btrfs: cache that we don't have security.capability set

When profiling a workload I noticed we were constantly calling getxattr.
These were mostly coming from __remove_privs, which will lookup if
security.capability exists to remove it.  However instrumenting getxattr
showed we get called nearly constantly on an idle machine on a lot of
accesses.

These are wasteful and not free.  Other security LSMs have a way to
cache their results, but capability doesn't have this, so it's asking us
all the time for the xattr.

Fix this by setting a flag in our inode that it doesn't have a
security.capability xattr.  We set this on new inodes and after a failed
lookup of security.capability.  If we set this xattr at all we'll clear
the flag.

I haven't found a test in fsperf that this makes a visible difference
on, but I assume fs_mark related tests would show it clearly.  This is a
perf report output of the smallfiles100k run where it shows 20% of our
time spent in __remove_privs because we're looking up the non-existent
xattr.

--21.86%--btrfs_write_check.constprop.0
  --21.62%--__file_remove_privs
    --21.55%--security_inode_need_killpriv
      --21.54%--cap_inode_need_killpriv
        --21.53%--__vfs_getxattr
          --20.89%--btrfs_getxattr

Obviously this is just CPU time in a mostly IO bound test, so the actual
effect of removing this callchain is minimal.  However in just normal
testing of an idle system tracing showed around 100 getxattr calls per
minute, and with this patch there are 0.
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/btrfs_inode.h

@@ -69,6 +69,8 @@ enum {
 	BTRFS_INODE_VERITY_IN_PROGRESS,
 	/* Set when this inode is a free space inode. */
 	BTRFS_INODE_FREE_SPACE_INODE,
+	/* Set when there are no capabilities in XATTs for the inode. */
+	BTRFS_INODE_NO_CAP_XATTR,
 };
 
 /* in memory btrfs inode */

fs/btrfs/inode.c

@@ -6225,6 +6225,13 @@ int btrfs_create_new_inode(struct btrfs_trans_handle *trans,
 	BTRFS_I(inode)->generation = trans->transid;
 	inode->i_generation = BTRFS_I(inode)->generation;
 
+	/*
+	 * We don't have any capability xattrs set here yet, shortcut any
+	 * queries for the xattrs here.  If we add them later via the inode
+	 * security init path or any other path this flag will be cleared.
+	 */
+	set_bit(BTRFS_INODE_NO_CAP_XATTR, &BTRFS_I(inode)->runtime_flags);
+
 	/*
 	 * Subvolumes don't inherit flags from their parent directory.
 	 * Originally this was probably by accident, but we probably can't

fs/btrfs/xattr.c

@@ -382,6 +382,53 @@ static int btrfs_xattr_handler_set(const struct xattr_handler *handler,
 	return btrfs_setxattr_trans(inode, name, buffer, size, flags);
 }
 
+static int btrfs_xattr_handler_get_security(const struct xattr_handler *handler,
+					    struct dentry *unused,
+					    struct inode *inode,
+					    const char *name, void *buffer,
+					    size_t size)
+{
+	int ret;
+	bool is_cap = false;
+
+	name = xattr_full_name(handler, name);
+
+	/*
+	 * security.capability doesn't cache the results, so calls into us
+	 * constantly to see if there's a capability xattr.  Cache the result
+	 * here in order to avoid wasting time doing lookups for xattrs we know
+	 * don't exist.
+	 */
+	if (strcmp(name, XATTR_NAME_CAPS) == 0) {
+		is_cap = true;
+		if (test_bit(BTRFS_INODE_NO_CAP_XATTR, &BTRFS_I(inode)->runtime_flags))
+			return -ENODATA;
+	}
+
+	ret = btrfs_getxattr(inode, name, buffer, size);
+	if (ret == -ENODATA && is_cap)
+		set_bit(BTRFS_INODE_NO_CAP_XATTR, &BTRFS_I(inode)->runtime_flags);
+	return ret;
+}
+
+static int btrfs_xattr_handler_set_security(const struct xattr_handler *handler,
+					    struct mnt_idmap *idmap,
+					    struct dentry *unused,
+					    struct inode *inode,
+					    const char *name,
+					    const void *buffer,
+					    size_t size, int flags)
+{
+	if (btrfs_root_readonly(BTRFS_I(inode)->root))
+		return -EROFS;
+
+	name = xattr_full_name(handler, name);
+	if (strcmp(name, XATTR_NAME_CAPS) == 0)
+		clear_bit(BTRFS_INODE_NO_CAP_XATTR, &BTRFS_I(inode)->runtime_flags);
+
+	return btrfs_setxattr_trans(inode, name, buffer, size, flags);
+}
+
 static int btrfs_xattr_handler_set_prop(const struct xattr_handler *handler,
 					struct mnt_idmap *idmap,
 					struct dentry *unused, struct inode *inode,
@@ -420,8 +467,8 @@ static int btrfs_xattr_handler_set_prop(const struct xattr_handler *handler,
 
 static const struct xattr_handler btrfs_security_xattr_handler = {
 	.prefix = XATTR_SECURITY_PREFIX,
-	.get = btrfs_xattr_handler_get,
-	.set = btrfs_xattr_handler_set,
+	.get = btrfs_xattr_handler_get_security,
+	.set = btrfs_xattr_handler_set_security,
 };
 
 static const struct xattr_handler btrfs_trusted_xattr_handler = {
@@ -473,6 +520,10 @@ static int btrfs_initxattrs(struct inode *inode,
 		}
 		strcpy(name, XATTR_SECURITY_PREFIX);
 		strcpy(name + XATTR_SECURITY_PREFIX_LEN, xattr->name);
+
+		if (strcmp(name, XATTR_NAME_CAPS) == 0)
+			clear_bit(BTRFS_INODE_NO_CAP_XATTR, &BTRFS_I(inode)->runtime_flags);
+
 		err = btrfs_setxattr(trans, inode, name, xattr->value,
 				     xattr->value_len, 0);
 		kfree(name);