[PATCH v2 2/3] nfsd: only keep unused entries on the LRU

[Jeff Layton <jlayton@kernel.org>]

Currently, nfsd_files live on the LRU once they are added until they are
unhashed. There's no need to keep ones that are actively in use there.

Before incrementing the refcount, do a lockless check for nf_lru being
empty. If it's not then attempt to remove the entry from the LRU. If
that's successful, claim the LRU reference and return it. If the removal
fails (or if the list_head was empty), then just increment the counter
as we normally would.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
---
 fs/nfsd/filecache.c | 23 ++++++++++++++++++++---
 1 file changed, 20 insertions(+), 3 deletions(-)

diff --git a/fs/nfsd/filecache.c b/fs/nfsd/filecache.c
index e63534f4b9f8..d2bbded805d4 100644
--- a/fs/nfsd/filecache.c
+++ b/fs/nfsd/filecache.c
@@ -420,14 +420,31 @@ nfsd_file_unhash(struct nfsd_file *nf)
 	return false;
 }
 
-struct nfsd_file *
-nfsd_file_get(struct nfsd_file *nf)
+static struct nfsd_file *
+__nfsd_file_get(struct nfsd_file *nf)
 {
 	if (likely(refcount_inc_not_zero(&nf->nf_ref)))
 		return nf;
 	return NULL;
 }
 
+struct nfsd_file *
+nfsd_file_get(struct nfsd_file *nf)
+{
+	/*
+	 * Do a lockless list_empty check first, before attempting to
+	 * remove it, so we can avoid the spinlock when it's not on the
+	 * list.
+	 *
+	 * If we successfully remove it from the LRU, then we can just
+	 * claim the LRU reference and return it. Otherwise, we need to
+	 * bump the counter the old-fashioned way.
+	 */
+	if (!list_empty(&nf->nf_lru) && nfsd_file_lru_remove(nf))
+		return nf;
+	return __nfsd_file_get(nf);
+}
+
 /**
  * nfsd_file_unhash_and_queue - unhash a file and queue it to the dispose list
  * @nf: nfsd_file to be unhashed and queued
@@ -449,7 +466,7 @@ nfsd_file_unhash_and_queue(struct nfsd_file *nf, struct list_head *dispose)
 		 * to take a reference. If that fails, just ignore
 		 * the file altogether.
 		 */
-		if (!nfsd_file_lru_remove(nf) && !nfsd_file_get(nf))
+		if (!nfsd_file_lru_remove(nf) && !__nfsd_file_get(nf))
 			return false;
 		list_add(&nf->nf_lru, dispose);
 		return true;
-- 
2.37.3