mm: improve documentation of page_order (6c14466c) · Commits · 戴 / test

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>

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)
		{

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