Allow auto-destruction of loop devices

This allows a flag to be set on loop devices so that when they are
closed for the last time, they'll self-destruct.

In general, so that we can automatically allocate loop devices (as with
losetup -f) and have them disappear when we're done with them.

In particular, right now, so that we can stop relying on the hackish
special-case in umount(8) which kills off loop devices which were set up by
'mount -oloop'.  That means we can stop putting crap in /etc/mtab which
doesn't belong there, which means it can be a symlink to /proc/mounts, which
means yet another writable file on the root filesystem is eliminated and the
'stateless' folks get happier...  and OLPC trac #356 can be closed.

The mount(8) side of that is at
http://marc.info/?l=util-linux-ng&m=119362955431694&w=2

[akpm@linux-foundation.org: coding-style fixes]
Signed-off-by: David Woodhouse <dwmw2@infradead.org>
Cc: Bernardo Innocenti <bernie@codewiz.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 9 insertions, 0 deletions

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index b8af22e610df..91ebb007416c 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -973,6 +973,10 @@ loop_set_status(struct loop_device *lo, const struct loop_info64 *info)
         lo->transfer = xfer->transfer;
         lo->ioctl = xfer->ioctl;
 
+        if ((lo->lo_flags & LO_FLAGS_AUTOCLEAR) !=
+             (info->lo_flags & LO_FLAGS_AUTOCLEAR))
+                lo->lo_flags ^= LO_FLAGS_AUTOCLEAR;
+
         lo->lo_encrypt_key_size = info->lo_encrypt_key_size;
         lo->lo_init[0] = info->lo_init[0];
         lo->lo_init[1] = info->lo_init[1];
@@ -1331,6 +1335,10 @@ static int lo_release(struct inode *inode, struct file *file)
 
         mutex_lock(&lo->lo_ctl_mutex);
         --lo->lo_refcnt;
+
+        if ((lo->lo_flags & LO_FLAGS_AUTOCLEAR) && !lo->lo_refcnt)
+                loop_clr_fd(lo, inode->i_bdev);
+
         mutex_unlock(&lo->lo_ctl_mutex);
 
         return 0;
diff --git a/include/linux/loop.h b/include/linux/loop.h
index 26a0a103898f..46169a7b559b 100644
--- a/include/linux/loop.h
+++ b/include/linux/loop.h
@@ -76,6 +76,7 @@ struct loop_device {
 enum {
         LO_FLAGS_READ_ONLY      = 1,
         LO_FLAGS_USE_AOPS       = 2,
+        LO_FLAGS_AUTOCLEAR      = 4,
 };
 
 #include <asm/posix_types.h>    /* for __kernel_old_dev_t */