[PATCH] bcache: fix available bucket accounting in bucket allocation

[Robert Pang]

In bch_bucket_alloc(), a cache set's avail_nbuckets is decremented whenever a
bucket is allocated, assuming that the bucket becomes unavailable once
allocated. However, when a bucket is allocated but the GC mark is unset or
reclaimable, it is available still. This false counter decrement leads to an
inflated in-use measurement, which triggers cache bypasses once CUTOFF_CACHE_ADD
is reached, persisting until the next garbage collection cycle.

Fix this by ensuring avail_nbuckets is only decremented when the allocated
bucket is already marked and not in the GC_MARK_RECLAIMABLE state.

Signed-off-by: Robert Pang <robertpang@xxxxxxxxxx>
---
 drivers/md/bcache/alloc.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/md/bcache/alloc.c b/drivers/md/bcache/alloc.c
index 3ec240ec685d..2d0cb9b414ac 100644
--- a/drivers/md/bcache/alloc.c
+++ b/drivers/md/bcache/alloc.c
@@ -446,7 +446,8 @@ long bch_bucket_alloc(struct cache *ca, unsigned int reserve, bool wait)
 		b->prio = INITIAL_PRIO;
 	}
 
-	if (ca->set->avail_nbuckets > 0) {
+	if (ca->set->avail_nbuckets > 0 &&
+	    GC_MARK(b) && GC_MARK(b) != GC_MARK_RECLAIMABLE) {
 		ca->set->avail_nbuckets--;
 		bch_update_bucket_in_use(ca->set, &ca->set->gc_stats);
 	}
-- 
2.56.0.398.g882cbad275-goog