[PATCH] fs: drop_caches: skip dropping pagecache which is always dirty

[cgel.zte@gmail.com]

From: Yang Yang <yang.yang29@zte.com.cn>

Pagecache of some kind of fs has PG_dirty bit set once it was
allocated, so it can't be dropped. These fs include ramfs and
tmpfs. This can make drop_pagecache_sb() more efficient.

Introduce a new fs flag to do this, and this new flag may be
used in other case in future.

Signed-off-by: Ran Xiaokai <ran.xiaokai@zte.com.cn>
Signed-off-by: Yang Yang <yang.yang29@zte.com.cn>
Signed-off-by: CGEL ZTE <cgel.zte@gmail.com>
---
 fs/drop_caches.c   | 7 +++++++
 fs/ramfs/inode.c   | 2 +-
 include/linux/fs.h | 1 +
 mm/shmem.c         | 2 +-
 4 files changed, 10 insertions(+), 2 deletions(-)

diff --git a/fs/drop_caches.c b/fs/drop_caches.c
index e619c31b6bd9..16956d5d3922 100644
--- a/fs/drop_caches.c
+++ b/fs/drop_caches.c
@@ -19,6 +19,13 @@ static void drop_pagecache_sb(struct super_block *sb, void *unused)
 {
 	struct inode *inode, *toput_inode = NULL;
 
+	/*
+	 * Pagecache of this kind of fs has PG_dirty bit set once it was
+	 * allocated, so it can't be dropped.
+	 */
+	if (sb->s_type->fs_flags & FS_ALWAYS_DIRTY)
+		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/ramfs/inode.c b/fs/ramfs/inode.c
index bc66d0173e33..5fb62d37618f 100644
--- a/fs/ramfs/inode.c
+++ b/fs/ramfs/inode.c
@@ -289,7 +289,7 @@ static struct file_system_type ramfs_fs_type = {
 	.init_fs_context = ramfs_init_fs_context,
 	.parameters	= ramfs_fs_parameters,
 	.kill_sb	= ramfs_kill_sb,
-	.fs_flags	= FS_USERNS_MOUNT,
+	.fs_flags	= FS_USERNS_MOUNT | FS_ALWAYS_DIRTY,
 };
 
 static int __init init_ramfs_fs(void)
diff --git a/include/linux/fs.h b/include/linux/fs.h
index e285bd9d6188..90cdd10d683e 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -2532,6 +2532,7 @@ struct file_system_type {
 #define FS_USERNS_MOUNT		8	/* Can be mounted by userns root */
 #define FS_DISALLOW_NOTIFY_PERM	16	/* Disable fanotify permission events */
 #define FS_ALLOW_IDMAP         32      /* FS has been updated to handle vfs idmappings. */
+#define FS_ALWAYS_DIRTY		64	/* Pagecache is always dirty. */
 #define FS_RENAME_DOES_D_MOVE	32768	/* FS will handle d_move() during rename() internally. */
 	int (*init_fs_context)(struct fs_context *);
 	const struct fs_parameter_spec *parameters;
diff --git a/mm/shmem.c b/mm/shmem.c
index 8baf26eda989..5d549f61735f 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -3974,7 +3974,7 @@ static struct file_system_type shmem_fs_type = {
 	.parameters	= shmem_fs_parameters,
 #endif
 	.kill_sb	= kill_litter_super,
-	.fs_flags	= FS_USERNS_MOUNT,
+	.fs_flags	= FS_USERNS_MOUNT | FS_ALWAYS_DIRTY,
 };
 
 void __init shmem_init(void)
-- 
2.25.1