[RFC PATCH] ovl: check underlying upper inode once in its lifetime

[Chen Hu]

ovl_permission() checks the underlying inode with the credential of
mounter. The cred, struct ovl_fs::creator_cred, is global per overlayfs
superblock. Performance degrades when concurrency increases on the cred,
to be specific, on ovl_fs::creator_cred::usage.

This patch (or hack to some extent) checks underlying upper inode once
in its lifetime, eliminates the cache line contention on
creator_cred::usage and gets 40%+ perf improvement on a 128 cores CPU.

CAUTION:
this may compromise the file permission check. Need to talk with
overlayfs experts.

Signed-off-by: Chen Hu <hu1.chen@xxxxxxxxx>
---
 fs/overlayfs/inode.c     | 5 ++++-
 fs/overlayfs/overlayfs.h | 1 +
 2 files changed, 5 insertions(+), 1 deletion(-)

diff --git a/fs/overlayfs/inode.c b/fs/overlayfs/inode.c
index 83ef66644c21..62ec99316c7a 100644
--- a/fs/overlayfs/inode.c
+++ b/fs/overlayfs/inode.c
@@ -307,7 +307,7 @@ int ovl_permission(struct mnt_idmap *idmap,
 	 * with creds of mounter
 	 */
 	err = generic_permission(&nop_mnt_idmap, inode, mask);
-	if (err)
+	if (err || ovl_test_flag(OVL_FASTPERM, inode))
 		return err;
 
 	old_cred = ovl_override_creds(inode->i_sb);
@@ -318,6 +318,9 @@ int ovl_permission(struct mnt_idmap *idmap,
 		mask |= MAY_READ;
 	}
 	err = inode_permission(mnt_idmap(realpath.mnt), realinode, mask);
+	if (err == 0 && upperinode)
+		/* This gets set once for the upper inode lifetime */
+		ovl_set_flag(OVL_FASTPERM, inode);
 	revert_creds(old_cred);
 
 	return err;
diff --git a/fs/overlayfs/overlayfs.h b/fs/overlayfs/overlayfs.h
index 9817b2dcb132..5b71aaa8f77c 100644
--- a/fs/overlayfs/overlayfs.h
+++ b/fs/overlayfs/overlayfs.h
@@ -53,6 +53,7 @@ enum ovl_inode_flag {
 	OVL_CONST_INO,
 	OVL_HAS_DIGEST,
 	OVL_VERIFIED_DIGEST,
+	OVL_FASTPERM,
 };
 
 enum ovl_entry_flag {
-- 
2.34.1