Re: [PATCH v2 2/2] mm/compaction: avoid rescanning pageblocks in isolate_freepages

Vlastimil Babka
2014-05-19T10:15:04Z

I wonder why nobody complained about the build warning... sorry.

----8<----
From: Vlastimil Babka <vbabka@suse.cz>
Date: Mon, 19 May 2014 12:02:38 +0200
Subject: mm-compaction-avoid-rescanning-pageblocks-in-isolate_freepages-fix
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Fix a (spurious) build warning:

mm/compaction.c:860:15: warning: ‘next_free_pfn’ may be used uninitialized in this function [-Wmaybe-uninitialized]

Seems like the compiler cannot prove that exiting the for loop without updating
next_free_pfn there will mean that the check for crossing the scanners will
trigger. So let's not confuse people who try to see why this warning occurs.

Instead of initializing next_free_pfn to zero with an explaining comment, just
drop the damned variable altogether and work with cc->free_pfn directly as
Nayoa originally suggested.

Suggested-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
---
mm/compaction.c | 6 ++----
1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/mm/compaction.c b/mm/compaction.c
index 8db9820..b0f939b 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -760,7 +760,6 @@ static void isolate_freepages(struct zone *zone,
unsigned long block_start_pfn; /* start of current pageblock */
unsigned long block_end_pfn; /* end of current pageblock */
unsigned long low_pfn; /* lowest pfn scanner is able to scan */
- unsigned long next_free_pfn; /* start pfn for scaning at next round */
int nr_freepages = cc->nr_freepages;
struct list_head *freelist = &cc->freepages;

@@ -822,7 +821,7 @@ static void isolate_freepages(struct zone *zone,
continue;

/* Found a block suitable for isolating free pages from */
- next_free_pfn = block_start_pfn;
+ cc->free_pfn = block_start_pfn;
isolated = isolate_freepages_block(cc, block_start_pfn,
block_end_pfn, freelist, false);
nr_freepages += isolated;
@@ -852,9 +851,8 @@ static void isolate_freepages(struct zone *zone,
* so that compact_finished() may detect this
*/
if (block_start_pfn < low_pfn)
- next_free_pfn = cc->migrate_pfn;
+ cc->free_pfn = cc->migrate_pfn;

- cc->free_pfn = next_free_pfn;
cc->nr_freepages = nr_freepages;
}

--
1.8.4.5