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@mina86.com 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;elsepage = pfn_to_page(pfn);On Mon, 12 Dec 2011 15:19:53 +0100, Mel Gorman mel@csn.ul.ie 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@csn.ul.ie 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