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Date: Fri, 17 Jan 2014 10:53:50 -0800 From: Laura Abbott <lauraa@...eaurora.org> To: Mel Gorman <mgorman@...e.de>, Andrew Morton <akpm@...ux-foundation.org> CC: Minchan Kim <minchan@...nel.org>, Xishi Qiu <qiuxishi@...wei.com>, riel@...hat.com, aquini@...hat.com, linux-mm@...ck.org, LKML <linux-kernel@...r.kernel.org> Subject: Re: [PATCH] mm: Improve documentation of page_order On 1/17/2014 6:32 AM, Mel Gorman wrote: > 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. > > Signed-off-by: Mel Gorman <mgorman@...e.de> > --- > 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), they must guarantee that page_zone(page)->lock here? > + * the page cannot be allocated or merged in parallel. > */ > static inline unsigned long page_order(struct page *page) > { > > -- > To unsubscribe, send a message with 'unsubscribe linux-mm' in > the body to majordomo@...ck.org. For more info on Linux MM, > see: http://www.linux-mm.org/ . > Don't email: <a href=mailto:"dont@...ck.org"> email@...ck.org </a> > -- Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, hosted by The Linux Foundation -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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