[PATCH 07/34] vmscan: shrinker->nr updates race and go wrong

Mel Gorman
2012-07-23T13:39:08Z

From: Dave Chinner <dchinner@redhat.com>

commit acf92b485cccf028177f46918e045c0c4e80ee10 upstream.

Stable note: Not tracked in Bugzilla. This patch reduces excessive
reclaim of slab objects reducing the amount of information
that has to be brought back in from disk.

shrink_slab() allows shrinkers to be called in parallel so the
struct shrinker can be updated concurrently. It does not provide any
exclusion for such updates, so we can get the shrinker->nr value
increasing or decreasing incorrectly.

As a result, when a shrinker repeatedly returns a value of -1 (e.g.
a VFS shrinker called w/ GFP_NOFS), the shrinker->nr goes haywire,
sometimes updating with the scan count that wasn't used, sometimes
losing it altogether. Worse is when a shrinker does work and that
update is lost due to racy updates, which means the shrinker will do
the work again!

Fix this by making the total_scan calculations independent of
shrinker->nr, and making the shrinker->nr updates atomic w.r.t. to
other updates via cmpxchg loops.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Mel Gorman <mgorman@suse.de>
---
mm/vmscan.c | 45 ++++++++++++++++++++++++++++++++-------------
1 file changed, 32 insertions(+), 13 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index d875058..31b551e 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -251,17 +251,29 @@ unsigned long shrink_slab(struct shrink_control *shrink,
unsigned long total_scan;
unsigned long max_pass;
int shrink_ret = 0;
+ long nr;
+ long new_nr;

+ /*
+ * copy the current shrinker scan count into a local variable
+ * and zero it so that other concurrent shrinker invocations
+ * don't also do this scanning work.
+ */
+ do {
+ nr = shrinker->nr;
+ } 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);
- shrinker->nr += delta;
- if (shrinker->nr < 0) {
+ total_scan += delta;
+ if (total_scan < 0) {
printk(KERN_ERR "shrink_slab: %pF negative objects to "
"delete nr=%ld\n",
- shrinker->shrink, shrinker->nr);
- shrinker->nr = max_pass;
+ shrinker->shrink, total_scan);
+ total_scan = max_pass;
}

/*
@@ -269,13 +281,10 @@ unsigned long shrink_slab(struct shrink_control *shrink,
* never try to free more than twice the estimate number of
* freeable entries.
*/
- if (shrinker->nr > max_pass * 2)
- shrinker->nr = max_pass * 2;
-
- total_scan = shrinker->nr;
- shrinker->nr = 0;
+ if (total_scan > max_pass * 2)
+ total_scan = max_pass * 2;

- trace_mm_shrink_slab_start(shrinker, shrink, total_scan,
+ trace_mm_shrink_slab_start(shrinker, shrink, nr,
nr_pages_scanned, lru_pages,
max_pass, delta, total_scan);

@@ -296,9 +305,19 @@ unsigned long shrink_slab(struct shrink_control *shrink,
cond_resched();
}

- shrinker->nr += total_scan;
- trace_mm_shrink_slab_end(shrinker, shrink_ret, total_scan,
- shrinker->nr);
+ /*
+ * move the unused scan count back into the shrinker in a
+ * manner that handles concurrent updates. If we exhausted the
+ * scan, there is no need to do an update.
+ */
+ do {
+ nr = shrinker->nr;
+ new_nr = total_scan + nr;
+ if (total_scan <= 0)
+ break;
+ } while (cmpxchg(&shrinker->nr, nr, new_nr) != nr);
+
+ trace_mm_shrink_slab_end(shrinker, shrink_ret, nr, new_nr);
}
up_read(&shrinker_rwsem);
out:
--
1.7.9.2