Re: [PATCH] mm: Improve documentation of page_order

[Mel Gorman]

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
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