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VFS: allow ->d_manage() to declare -EISDIR in rcu_walk mode.

PATCH Superseded NeilBrown 4 participants 7 messages Jul 30, 2014 – Aug 4, 2014

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NeilBrown

author
#0 lore raw

PATCH 1/1 VFS: allow ->d_manage() to declare -EISDIR in rcu_walk mode.

In REF-walk mode, ->d_manage can return -EISDIR to indicate that the dentry is not really a mount trap (or even a mount point) and that any mounts or any DCACHE_NEED_AUTOMOUNT flag should be ignored.

RCU-walk mode doesn't currently support this, so if there is a dentry with DCACHE_NEED_AUTOMOUNT set but which shouldn't be a mount-trap, lookup_fast() will always drop in REF-walk mode.

With this patch, an -EISDIR from ->d_manage will always cause mounts and automounts to be ignored, both in REF-walk and RCU-walk.

Cc Ian Kent <[email protected]>
Signed-off-by NeilBrown <[email protected]>

---
Hi Al,
this patch is needed before I can make autofs4 fully support RCU-walk.
There are cases currently were directories have DCACHE_NEED_AUTOMOUNT but for which no automount is required.

Thanks,
NeilBrown

Documentation/filesystems/vfs.txt +2 -1
index a1d0d7a30165..61d65cc65c54 100644
@@ -1053,7 +1053,8 @@ struct dentry_operations {
 	If the 'rcu_walk' parameter is true, then the caller is doing a
 	pathwalk in RCU-walk mode.  Sleeping is not permitted in this mode,
 	and the caller can be asked to leave it and call again by returning
-	-ECHILD.
+	-ECHILD.  -EISDIR may also be returned to tell pathwalk to
+	ignore d_automount or any mounts.
 
 	This function is only used if DCACHE_MANAGE_TRANSIT is set on the
 	dentry being transited from.
fs/namei.c +14 -11
index 985c6f368485..0abfea5697e0 100644
@@ -1091,10 +1091,10 @@ int follow_down_one(struct path *path)
 }
 EXPORT_SYMBOL(follow_down_one);
 
-static inline bool managed_dentry_might_block(struct dentry *dentry)
+static inline int managed_dentry_rcu(struct dentry *dentry)
 {
-	return (dentry->d_flags & DCACHE_MANAGE_TRANSIT &&
-		dentry->d_op->d_manage(dentry, true) < 0);
+	return (dentry->d_flags & DCACHE_MANAGE_TRANSIT) ?
+		dentry->d_op->d_manage(dentry, true) : 0;
 }
 
 /*

@@ -1110,11 +1110,16 @@ static bool __follow_mount_rcu(struct nameidata *nd, struct path *path,
 		 * Don't forget we might have a non-mountpoint managed dentry
 		 * that wants to block transit.
 		 */
-		if (unlikely(managed_dentry_might_block(path->dentry)))
+		switch (managed_dentry_rcu(path->dentry)) {
+		case -ECHILD:
+		default:
 			return false;
+		case -EISDIR:
+			return true;
+		}
 
 		if (!d_mountpoint(path->dentry))
-			return true;
+			return !(path->dentry->d_flags & DCACHE_NEED_AUTOMOUNT);
 
 		mounted = __lookup_mnt(path->mnt, path->dentry);
 		if (!mounted)

@@ -1130,7 +1135,8 @@ static bool __follow_mount_rcu(struct nameidata *nd, struct path *path,
 		 */
 		*inode = path->dentry->d_inode;
 	}
-	return read_seqretry(&mount_lock, nd->m_seq);
+	return read_seqretry(&mount_lock, nd->m_seq) &&
+		!(path->dentry->d_flags & DCACHE_NEED_AUTOMOUNT);
 }
 
 static int follow_dotdot_rcu(struct nameidata *nd)

@@ -1402,11 +1408,8 @@ static int lookup_fast(struct nameidata *nd,
 		}
 		path->mnt = mnt;
 		path->dentry = dentry;
-		if (unlikely(!__follow_mount_rcu(nd, path, inode)))
-			goto unlazy;
-		if (unlikely(path->dentry->d_flags & DCACHE_NEED_AUTOMOUNT))
-			goto unlazy;
-		return 0;
+		if (likely(__follow_mount_rcu(nd, path, inode)))
+			return 0;
 unlazy:
 		if (unlazy_walk(nd, dentry))
 			return -ECHILD;
author
#2 lore raw
Quoted 1 line · On Wed, 30 Jul 2014 13:16:52 -0700 Christoph Hellwig <[email protected]> wrote:

This looks good enough that I'll apply it to the interim vfs queue.

Thanks!

NeilBrown

#3 lore raw
In reply to NeilBrown

fs/namei.c static bool __follow_mount_rcu(struct nameidata *nd, struct path *path, · line 1120

Quoted 13 lines · NeilBrown
fs/namei.c +6 -1
@@ -1110,11 +1110,16 @@ static bool __follow_mount_rcu(struct nameidata *nd, struct path *path,
 		 * Don't forget we might have a non-mountpoint managed dentry
 		 * that wants to block transit.
 		 */
-		if (unlikely(managed_dentry_might_block(path->dentry)))
+		switch (managed_dentry_rcu(path->dentry)) {
+		case -ECHILD:
+		default:
 			return false;
+		case -EISDIR:
+			return true;
+		}

Smatch says that any lines after that switch statement are unreachable.

Is the "default" intended?

fs/namei.c static bool __follow_mount_rcu(struct nameidata *nd, struct path *path, · line 1125

Quoted 6 lines
fs/namei.c +1 -1
 		if (!d_mountpoint(path->dentry))
-			return true;
+			return !(path->dentry->d_flags & DCACHE_NEED_AUTOMOUNT);
 
 		mounted = __lookup_mnt(path->mnt, path->dentry);
 		if (!mounted)

regards,
dan carpenter

author
#4 lore raw
Quoted 17 lines · On Fri, 1 Aug 2014 11:33:18 +0300 Dan Carpenter <[email protected]>…

fs/namei.c static bool __follow_mount_rcu(struct nameidata *nd, struct path *path, · line 1120

Quoted 13 lines · NeilBrown
fs/namei.c +6 -1
@@ -1110,11 +1110,16 @@ static bool __follow_mount_rcu(struct nameidata *nd, struct path *path,
 		 * Don't forget we might have a non-mountpoint managed dentry
 		 * that wants to block transit.
 		 */
-		if (unlikely(managed_dentry_might_block(path->dentry)))
+		switch (managed_dentry_rcu(path->dentry)) {
+		case -ECHILD:
+		default:
 			return false;
+		case -EISDIR:
+			return true;
+		}

Smatch says that any lines after that switch statement are unreachable.

Quoted 1 line

Is the "default" intended?

Ugh. It was an after thought.

The defined error codes are -ECHILD and -EISDIR.
But current code actually treats any other negative number as -ECHILD. So I stuck in the default .... which of course was wrong.
I need to add
case 0:

       break;

I'll send that out on Monday.

Thanks heaps.

NeilBrown

Quoted 10 lines

fs/namei.c static bool __follow_mount_rcu(struct nameidata *nd, struct path *path, · line 1125

Quoted 7 lines
fs/namei.c +1 -1
 		if (!d_mountpoint(path->dentry))
-			return true;
+			return !(path->dentry->d_flags & DCACHE_NEED_AUTOMOUNT);
 
 		mounted = __lookup_mnt(path->mnt, path->dentry);
 		if (!mounted)

regards,
dan carpenter

author
#5 lore raw

PATCH v2 1/1 VFS: allow ->d_manage() to declare -EISDIR in rcu_walk mode.

In REF-walk mode, ->d_manage can return -EISDIR to indicate that the dentry is not really a mount trap (or even a mount point) and that any mounts or any DCACHE_NEED_AUTOMOUNT flag should be ignored.

RCU-walk mode doesn't currently support this, so if there is a dentry with DCACHE_NEED_AUTOMOUNT set but which shouldn't be a mount-trap, lookup_fast() will always drop in REF-walk mode.

With this patch, an -EISDIR from ->d_manage will always cause mounts and automounts to be ignored, both in REF-walk and RCU-walk.

Bug-fixed-by Dan Carpenter <[email protected]>
Cc Ian Kent <[email protected]>
Signed-off-by NeilBrown <[email protected]>

---
Previous version of this patch had a bug causing code to be unreachable. Please replace with this version.

Thanks,
NeilBrown

Documentation/filesystems/vfs.txt +2 -1
index a1d0d7a30165..61d65cc65c54 100644
@@ -1053,7 +1053,8 @@ struct dentry_operations {
 	If the 'rcu_walk' parameter is true, then the caller is doing a
 	pathwalk in RCU-walk mode.  Sleeping is not permitted in this mode,
 	and the caller can be asked to leave it and call again by returning
-	-ECHILD.
+	-ECHILD.  -EISDIR may also be returned to tell pathwalk to
+	ignore d_automount or any mounts.
 
 	This function is only used if DCACHE_MANAGE_TRANSIT is set on the
 	dentry being transited from.
fs/namei.c +16 -11
index 9eb787e5c167..98ce4cffe51c 100644
@@ -1091,10 +1091,10 @@ int follow_down_one(struct path *path)
 }
 EXPORT_SYMBOL(follow_down_one);
 
-static inline bool managed_dentry_might_block(struct dentry *dentry)
+static inline int managed_dentry_rcu(struct dentry *dentry)
 {
-	return (dentry->d_flags & DCACHE_MANAGE_TRANSIT &&
-		dentry->d_op->d_manage(dentry, true) < 0);
+	return (dentry->d_flags & DCACHE_MANAGE_TRANSIT) ?
+		dentry->d_op->d_manage(dentry, true) : 0;
 }
 
 /*

@@ -1110,11 +1110,18 @@ static bool __follow_mount_rcu(struct nameidata *nd, struct path *path,
 		 * Don't forget we might have a non-mountpoint managed dentry
 		 * that wants to block transit.
 		 */
-		if (unlikely(managed_dentry_might_block(path->dentry)))
+		switch (managed_dentry_rcu(path->dentry)) {
+		case -ECHILD:
+		default:
 			return false;
+		case -EISDIR:
+			return true;
+		case 0:
+			break;
+		}
 
 		if (!d_mountpoint(path->dentry))
-			return true;
+			return !(path->dentry->d_flags & DCACHE_NEED_AUTOMOUNT);
 
 		mounted = __lookup_mnt(path->mnt, path->dentry);
 		if (!mounted)

@@ -1130,7 +1137,8 @@ static bool __follow_mount_rcu(struct nameidata *nd, struct path *path,
 		 */
 		*inode = path->dentry->d_inode;
 	}
-	return read_seqretry(&mount_lock, nd->m_seq);
+	return read_seqretry(&mount_lock, nd->m_seq) &&
+		!(path->dentry->d_flags & DCACHE_NEED_AUTOMOUNT);
 }
 
 static int follow_dotdot_rcu(struct nameidata *nd)

@@ -1402,11 +1410,8 @@ static int lookup_fast(struct nameidata *nd,
 		}
 		path->mnt = mnt;
 		path->dentry = dentry;
-		if (unlikely(!__follow_mount_rcu(nd, path, inode)))
-			goto unlazy;
-		if (unlikely(path->dentry->d_flags & DCACHE_NEED_AUTOMOUNT))
-			goto unlazy;
-		return 0;
+		if (likely(__follow_mount_rcu(nd, path, inode)))
+			return 0;
 unlazy:
 		if (unlazy_walk(nd, dentry))
 			return -ECHILD;
maintainer
#6 lore raw
Quoted 6 lines · NeilBrown

In REF-walk mode, ->d_manage can return -EISDIR to indicate that the dentry is not really a mount trap (or even a mount point) and that any mounts or any DCACHE_NEED_AUTOMOUNT flag should be ignored.

Quoted 6 lines

RCU-walk mode doesn't currently support this, so if there is a dentry with DCACHE_NEED_AUTOMOUNT set but which shouldn't be a mount-trap, lookup_fast() will always drop in REF-walk mode.

With this patch, an -EISDIR from ->d_manage will always cause mounts and automounts to be ignored, both in REF-walk and RCU-walk.

In queue. Right now I'm trying to linearize the damn acct fixes from late April, needed to get Eric's umount-on-rmdir series into the mix safely ;-/ Hopefully I'll have something pushable by Tuesday; your patch definitely will be in the pile.