On Tue 31-05-11 00:16:33, Minchan Kim wrote:
> > /* Direct compactor: Is a suitable page free? */
> > for (order = cc->order; order < MAX_ORDER; order++) {
> > /* Job done if page is free of the right migratetype */
>
> It looks good to me.
> Let's think about another place, compaction_suitable.
Good spotted.
> It has same problem so we can move the check right before zone_watermark_ok.
> As I look it more, I thought we need free pages for compaction so we would
> be better to give up early if we can't get enough free pages. But I changed
> my mind. It's a totally user request and we can get free pages in migration
> progress(ex, other big memory hogger might free his big rss).
> So my conclusion is that we should do *best effort* than early give up.
Agreed
> If you agree with me, how about resending patch with compaction_suitable fix?
Here we go. Thanks
---
mm: compaction: fix special case -1 order checks
56de7263 (mm: compaction: direct compact when a high-order allocation
fails) introduced a check for cc->order == -1 in compact_finished. We
should continue compacting in that case because the request came from
userspace and there is no particular order to compact for.
Similar check has been added by 82478fb7 (mm: compaction:
prevent division-by-zero during user-requested compaction) for
compaction_suitable.
The check is, however, done after zone_watermark_ok which uses order as
a right hand argument for shifts. Not only watermark check is pointless
if we can break out without it but it also uses 1 << -1 which is not
well defined (at least from C standard). Let's move the -1 check above
zone_watermark_ok.
[Minchan Kim <minchan.kim@xxxxxxxxx> - caught compaction_suitable]
Signed-off-by: Michal Hocko <mhocko@xxxxxxx>
Cc: Mel Gorman <mgorman@xxxxxxx>
---
compaction.c | 14 +++++++-------
1 file changed, 7 insertions(+), 7 deletions(-)
Index: linus_tree/mm/compaction.c
===================================================================
--- linus_tree.orig/mm/compaction.c 2011-05-30 14:19:58.000000000 +0200
+++ linus_tree/mm/compaction.c 2011-05-30 17:16:02.000000000 +0200
@@ -420,13 +420,6 @@ static int compact_finished(struct zone
if (cc->free_pfn <= cc->migrate_pfn)
return COMPACT_COMPLETE;
- /* Compaction run is not finished if the watermark is not met */
- watermark = low_wmark_pages(zone);
- watermark += (1 << cc->order);
-
- if (!zone_watermark_ok(zone, cc->order, watermark, 0, 0))
- return COMPACT_CONTINUE;
-
/*
* order == -1 is expected when compacting via
* /proc/sys/vm/compact_memory
@@ -434,6 +427,13 @@ static int compact_finished(struct zone
if (cc->order == -1)
return COMPACT_CONTINUE;
+ /* Compaction run is not finished if the watermark is not met */
+ watermark = low_wmark_pages(zone);
+ watermark += (1 << cc->order);
+
+ if (!zone_watermark_ok(zone, cc->order, watermark, 0, 0))
+ return COMPACT_CONTINUE;
+
/* Direct compactor: Is a suitable page free? */
for (order = cc->order; order < MAX_ORDER; order++) {
/* Job done if page is free of the right migratetype */
@@ -461,6 +461,13 @@ unsigned long compaction_suitable(struct
unsigned long watermark;
/*
+ * order == -1 is expected when compacting via
+ * /proc/sys/vm/compact_memory
+ */
+ if (order == -1)
+ return COMPACT_CONTINUE;
+
+ /*
* Watermarks for order-0 must be met for compaction. Note the 2UL.
* This is because during migration, copies of pages need to be
* allocated and for a short time, the footprint is higher
@@ -470,13 +477,6 @@ unsigned long compaction_suitable(struct
return COMPACT_SKIPPED;
/*
- * order == -1 is expected when compacting via
- * /proc/sys/vm/compact_memory
- */
- if (order == -1)
- return COMPACT_CONTINUE;
-
- /*
* fragmentation index determines if allocation failures are due to
* low memory or external fragmentation
*
--
Michal Hocko
SUSE Labs
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9
Czech Republic
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