From 36c9ea9837c1cb21c778781495101eaff7e5eb56 Mon Sep 17 00:00:00 2001 From: Kent Overstreet Date: Mon, 3 Jun 2013 13:04:56 -0700 Subject: [PATCH] bcache: Document shrinker reserve better Signed-off-by: Kent Overstreet --- drivers/md/bcache/btree.c | 7 +++++++ 1 file changed, 7 insertions(+) diff --git a/drivers/md/bcache/btree.c b/drivers/md/bcache/btree.c index a6ad49ac5f2b..04636a70ffaa 100644 --- a/drivers/md/bcache/btree.c +++ b/drivers/md/bcache/btree.c @@ -622,6 +622,13 @@ static int bch_mca_shrink(struct shrinker *shrink, struct shrink_control *sc) else if (!mutex_trylock(&c->bucket_lock)) return -1; + /* + * It's _really_ critical that we don't free too many btree nodes - we + * have to always leave ourselves a reserve. The reserve is how we + * guarantee that allocating memory for a new btree node can always + * succeed, so that inserting keys into the btree can always succeed and + * IO can always make forward progress: + */ nr /= c->btree_pages; nr = min_t(unsigned long, nr, mca_can_free(c));