mm: improve documentation of page_order

Developers occasionally try and optimise PFN scanners by using
page_order but miss that in general it requires zone->lock.  This has
happened twice for compaction.c and rejected both times.  This patch
clarifies the documentation of page_order and adds a note to
compaction.c why page_order is not used.

[akpm@linux-foundation.org: tweaks]
[lauraa@codeaurora.org: Corrected a page_zone(page)->lock reference]
Signed-off-by: Mel Gorman <mgorman@suse.de>
Acked-by: Rafael Aquini <aquini@redhat.com>
Acked-by: Minchan Kim <minchan@kernel.org>
Cc: Laura Abbott <lauraa@codeaurora.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/compaction.c b/mm/compaction.c
index e0ab02d..b48c525 100644 (file)
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -523,7 +523,10 @@ isolate_migratepages_range(struct zone *zone, struct compact_control *cc,
                if (!isolation_suitable(cc, page))
                        goto next_pageblock;
 
-               /* Skip if free */
+               /*
+                * Skip if free. page_order cannot be used without zone->lock
+                * as nothing prevents parallel allocations or buddy merging.
+                */
                if (PageBuddy(page))
                        continue;
 

diff --cc mm/internal.h
Simple merge