Messages in this thread Patch in this message | | | Date | Tue, 21 Jan 2014 11:05:15 +0000 | From | Mel Gorman <> | Subject | [PATCH] mm: Improve documentation of page_order v2 |
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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.
[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> --- 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)->lock, they must guarantee that the + * page cannot be allocated or merged in parallel. */ static inline unsigned long page_order(struct page *page) {
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