[PATCH 2/2] Btrfs: fix wrong the mount information in /proc

[PATCH 2/2] Btrfs: fix wrong the mount information in /proc

[Miao Xie]

If we remove the disk that is specified when mounting, the mount information
in /proc can not be updated. It will make us can not add that disk back into
the filesystem. This patch fix this problem by implement the show_devname()
interface and choose the name of the device with minimum device id to show.

Signed-off-by: Miao Xie <miaox@cn.fujitsu.com>
---
 fs/btrfs/super.c |   31 +++++++++++++++++++++++++++++++
 1 files changed, 31 insertions(+), 0 deletions(-)

diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index c5f8fca..99e5671 100644
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -41,6 +41,7 @@
 #include <linux/slab.h>
 #include <linux/cleancache.h>
 #include <linux/ratelimit.h>
+#include "../mount.h"
 #include "compat.h"
 #include "delayed-inode.h"
 #include "ctree.h"
@@ -892,6 +893,35 @@ static int btrfs_show_options(struct seq_file *seq, struct dentry *dentry)
 	return 0;
 }
 
+static int btrfs_show_devname(struct seq_file *m, struct vfsmount *mnt)
+{
+	struct btrfs_fs_info *fs_info = btrfs_sb(mnt->mnt_sb);
+	struct mount *r = real_mount(mnt);
+	struct btrfs_fs_devices *cur_devices;
+	struct btrfs_device *dev;
+	struct list_head *head;
+	struct btrfs_device *first_dev = NULL;
+
+	mutex_lock(&fs_info->fs_devices->device_list_mutex);
+	cur_devices = fs_info->fs_devices;
+	while (cur_devices) {
+		head = &cur_devices->devices;
+		list_for_each_entry(dev, head, dev_list) {
+			if (!strcmp(r->mnt_devname, dev->name)) {
+				seq_escape(m, r->mnt_devname, " \t\n\\");
+				goto out;
+			}
+			if (!first_dev || dev->devid < first_dev->devid)
+				first_dev = dev;
+		}
+		cur_devices = cur_devices->seed;
+	}
+	seq_escape(m, first_dev->name, " \t\n\\");
+out:
+	mutex_unlock(&fs_info->fs_devices->device_list_mutex);
+	return 0;
+}
+
 static int btrfs_test_super(struct super_block *s, void *data)
 {
 	struct btrfs_fs_info *p = data;
@@ -1467,6 +1497,7 @@ static const struct super_operations btrfs_super_ops = {
 	.put_super	= btrfs_put_super,
 	.sync_fs	= btrfs_sync_fs,
 	.show_options	= btrfs_show_options,
+	.show_devname	= btrfs_show_devname,
 	.write_inode	= btrfs_write_inode,
 	.dirty_inode	= btrfs_fs_dirty_inode,
 	.alloc_inode	= btrfs_alloc_inode,
-- 
1.7.6.5

Re: [PATCH 2/2] Btrfs: fix wrong the mount information in /proc

[Al Viro]

On Thu, May 31, 2012 at 06:25:14PM +0800, Miao Xie wrote:
> +#include "../mount.h"

No.

> +	struct mount *r = real_mount(mnt);

And even more so.  Find a way to do that without layering violations or
live with the current behaviour.

Both patches NAKed with extreme prejudice.  Don't bring them back, you
are *not* going to get them in.  Any filesystem code that wants to do
that kind of thing is seriously out of luck.

Re: [PATCH 2/2] Btrfs: fix wrong the mount information in /proc

[Al Viro]

On Thu, May 31, 2012 at 03:36:05PM +0100, Al Viro wrote:
> On Thu, May 31, 2012 at 06:25:14PM +0800, Miao Xie wrote:
> > +#include "../mount.h"
> 
> No.
> 
> > +	struct mount *r = real_mount(mnt);
> 
> And even more so.  Find a way to do that without layering violations or
> live with the current behaviour.
> 
> Both patches NAKed with extreme prejudice.  Don't bring them back, you
> are *not* going to get them in.  Any filesystem code that wants to do
> that kind of thing is seriously out of luck.

BTW, resulting behaviour is bogus, regardless of layering violations  -
note that device_list_add() renames existing btrfs_device.  *And* does so
without holding ->device_list_mutex, so your patch would've cheerfully
walked right into strcmp() vs. kfree() races.  And while the order of
kfree vs. reassignment in device_list_add() is clearly wrong, flipping it
won't fix that problem.

Try to formulate the rules for names you are generating - when do they
change, what can we say about two mounts by comparing these names,
when can two that used to give different names start giving the same
and vice versa.  You won't be happy with the result.

Re: [PATCH 2/2] Btrfs: fix wrong the mount information in /proc

[Josef Bacik]

On Thu, May 31, 2012 at 04:53:04PM +0100, Al Viro wrote:
> On Thu, May 31, 2012 at 03:36:05PM +0100, Al Viro wrote:
> > On Thu, May 31, 2012 at 06:25:14PM +0800, Miao Xie wrote:
> > > +#include "../mount.h"
> > 
> > No.
> > 
> > > +	struct mount *r = real_mount(mnt);
> > 
> > And even more so.  Find a way to do that without layering violations or
> > live with the current behaviour.
> > 
> > Both patches NAKed with extreme prejudice.  Don't bring them back, you
> > are *not* going to get them in.  Any filesystem code that wants to do
> > that kind of thing is seriously out of luck.
> 
> BTW, resulting behaviour is bogus, regardless of layering violations  -
> note that device_list_add() renames existing btrfs_device.  *And* does so
> without holding ->device_list_mutex, so your patch would've cheerfully
> walked right into strcmp() vs. kfree() races.  And while the order of
> kfree vs. reassignment in device_list_add() is clearly wrong, flipping it
> won't fix that problem.
> 
> Try to formulate the rules for names you are generating - when do they
> change, what can we say about two mounts by comparing these names,
> when can two that used to give different names start giving the same
> and vice versa.  You won't be happy with the result.

I'm fixing this up right now and then I'll also fix the show_devname thing, we
can just keep track of which device mount() was called with so that we can print
that out and then if that device is ever removed we can just print out something
different.  Thanks,

Josef