vmscan: fix initial shrinker size handling

A shrinker function can return -1, means that it cannot do anything
without a risk of deadlock.  For example prune_super() does this if it
cannot grab a superblock refrence, even if nr_to_scan=0.  Currently we
interpret this -1 as a ULONG_MAX size shrinker and evaluate `total_scan'
according to this.  So the next time around this shrinker can cause
really big pressure.  Let's skip such shrinkers instead.

Also make total_scan signed, otherwise the check (total_scan < 0) below
never works.

Signed-off-by: Konstantin Khlebnikov <khlebnikov@openvz.org>
Cc: Dave Chinner <david@fromorbit.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 6 insertions, 3 deletions

diff --git a/mm/vmscan.c b/mm/vmscan.c
index a1893c05079..f5255442ae2 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -247,14 +247,18 @@ unsigned long shrink_slab(struct shrink_control *shrink,
 
         list_for_each_entry(shrinker, &shrinker_list, list) {
                 unsigned long long delta;
-                unsigned long total_scan;
-                unsigned long max_pass;
+                long total_scan;
+                long max_pass;
                 int shrink_ret = 0;
                 long nr;
                 long new_nr;
                 long batch_size = shrinker->batch ? shrinker->batch
                                                   : SHRINK_BATCH;
 
+                max_pass = do_shrinker_shrink(shrinker, shrink, 0);
+                if (max_pass <= 0)
+                        continue;
+
                 /*
                  * copy the current shrinker scan count into a local variable
                  * and zero it so that other concurrent shrinker invocations
@@ -265,7 +269,6 @@ unsigned long shrink_slab(struct shrink_control *shrink,
                 } while (cmpxchg(&shrinker->nr, nr, 0) != nr);
 
                 total_scan = nr;
-                max_pass = do_shrinker_shrink(shrinker, shrink, 0);
                 delta = (4 * nr_pages_scanned) / shrinker->seeks;
                 delta *= max_pass;
                 do_div(delta, lru_pages + 1);