = pfn_to_page(low_pfn);
> >
> >Can we move (page_zone != zone) check here as isolate_freepages() does?
> 
> Duplicate perhaps, not sure about move.

Sorry for my unclearness.
I meant that we had better do this check in per-pageblock loop (as the free
scanner does) instead of in per-pfn loop (as we do now.)

> Does CMA make sure that all pages
> are in the same zone?

It seems not, CMA just specifies start pfn and end pfn, so it can cover
multiple zones.
And we also have a case of node overlapping as commented in commit dc9086004
"mm: compaction: check for overlapping nodes during isolation for migration".
So we need this check in compaction side.

Thanks,
Naoya Horiguchi

> Common sense tells me it would be useless otherwise,
> but I haven't checked if we can rely on it.

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