From: Trond Myklebust <trond.myklebust@hammerspace.com>

Date: Fri, 6 Nov 2020 20:47:05 -0500

Subject: [PATCH] NFS: Do uncached readdir when we're seeking a cookie in an

 empty page cache

Git-commit: 794092c57f89c2c833da00f82f38a0afcb4033bc

Patch-mainline: v5.11-rc1

References: bsc#1191628 bsc#1192549

If the directory is changing, causing the page cache to get invalidated

while we are listing the contents, then the NFS client is currently forced

to read in the entire directory contents from scratch, because it needs

to perform a linear search for the readdir cookie. While this is not

an issue for small directories, it does not scale to directories with

millions of entries.

In order to be able to deal with large directories that are changing,

add a heuristic to ensure that if the page cache is empty, and we are

searching for a cookie that is not the zero cookie, we just default to

performing uncached readdir.

Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>

Reviewed-by: Benjamin Coddington <bcodding@redhat.com>

Tested-by: Benjamin Coddington <bcodding@redhat.com>

Tested-by: Dave Wysochanski <dwysocha@redhat.com>

Acked-by: NeilBrown <neilb@suse.com>

---

 fs/nfs/dir.c |   17 +++++++++++++++++

 1 file changed, 17 insertions(+)

--- a/fs/nfs/dir.c

+++ b/fs/nfs/dir.c

@@ -743,12 +743,29 @@ error:

 	return res;

 }

+static bool nfs_readdir_dont_search_cache(nfs_readdir_descriptor_t *desc)

+{

+	struct address_space *mapping = desc->file->f_mapping;

+	struct inode *dir = file_inode(desc->file);

+	unsigned int dtsize = NFS_SERVER(dir)->dtsize;

+	loff_t size = i_size_read(dir);

+

+	/*

+	 * Default to uncached readdir if the page cache is empty, and

+	 * we're looking for a non-zero cookie in a large directory.

+	 */

+	return *desc->dir_cookie != 0 && mapping->nrpages == 0 && size > dtsize;

+}

+

 /* Search for desc->dir_cookie from the beginning of the page cache */

 static inline

 int readdir_search_pagecache(nfs_readdir_descriptor_t *desc)

 {

 	int res;

+	if (nfs_readdir_dont_search_cache(desc))

+		return -EBADCOOKIE;

+

 	if (desc->page_index == 0) {

 		desc->current_index = 0;

 		desc->last_cookie = 0;