[PATCH v4 19/21] NFS: Optimisations for monotonically increasing readdir cookies

[trondmy@xxxxxxxxxx]

From: Trond Myklebust <trond.myklebust@xxxxxxxxxxxxxxx>

If the server is handing out monotonically increasing readdir cookie values,
then we can optimise away searches through pages that contain cookies that
lie outside our search range.

Signed-off-by: Trond Myklebust <trond.myklebust@xxxxxxxxxxxxxxx>
---
 fs/nfs/dir.c | 23 ++++++++++++++++++++++-
 1 file changed, 22 insertions(+), 1 deletion(-)

diff --git a/fs/nfs/dir.c b/fs/nfs/dir.c
index 454377228167..b6c3501e8f61 100644
--- a/fs/nfs/dir.c
+++ b/fs/nfs/dir.c
@@ -140,7 +140,8 @@ struct nfs_cache_array {
 	u64 last_cookie;
 	unsigned int size;
 	unsigned char page_full : 1,
-		      page_is_eof : 1;
+		      page_is_eof : 1,
+		      cookies_are_ordered : 1;
 	struct nfs_cache_array_entry array[];
 };
 
@@ -178,6 +179,7 @@ static void nfs_readdir_page_init_array(struct page *page, u64 last_cookie)
 	array = kmap_atomic(page);
 	nfs_readdir_array_init(array);
 	array->last_cookie = last_cookie;
+	array->cookies_are_ordered = 1;
 	kunmap_atomic(array);
 }
 
@@ -269,6 +271,8 @@ int nfs_readdir_add_to_array(struct nfs_entry *entry, struct page *page)
 	cache_entry->name_len = entry->len;
 	cache_entry->name = name;
 	array->last_cookie = entry->cookie;
+	if (array->last_cookie <= cache_entry->cookie)
+		array->cookies_are_ordered = 0;
 	array->size++;
 	if (entry->eof != 0)
 		nfs_readdir_array_set_eof(array);
@@ -395,6 +399,19 @@ nfs_readdir_inode_mapping_valid(struct nfs_inode *nfsi)
 	return !test_bit(NFS_INO_INVALIDATING, &nfsi->flags);
 }
 
+static bool nfs_readdir_array_cookie_in_range(struct nfs_cache_array *array,
+					      u64 cookie)
+{
+	if (!array->cookies_are_ordered)
+		return true;
+	/* Optimisation for monotonically increasing cookies */
+	if (cookie >= array->last_cookie)
+		return false;
+	if (array->size && cookie < array->array[0].cookie)
+		return false;
+	return true;
+}
+
 static int nfs_readdir_search_for_cookie(struct nfs_cache_array *array,
 					 struct nfs_readdir_descriptor *desc)
 {
@@ -402,6 +419,9 @@ static int nfs_readdir_search_for_cookie(struct nfs_cache_array *array,
 	loff_t new_pos;
 	int status = -EAGAIN;
 
+	if (!nfs_readdir_array_cookie_in_range(array, desc->dir_cookie))
+		goto check_eof;
+
 	for (i = 0; i < array->size; i++) {
 		if (array->array[i].cookie == desc->dir_cookie) {
 			struct nfs_inode *nfsi = NFS_I(file_inode(desc->file));
@@ -435,6 +455,7 @@ static int nfs_readdir_search_for_cookie(struct nfs_cache_array *array,
 			return 0;
 		}
 	}
+check_eof:
 	if (array->page_is_eof) {
 		status = -EBADCOOKIE;
 		if (desc->dir_cookie == array->last_cookie)
-- 
2.28.0