On Fri, Jan 17, 2014 at 10:53:50AM -0800, Laura Abbott wrote: > On 1/17/2014 6:32 AM, Mel Gorman wrote: > >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. > > > >Signed-off-by: Mel Gorman <mgorman@xxxxxxx> > >--- > > mm/compaction.c | 5 ++++- > > mm/internal.h | 8 +++++--- > > 2 files changed, 9 insertions(+), 4 deletions(-) > > > >diff --git a/mm/compaction.c b/mm/compaction.c > >index f58bcd0..f91d26b 100644 > >--- a/mm/compaction.c > >+++ b/mm/compaction.c > >@@ -522,7 +522,10 @@ 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 --git a/mm/internal.h b/mm/internal.h > >index 684f7aa..09cd8be 100644 > >--- a/mm/internal.h > >+++ b/mm/internal.h > >@@ -144,9 +144,11 @@ isolate_migratepages_range(struct zone *zone, struct compact_control *cc, > > #endif > > > > /* > >- * function for dealing with page's order in buddy system. > >- * zone->lock is already acquired when we use these. > >- * So, we don't need atomic page->flags operations here. > >+ * This functions returns the order of a free page in the buddy system. > >+ * In general, page_zone(page)->lock must be held by the caller to prevent > >+ * the page being allocated in parallel and returning garbage as the order. > >+ * If the caller does not hold page_zone(page), they must guarantee that > page_zone(page)->lock here? *sigh* Yes, thanks. -- 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/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>