[PATCH 2/2] ovl: enable RCU'd ->get_acl()
From: Miklos Szeredi
Date: Tue Aug 10 2021 - 08:08:27 EST
Overlayfs does not cache ACL's (to avoid double caching). Instead it just
calls the underlying filesystem's i_op->get_acl(), which will return the
cached value, if possible.
In rcu path walk, however, get_cached_acl_rcu() is employed to get the
value from the cache, which will fail on overlayfs resulting in dropping
out of rcu walk mode. This can result in a big performance hit in certain
situations.
Fix by calling ->get_acl() with LOOKUP_RCU flag in case of ACL_DONT_CACHE
(which indicates pass-through)
Reported-by: garyhuang <zjh.20052005@xxxxxxx>
Signed-off-by: Miklos Szeredi <mszeredi@xxxxxxxxxx>
---
fs/overlayfs/inode.c | 7 ++++---
fs/posix_acl.c | 8 +++++++-
include/linux/fs.h | 5 +++++
3 files changed, 16 insertions(+), 4 deletions(-)
diff --git a/fs/overlayfs/inode.c b/fs/overlayfs/inode.c
index 727154a1d3ce..6a55231b262a 100644
--- a/fs/overlayfs/inode.c
+++ b/fs/overlayfs/inode.c
@@ -13,6 +13,7 @@
#include <linux/fiemap.h>
#include <linux/fileattr.h>
#include <linux/security.h>
+#include <linux/namei.h>
#include "overlayfs.h"
@@ -454,12 +455,12 @@ struct posix_acl *ovl_get_acl(struct inode *inode, int type, int flags)
const struct cred *old_cred;
struct posix_acl *acl;
- if (flags)
- return ERR_PTR(-EINVAL);
-
if (!IS_ENABLED(CONFIG_FS_POSIX_ACL) || !IS_POSIXACL(realinode))
return NULL;
+ if (flags & LOOKUP_RCU)
+ return get_cached_acl_rcu(realinode, type);
+
old_cred = ovl_override_creds(inode->i_sb);
acl = get_acl(realinode, type);
revert_creds(old_cred);
diff --git a/fs/posix_acl.c b/fs/posix_acl.c
index 6b7f793e2b6f..4d1c6c266cf0 100644
--- a/fs/posix_acl.c
+++ b/fs/posix_acl.c
@@ -22,6 +22,7 @@
#include <linux/xattr.h>
#include <linux/export.h>
#include <linux/user_namespace.h>
+#include <linux/namei.h>
static struct posix_acl **acl_by_type(struct inode *inode, int type)
{
@@ -56,7 +57,12 @@ EXPORT_SYMBOL(get_cached_acl);
struct posix_acl *get_cached_acl_rcu(struct inode *inode, int type)
{
- return rcu_dereference(*acl_by_type(inode, type));
+ struct posix_acl *acl = rcu_dereference(*acl_by_type(inode, type));
+
+ if (acl == ACL_DONT_CACHE)
+ acl = inode->i_op->get_acl(inode, type, LOOKUP_RCU);
+
+ return acl;
}
EXPORT_SYMBOL(get_cached_acl_rcu);
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 1c56d4fc4efe..20b7db2d0a85 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -581,6 +581,11 @@ static inline void mapping_allow_writable(struct address_space *mapping)
struct posix_acl;
#define ACL_NOT_CACHED ((void *)(-1))
+/*
+ * ACL_DONT_CACHE is for stacked filesystems, that rely on underlying fs to
+ * cache the ACL. This also means that ->get_acl() can be called in RCU mode
+ * with the LOOKUP_RCU flag.
+ */
#define ACL_DONT_CACHE ((void *)(-3))
static inline struct posix_acl *
--
2.31.1