linux-kernel - Re: [PATCH] mm/zswap: use workqueue to destroy pool

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Date:	Thu, 28 Apr 2016 13:09:49 +0900
From:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:	Dan Streetman <ddstreet@...e.org>, Yu Zhao <yuzhao@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Seth Jennings <sjenning@...hat.com>,
	Minchan Kim <minchan@...nel.org>,
	Nitin Gupta <ngupta@...are.org>, Linux-MM <linux-mm@...ck.org>,
	Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Dan Streetman <dan.streetman@...onical.com>
Subject: Re: [PATCH] mm/zswap: use workqueue to destroy pool

On (04/28/16 10:40), Sergey Senozhatsky wrote:
[..]
> the bigger issue here (and I was thinking at some point of fixing it,
> but then I grepped to see how many API users are in there, and I gave
> up) is that it seems we have no way to check if the dir exists in debugfs.

well, unless we want to do something like below. but I don't think Greg
will not buy it and the basic rule is to be careful in the driver code
and avoid any collisions.

---

 fs/debugfs/inode.c      | 48 ++++++++++++++++++++++++++++++++++++++++++++++++
 include/linux/debugfs.h |  7 +++++++
 2 files changed, 55 insertions(+)

diff --git a/fs/debugfs/inode.c b/fs/debugfs/inode.c
index 8580831..76cf851 100644
--- a/fs/debugfs/inode.c
+++ b/fs/debugfs/inode.c
@@ -709,6 +709,54 @@ exit:
 EXPORT_SYMBOL_GPL(debugfs_rename);
 
 /**
+ * debugfs_entry_exists - lookup file/directory name
+ *
+ * @name: a pointer to a string containing the name of the file/directory
+ *        to lookup.
+ * @parent: a pointer to the parent dentry.  This should be a directory
+ *          dentry if set. If this parameter is NULL, then the root of the
+ *          debugfs filesystem will be used.
+ *
+ * This function lookup a file/directory name in debugfs. If the
+ * name corresponds to positive dentry, the function will return %0.
+ *
+ * If debugfs is not enabled in the kernel, the value -%ENODEV will be
+ * returned.
+ */
+int debugfs_entry_exists(const char *name, struct dentry *parent)
+{
+	struct dentry *dentry;
+	int error;
+
+	if (IS_ERR(parent))
+		return PTR_ERR(parent);
+
+	error = simple_pin_fs(&debug_fs_type, &debugfs_mount,
+			      &debugfs_mount_count);
+	if (error)
+		return error;
+
+	if (!parent)
+		parent = debugfs_mount->mnt_root;
+
+	error = -EINVAL;
+	inode_lock(d_inode(parent));
+	dentry = lookup_one_len(name, parent, strlen(name));
+	if (IS_ERR(dentry)) {
+		error = PTR_ERR(dentry);
+	} else {
+		if (d_really_is_positive(dentry))
+			error = 0;
+		dput(dentry);
+	}
+
+	inode_unlock(d_inode(parent));
+	simple_release_fs(&debugfs_mount, &debugfs_mount_count);
+	return error;
+}
+EXPORT_SYMBOL_GPL(debugfs_entry_exists);
+
+/**
  * debugfs_initialized - Tells whether debugfs has been registered
  */
 bool debugfs_initialized(void)
diff --git a/include/linux/debugfs.h b/include/linux/debugfs.h
index 981e53a..5b6321e 100644
--- a/include/linux/debugfs.h
+++ b/include/linux/debugfs.h
@@ -124,6 +124,8 @@ ssize_t debugfs_read_file_bool(struct file *file, char __user *user_buf,
 ssize_t debugfs_write_file_bool(struct file *file, const char __user *user_buf,
 				size_t count, loff_t *ppos);
 
+int debugfs_entry_exists(const char *name, struct dentry *parent);
+
 #else
 
 #include <linux/err.h>
@@ -312,6 +314,11 @@ static inline ssize_t debugfs_write_file_bool(struct file *file,
 	return -ENODEV;
 }
 
+static inline int debugfs_entry_exists(const char *name, struct dentry *parent)
+{
+	return -ENODEV;
+}
+
 #endif
 
 #endif