[PATCH 5/6] nfsd: filecache: don't repeatedly add/remove files on the lru list

[NeilBrown <neilb@suse.de>]

There is no need to remove a file from the lru every time we access it,
and then add it back.  It is sufficient to set the REFERENCED flag every
time we put the file.  The order in the lru of REFERENCED files is
largely irrelevant as they will all be moved to the end.

With this patch, files are only added when they are allocated (if
want_gc) and they are only removed by the list_lru_(shrink_)walk
callback or when forcibly removing a file.

This should reduce contention on the list_lru spinlock(s) and reduce
memory traffic a little.

Signed-off-by: NeilBrown <neilb@suse.de>
---
 fs/nfsd/filecache.c | 51 +++++++++++++++------------------------------
 1 file changed, 17 insertions(+), 34 deletions(-)

diff --git a/fs/nfsd/filecache.c b/fs/nfsd/filecache.c
index 9faf469354a5..d1ce0bc86ff7 100644
--- a/fs/nfsd/filecache.c
+++ b/fs/nfsd/filecache.c
@@ -318,15 +318,14 @@ nfsd_file_check_writeback(struct nfsd_file *nf)
 		mapping_tagged(mapping, PAGECACHE_TAG_WRITEBACK);
 }
 
-static bool nfsd_file_lru_add(struct nfsd_file *nf)
+static void nfsd_file_lru_add(struct nfsd_file *nf)
 {
-	set_bit(NFSD_FILE_REFERENCED, &nf->nf_flags);
-	set_bit(NFSD_FILE_RECENT, &nf->nf_flags);
-	if (list_lru_add_obj(&nfsd_file_lru, &nf->nf_lru)) {
+	refcount_inc(&nf->nf_ref);
+	if (list_lru_add_obj(&nfsd_file_lru, &nf->nf_lru))
 		trace_nfsd_file_lru_add(nf);
-		return true;
-	}
-	return false;
+	else
+		WARN_ON(1);
+	nfsd_file_schedule_laundrette();
 }
 
 static bool nfsd_file_lru_remove(struct nfsd_file *nf)
@@ -362,20 +361,10 @@ nfsd_file_put(struct nfsd_file *nf)
 
 	if (test_bit(NFSD_FILE_GC, &nf->nf_flags) &&
 	    test_bit(NFSD_FILE_HASHED, &nf->nf_flags)) {
-		/*
-		 * If this is the last reference (nf_ref == 1), then try to
-		 * transfer it to the LRU.
-		 */
-		if (refcount_dec_not_one(&nf->nf_ref))
-			return;
-
-		/* Try to add it to the LRU.  If that fails, decrement. */
-		if (nfsd_file_lru_add(nf)) {
-			nfsd_file_schedule_laundrette();
-			return;
-		}
-
+		set_bit(NFSD_FILE_REFERENCED, &nf->nf_flags);
+		set_bit(NFSD_FILE_RECENT, &nf->nf_flags);
 	}
+
 	if (refcount_dec_and_test(&nf->nf_ref))
 		nfsd_file_free(nf);
 }
@@ -510,13 +499,12 @@ nfsd_file_lru_cb(struct list_head *item, struct list_lru_one *lru,
 	}
 
 	/*
-	 * Put the reference held on behalf of the LRU. If it wasn't the last
-	 * one, then just remove it from the LRU and ignore it.
+	 * Put the reference held on behalf of the LRU if it is the last
+	 * reference, else rotate.
 	 */
-	if (!refcount_dec_and_test(&nf->nf_ref)) {
+	if (!refcount_dec_if_one(&nf->nf_ref)) {
 		trace_nfsd_file_gc_in_use(nf);
-		list_lru_isolate(lru, &nf->nf_lru);
-		return LRU_REMOVED;
+		return LRU_ROTATE;
 	}
 
 	/* Refcount went to zero. Unhash it and queue it to the dispose list */
@@ -1046,16 +1034,8 @@ nfsd_file_do_acquire(struct svc_rqst *rqstp, struct net *net,
 	nf = nfsd_file_lookup_locked(net, current_cred(), inode, need, want_gc);
 	rcu_read_unlock();
 
-	if (nf) {
-		/*
-		 * If the nf is on the LRU then it holds an extra reference
-		 * that must be put if it's removed. It had better not be
-		 * the last one however, since we should hold another.
-		 */
-		if (nfsd_file_lru_remove(nf))
-			refcount_dec(&nf->nf_ref);
+	if (nf)
 		goto wait_for_construction;
-	}
 
 	new = nfsd_file_alloc(net, inode, need, want_gc);
 	if (!new) {
@@ -1149,6 +1129,9 @@ nfsd_file_do_acquire(struct svc_rqst *rqstp, struct net *net,
 	 */
 	if (status != nfs_ok || inode->i_nlink == 0)
 		nfsd_file_unhash(nf);
+	else if (want_gc)
+		nfsd_file_lru_add(nf);
+
 	clear_and_wake_up_bit(NFSD_FILE_PENDING, &nf->nf_flags);
 	if (status == nfs_ok)
 		goto out;
-- 
2.47.1