[PATCH v3 02/32] vmscan: take at least one pass with shrinkers

[Glauber Costa <glommer@parallels.com>]

In very low free kernel memory situations, it may be the case that we
have less objects to free than our initial batch size. If this is the
case, it is better to shrink those, and open space for the new workload
then to keep them and fail the new allocations.

More specifically, this happens because we encode this in a loop with
the condition: "while (total_scan >= batch_size)". So if we are in such
a case, we'll not even enter the loop.

This patch modifies turns it into a do () while {} loop, that will
guarantee that we scan it at least once, while keeping the behaviour
exactly the same for the cases in which total_scan > batch_size.

Signed-off-by: Glauber Costa <glommer@parallels.com>
Reviewed-by: Dave Chinner <david@fromorbit.com>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>
CC: "Theodore Ts'o" <tytso@mit.edu>
CC: Al Viro <viro@zeniv.linux.org.uk>
---
 mm/vmscan.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 88c5fed..fc6d45a 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -280,7 +280,7 @@ unsigned long shrink_slab(struct shrink_control *shrink,
 					nr_pages_scanned, lru_pages,
 					max_pass, delta, total_scan);
 
-		while (total_scan >= batch_size) {
+		do {
 			int nr_before;
 
 			nr_before = do_shrinker_shrink(shrinker, shrink, 0);
@@ -294,7 +294,7 @@ unsigned long shrink_slab(struct shrink_control *shrink,
 			total_scan -= batch_size;
 
 			cond_resched();
-		}
+		} while (total_scan >= batch_size);
 
 		/*
 		 * move the unused scan count back into the shrinker in a
-- 
1.8.1.4

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