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mm: improve documentation of page_order

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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.

[[email protected]: tweaks]
[[email protected]: Corrected a page_zone(page)->lock reference]
Signed-off-by: Mel Gorman <[email protected]>
Acked-by: Rafael Aquini <[email protected]>
Acked-by: Minchan Kim <[email protected]>
Cc: Laura Abbott <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>
This commit is contained in:
Mel Gorman 2014-01-23 15:53:38 -08:00 • committed by Linus Torvalds
commit 6c14466cc0
2 changed files with 9 additions and 4 deletions
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5
mm/compaction.c
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@ -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;