diff options
| author | Mel Gorman <mgorman@suse.de> | 2014-01-23 15:53:38 -0800 |
|---|---|---|
| committer | Linus Torvalds <torvalds@linux-foundation.org> | 2014-01-23 16:36:53 -0800 |
| commit | 6c14466cc00ff13121ae782d33d9df0fde20b124 (patch) | |
| tree | 65cf09a45f47de60638fdd1fe3140f9b8bdc27b9 /mm | |
| parent | 0eef615665ede1e0d603ea9ecca88c1da6f02234 (diff) | |
| download | lwn-6c14466cc00ff13121ae782d33d9df0fde20b124.tar.gz lwn-6c14466cc00ff13121ae782d33d9df0fde20b124.zip | |
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>
Diffstat (limited to 'mm')
| -rw-r--r-- | mm/compaction.c | 5 | ||||
| -rw-r--r-- | mm/internal.h | 8 |
2 files changed, 9 insertions, 4 deletions
diff --git a/mm/compaction.c b/mm/compaction.c index e0ab02d70f13..b48c5259ea33 100644 --- 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 --git a/mm/internal.h b/mm/internal.h index 7e145e8cd1e6..612c14f5e0f5 100644 --- a/mm/internal.h +++ b/mm/internal.h @@ -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) { |