[RFC PATCH v2 1/3] fs/drop_caches: skip filesystems without page cache

[Huang Shijie <huangsj@hygon.cn>]

Add a new flag SB_I_NO_PAGECACHE for superblock.

Skip scanning the inode lists of filesystems that have no page cache
in drop_pagecache_sb(), as indicated by the SB_I_NO_PAGECACHE flag.

This patch only changes the procfs.

Signed-off-by: Huang Shijie <huangsj@hygon.cn>
---
 fs/drop_caches.c               | 3 +++
 fs/proc/root.c                 | 2 +-
 include/linux/fs/super_types.h | 1 +
 3 files changed, 5 insertions(+), 1 deletion(-)

diff --git a/fs/drop_caches.c b/fs/drop_caches.c
index 49f56a598ecb..27caf748b3c5 100644
--- a/fs/drop_caches.c
+++ b/fs/drop_caches.c
@@ -20,6 +20,9 @@ static void drop_pagecache_sb(struct super_block *sb, void *unused)
 {
 	struct inode *inode, *toput_inode = NULL;
 
+	if (sb->s_iflags & SB_I_NO_PAGECACHE)
+		return;
+
 	spin_lock(&sb->s_inode_list_lock);
 	list_for_each_entry(inode, &sb->s_inodes, i_sb_list) {
 		spin_lock(&inode->i_lock);
diff --git a/fs/proc/root.c b/fs/proc/root.c
index 99adddfeb4a4..d45f5af5ab53 100644
--- a/fs/proc/root.c
+++ b/fs/proc/root.c
@@ -266,7 +266,7 @@ static int proc_fill_super(struct super_block *s, struct fs_context *fc)
 		return ret;
 
 	/* User space would break if executables or devices appear on proc */
-	s->s_iflags |= SB_I_NOEXEC | SB_I_NODEV;
+	s->s_iflags |= SB_I_NOEXEC | SB_I_NODEV | SB_I_NO_PAGECACHE;
 	s->s_flags |= SB_NODIRATIME | SB_NOSUID | SB_NOEXEC;
 	s->s_blocksize = 1024;
 	s->s_blocksize_bits = 10;
diff --git a/include/linux/fs/super_types.h b/include/linux/fs/super_types.h
index ecd96aeb1cee..1a596caf58a8 100644
--- a/include/linux/fs/super_types.h
+++ b/include/linux/fs/super_types.h
@@ -352,5 +352,6 @@ struct super_block {
 #define SB_I_NOIDMAP	0x00002000	/* No idmapped mounts on this superblock */
 #define SB_I_ALLOW_HSM	0x00004000	/* Allow HSM events on this superblock */
 #define SB_I_NO_DATA_INTEGRITY	0x00008000 /* fs cannot guarantee data persistence on sync */
+#define SB_I_NO_PAGECACHE	0x00010000 /* this file system has no page cache */
 
 #endif /* _LINUX_FS_SUPER_TYPES_H */
-- 
2.53.0