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Date:   Sun, 15 Apr 2018 15:21:01 +0100
From:   Al Viro <viro@ZenIV.linux.org.uk>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Nikolay Borisov <nborisov@suse.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Khazhismel Kumykov <khazhy@google.com>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        David Rientjes <rientjes@google.com>,
        Goldwyn Rodrigues <rgoldwyn@suse.de>,
        Jeff Mahoney <jeffm@suse.com>,
        Davidlohr Bueso <dave@stgolabs.net>
Subject: Re: [PATCH] fs/dcache.c: re-add cond_resched() in
 shrink_dcache_parent()
Message-ID: <20180415142101.GZ30522@ZenIV.linux.org.uk>
References: <20180413181350.88831-1-khazhy@google.com>
 <20180413202823.204377-1-khazhy@google.com>
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 <3362fb2d-85ff-86af-399f-698c986e46cc@suse.com>
 <20180414080206.GV30522@ZenIV.linux.org.uk>
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On Sun, Apr 15, 2018 at 03:39:44AM +0100, Al Viro wrote:

> I really wonder if we should just do the following in
> d_invalidate():
> 	* grab ->d_lock on victim, check if it's unhashed,
> unlock and bugger off if it is.  Otherwise, unhash and unlock.
> >From that point on any d_set_mounted() in the subtree will
> fail.
> 	* shrink_dcache_parent() to reduce the subtree size.
> 	* go through the (hopefully shrunk) subtree, picking
> mountpoints.  detach_mounts() for each of them.
> 	* shrink_dcache_parent() if any points had been
> encountered, to kick the now-unpinned stuff.
> 
> As a side benefit, we could probably be gentler on rename_lock
> in d_set_mounted() after that change...

Something like this, IOW:

diff --git a/fs/dcache.c b/fs/dcache.c
index 86d2de63461e..e61c07ff2d95 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -1230,13 +1230,11 @@ enum d_walk_ret {
  * @parent:	start of walk
  * @data:	data passed to @enter() and @finish()
  * @enter:	callback when first entering the dentry
- * @finish:	callback when successfully finished the walk
  *
  * The @enter() and @finish() callbacks are called with d_lock held.
  */
 static void d_walk(struct dentry *parent, void *data,
-		   enum d_walk_ret (*enter)(void *, struct dentry *),
-		   void (*finish)(void *))
+		   enum d_walk_ret (*enter)(void *, struct dentry *))
 {
 	struct dentry *this_parent;
 	struct list_head *next;
@@ -1325,8 +1323,6 @@ static void d_walk(struct dentry *parent, void *data,
 	if (need_seqretry(&rename_lock, seq))
 		goto rename_retry;
 	rcu_read_unlock();
-	if (finish)
-		finish(data);
 
 out_unlock:
 	spin_unlock(&this_parent->d_lock);
@@ -1375,7 +1371,7 @@ int path_has_submounts(const struct path *parent)
 	struct check_mount data = { .mnt = parent->mnt, .mounted = 0 };
 
 	read_seqlock_excl(&mount_lock);
-	d_walk(parent->dentry, &data, path_check_mount, NULL);
+	d_walk(parent->dentry, &data, path_check_mount);
 	read_sequnlock_excl(&mount_lock);
 
 	return data.mounted;
@@ -1483,7 +1479,7 @@ void shrink_dcache_parent(struct dentry *parent)
 		data.start = parent;
 		data.found = 0;
 
-		d_walk(parent, &data, select_collect, NULL);
+		d_walk(parent, &data, select_collect);
 		if (!data.found)
 			break;
 
@@ -1518,7 +1514,7 @@ static enum d_walk_ret umount_check(void *_data, struct dentry *dentry)
 static void do_one_tree(struct dentry *dentry)
 {
 	shrink_dcache_parent(dentry);
-	d_walk(dentry, dentry, umount_check, NULL);
+	d_walk(dentry, dentry, umount_check);
 	d_drop(dentry);
 	dput(dentry);
 }
@@ -1542,78 +1538,48 @@ void shrink_dcache_for_umount(struct super_block *sb)
 	}
 }
 
-struct detach_data {
-	struct select_data select;
-	struct dentry *mountpoint;
-};
-static enum d_walk_ret detach_and_collect(void *_data, struct dentry *dentry)
+static enum d_walk_ret find_submount(void *_data, struct dentry *dentry)
 {
-	struct detach_data *data = _data;
-
 	if (d_mountpoint(dentry)) {
 		__dget_dlock(dentry);
-		data->mountpoint = dentry;
+		*(struct dentry **)_data = dentry;
 		return D_WALK_QUIT;
 	}
 
-	return select_collect(&data->select, dentry);
-}
-
-static void check_and_drop(void *_data)
-{
-	struct detach_data *data = _data;
-
-	if (!data->mountpoint && list_empty(&data->select.dispose))
-		__d_drop(data->select.start);
+	return D_WALK_CONTINUE;
 }
 
 /**
  * d_invalidate - detach submounts, prune dcache, and drop
  * @dentry: dentry to invalidate (aka detach, prune and drop)
- *
- * no dcache lock.
- *
- * The final d_drop is done as an atomic operation relative to
- * rename_lock ensuring there are no races with d_set_mounted.  This
- * ensures there are no unhashed dentries on the path to a mountpoint.
  */
 void d_invalidate(struct dentry *dentry)
 {
-	/*
-	 * If it's already been dropped, return OK.
-	 */
+	bool had_submounts = false;
 	spin_lock(&dentry->d_lock);
 	if (d_unhashed(dentry)) {
 		spin_unlock(&dentry->d_lock);
 		return;
 	}
+	__d_drop(dentry);
 	spin_unlock(&dentry->d_lock);
 
 	/* Negative dentries can be dropped without further checks */
-	if (!dentry->d_inode) {
-		d_drop(dentry);
+	if (!dentry->d_inode)
 		return;
-	}
 
+	shrink_dcache_parent(dentry);
 	for (;;) {
-		struct detach_data data;
-
-		data.mountpoint = NULL;
-		INIT_LIST_HEAD(&data.select.dispose);
-		data.select.start = dentry;
-		data.select.found = 0;
-
-		d_walk(dentry, &data, detach_and_collect, check_and_drop);
-
-		if (!list_empty(&data.select.dispose))
-			shrink_dentry_list(&data.select.dispose);
-		else if (!data.mountpoint)
+		struct dentry *victim = NULL;
+		d_walk(dentry, &victim, find_submount);
+		if (!victim) {
+			if (had_submounts)
+				shrink_dcache_parent(dentry);
 			return;
-
-		if (data.mountpoint) {
-			detach_mounts(data.mountpoint);
-			dput(data.mountpoint);
 		}
+		had_submounts = true;
+		detach_mounts(victim);
+		dput(victim);
 	}
 }
 EXPORT_SYMBOL(d_invalidate);
@@ -3112,7 +3078,7 @@ static enum d_walk_ret d_genocide_kill(void *data, struct dentry *dentry)
 
 void d_genocide(struct dentry *parent)
 {
-	d_walk(parent, parent, d_genocide_kill, NULL);
+	d_walk(parent, parent, d_genocide_kill);
 }
 
 EXPORT_SYMBOL(d_genocide);