autofs: add strictexpire mount option

Commit 092a5345 ("autofs: take more care to not update last_used on
path walk") helped to (partially) resolve a problem where automounts
were not expiring due to aggressive accesses from user space.

This patch was later reverted because, for very large environments, it
meant more mount requests from clients and when there are a lot of
clients this caused a fairly significant increase in server load.

But there is a need for both types of expire check, depending on use
case, so add a mount option to allow for strict update of last use of
autofs dentrys (which just means not updating the last use on path walk
access).

Link: http://lkml.kernel.org/r/154296973880.9889.14085372741514507967.stgit@pluto-themaw-netSigned-off-by: Ian Kent <raven@themaw.net>
Cc: Al Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/autofs/autofs_i.h

@@ -104,6 +104,7 @@ struct autofs_wait_queue {
 #define AUTOFS_SBI_MAGIC 0x6d4a556d
 
 #define AUTOFS_SBI_CATATONIC	0x0001
+#define AUTOFS_SBI_STRICTEXPIRE 0x0002
 
 struct autofs_sb_info {
 	u32 magic;

fs/autofs/inode.c

@@ -87,6 +87,8 @@ static int autofs_show_options(struct seq_file *m, struct dentry *root)
 		seq_printf(m, ",direct");
 	else
 		seq_printf(m, ",indirect");
+	if (sbi->flags & AUTOFS_SBI_STRICTEXPIRE)
+		seq_printf(m, ",strictexpire");
 #ifdef CONFIG_CHECKPOINT_RESTORE
 	if (sbi->pipe)
 		seq_printf(m, ",pipe_ino=%ld", file_inode(sbi->pipe)->i_ino);
@@ -109,7 +111,7 @@ static const struct super_operations autofs_sops = {
 };
 
 enum {Opt_err, Opt_fd, Opt_uid, Opt_gid, Opt_pgrp, Opt_minproto, Opt_maxproto,
-	Opt_indirect, Opt_direct, Opt_offset};
+	Opt_indirect, Opt_direct, Opt_offset, Opt_strictexpire};
 
 static const match_table_t tokens = {
 	{Opt_fd, "fd=%u"},
@@ -121,6 +123,7 @@ static const match_table_t tokens = {
 	{Opt_indirect, "indirect"},
 	{Opt_direct, "direct"},
 	{Opt_offset, "offset"},
+	{Opt_strictexpire, "strictexpire"},
 	{Opt_err, NULL}
 };
 
@@ -200,6 +203,9 @@ static int parse_options(char *options,
 		case Opt_offset:
 			set_autofs_type_offset(&sbi->type);
 			break;
+		case Opt_strictexpire:
+			sbi->flags |= AUTOFS_SBI_STRICTEXPIRE;
+			break;
 		default:
 			return 1;
 		}

fs/autofs/root.c

@@ -275,7 +275,10 @@ static int autofs_mount_wait(const struct path *path, bool rcu_walk)
 		pr_debug("waiting for mount name=%pd\n", path->dentry);
 		status = autofs_wait(sbi, path, NFY_MOUNT);
 		pr_debug("mount wait done status=%d\n", status);
+		ino->last_used = jiffies;
+		return status;
 	}
+	if (!(sbi->flags & AUTOFS_SBI_STRICTEXPIRE))
 		ino->last_used = jiffies;
 	return status;
 }

include/uapi/linux/auto_fs.h

@@ -23,7 +23,7 @@
 #define AUTOFS_MIN_PROTO_VERSION	3
 #define AUTOFS_MAX_PROTO_VERSION	5
 
-#define AUTOFS_PROTO_SUBVERSION		3
+#define AUTOFS_PROTO_SUBVERSION		4
 
 /*
  * The wait_queue_token (autofs_wqt_t) is part of a structure which is passed