On Mon, Dec 12, 2011 at 03:51:55PM +0100, Michal Nazarewicz wrote: > >On Fri, Nov 18, 2011 at 05:43:10PM +0100, Marek Szyprowski wrote: > >>From: Michal Nazarewicz <mina86@xxxxxxxxxx> > >>diff --git a/mm/compaction.c b/mm/compaction.c > >>index 6afae0e..09c9702 100644 > >>--- a/mm/compaction.c > >>+++ b/mm/compaction.c > >>@@ -111,7 +111,10 @@ skip: > >> > >> next: > >> pfn += isolated; > >>- page += isolated; > >>+ if (zone_pfn_same_memmap(pfn - isolated, pfn)) > >>+ page += isolated; > >>+ else > >>+ page = pfn_to_page(pfn); > >> } > > On Mon, 12 Dec 2011 15:19:53 +0100, Mel Gorman <mel@xxxxxxxxx> wrote: > >Is this necessary? > > > >We are isolating pages, the largest of which is a MAX_ORDER_NR_PAGES > >page. [...] > > On Mon, 12 Dec 2011 15:40:30 +0100, Mel Gorman <mel@xxxxxxxxx> wrote: > >To be clear, I'm referring to a single page being isolated here. It may > >or may not be a high-order page but it's still going to be less then > >MAX_ORDER_NR_PAGES so you should be able check when a new block is > >entered and pfn_to_page is necessary. > > Do you mean something like: > > if (same pageblock) > just do arithmetic; > else > use pfn_to_page; > something like the following untested snippet. /* * Resolve pfn_to_page every MAX_ORDER_NR_PAGES to handle the case where * memmap is not contiguous such as with SPARSEMEM memory model without * VMEMMAP */ pfn += isolated; page += isolated; if ((pfn & ~(MAX_ORDER_NR_PAGES-1)) == 0) page = pfn_to_page(pfn); That would be closer to what other PFN walkers do > ? > > I've discussed it with Dave and he suggested that approach as an > optimisation since in some configurations zone_pfn_same_memmap() > is always true thus compiler will strip the else part, whereas > same pageblock test will be false on occasions regardless of kernel > configuration. > Ok, while I recognise it's an optimisation, it's a very small optimisation and I'm not keen on introducing something new for CMA that has been coped with in the past by always walking PFNs in pageblock-sized ranges with pfn_valid checks where necessary. See setup_zone_migrate_reserve as one example where pfn_to_page is only called once per pageblock and calls pageblock_is_reserved() for examining pages within a pageblock. Still, if you really want the helper, at least keep it in compaction.c as there should be no need to have it in mmzone.h -- Mel Gorman SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>