linux-kernel - Re: [PATCH 0/9] [RFC v2] safely drop directory dentry on failed revalidate

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Message-ID: <20130821210037.GO27005@ZenIV.linux.org.uk>
Date:	Wed, 21 Aug 2013 22:00:38 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Ryusuke Konishi <konishi.ryusuke@....ntt.co.jp>
Cc:	Miklos Szeredi <miklos@...redi.hu>, rwheeler@...hat.com,
	avati@...hat.com, bfoster@...hat.com, dhowells@...hat.com,
	eparis@...hat.com, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, mszeredi@...e.cz, raven@...maw.net
Subject: Re: [PATCH 0/9] [RFC v2] safely drop directory dentry on failed
 revalidate

On Thu, Aug 22, 2013 at 05:04:59AM +0900, Ryusuke Konishi wrote:
> On Wed, 21 Aug 2013 06:40:56 +0100, Al Viro wrote:
> > 	And I would like to understand what nilfs one is trying to do...
> > Unless I'm seriously misreading that code, it's *not* on any kind of a hot
> > path, so I really wonder why don't we simply do shrink_dcache_parent() +
> > check if d_count has dropped to 1, without trying to look for submounts
> > first - if we have any, shrink_dcache_parent() is simply going to leave
> > us with d_count > 1 and that's it.  Actually, it's cheaper that way -
> > no need to walk the tree twice.
> 
> > 	We are really getting too many tree walkers in fs/dcache.c and
> > all that duplication is the prime breeding ground for bugs ;-/
> 
> I agree that we can eliminate have_submounts() from nilfs.  Please
> apply the following patch if you hope so.
> 
> > Moreover, checking for d_count == 1
> > case first is also pointless - in that case we have no children at all
> > and shrink_dcache_parent() will return immediately.
> 
> This also looks true.  I will confirm whether we can remove the
> pre-check for d_count == 1 case.

Umm...  How about the following, then?

diff --git a/fs/nilfs2/super.c b/fs/nilfs2/super.c
index af3ba04..7ac2a12 100644
--- a/fs/nilfs2/super.c
+++ b/fs/nilfs2/super.c
@@ -994,23 +994,16 @@ static int nilfs_attach_snapshot(struct super_block *s, __u64 cno,
 	return ret;
 }
 
-static int nilfs_tree_was_touched(struct dentry *root_dentry)
-{
-	return d_count(root_dentry) > 1;
-}
-
 /**
- * nilfs_try_to_shrink_tree() - try to shrink dentries of a checkpoint
+ * nilfs_tree_is_busy() - try to shrink dentries of a checkpoint
  * @root_dentry: root dentry of the tree to be shrunk
  *
  * This function returns true if the tree was in-use.
  */
-static int nilfs_try_to_shrink_tree(struct dentry *root_dentry)
+static bool nilfs_tree_is_busy(struct dentry *root_dentry)
 {
-	if (have_submounts(root_dentry))
-		return true;
 	shrink_dcache_parent(root_dentry);
-	return nilfs_tree_was_touched(root_dentry);
+	return d_count(root_dentry) > 1;
 }
 
 int nilfs_checkpoint_is_mounted(struct super_block *sb, __u64 cno)
@@ -1034,8 +1027,7 @@ int nilfs_checkpoint_is_mounted(struct super_block *sb, __u64 cno)
 		if (inode) {
 			dentry = d_find_alias(inode);
 			if (dentry) {
-				if (nilfs_tree_was_touched(dentry))
-					ret = nilfs_try_to_shrink_tree(dentry);
+				ret = nilfs_tree_is_busy(dentry);
 				dput(dentry);
 			}
 			iput(inode);
@@ -1331,11 +1323,8 @@ nilfs_mount(struct file_system_type *fs_type, int flags,
 
 		s->s_flags |= MS_ACTIVE;
 	} else if (!sd.cno) {
-		int busy = false;
-
-		if (nilfs_tree_was_touched(s->s_root)) {
-			busy = nilfs_try_to_shrink_tree(s->s_root);
-			if (busy && (flags ^ s->s_flags) & MS_RDONLY) {
+		if (nilfs_tree_is_busy(s->s_root)) {
+			if ((flags ^ s->s_flags) & MS_RDONLY) {
 				printk(KERN_ERR "NILFS: the device already "
 				       "has a %s mount.\n",
 				       (s->s_flags & MS_RDONLY) ?
@@ -1343,8 +1332,7 @@ nilfs_mount(struct file_system_type *fs_type, int flags,
 				err = -EBUSY;
 				goto failed_super;
 			}
-		}
-		if (!busy) {
+		} else {
 			/*
 			 * Try remount to setup mount states if the current
 			 * tree is not mounted and only snapshots use this sb.
--
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