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Commit 6c14466c authored by Mel Gorman's avatar Mel Gorman Committed by Linus Torvalds
Browse files

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: default avatarMel Gorman <mgorman@suse.de>
Acked-by: default avatarRafael Aquini <aquini@redhat.com>
Acked-by: default avatarMinchan Kim <minchan@kernel.org>
Cc: Laura Abbott <lauraa@codeaurora.org>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 0eef6156

mm/compaction.c

+4 −1
Original line number Diff line number Diff line
@@ -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;

mm/internal.h

+5 −3
Original line number Diff line number Diff line
@@ -143,9 +143,11 @@ 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 function 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 from being allocated in parallel and returning garbage as the order.

 * If a caller does not hold page_zone(page)->lock, it must guarantee that the

 * page cannot be allocated or merged in parallel.

 */

static inline unsigned long page_order(struct page *page)

{