ext4: update the s_last_mounted field in the superblock

This field can be very helpful when a system administrator is trying
to sort through large numbers of block devices or filesystem images.
What is stored in this field can be ambiguous if multiple filesystem
namespaces are in play; what we store in practice is the mountpoint
interpreted by the process's namespace which first opens a file in the
filesystem.
Signed-off-by: N"Theodore Ts'o" <tytso@mit.edu>

fs/ext4/ext4.h

@@ -834,6 +834,12 @@ struct ext4_super_block {
 };
 
 #ifdef __KERNEL__
+
+/*
+ * Mount flags
+ */
+#define EXT4_MF_MNTDIR_SAMPLED	0x0001
+
 /*
  * fourth extended-fs super-block data in memory
  */
@@ -853,6 +859,7 @@ struct ext4_sb_info {
 	struct ext4_super_block *s_es;	/* Pointer to the super block in the buffer */
 	struct buffer_head **s_group_desc;
 	unsigned int s_mount_opt;
+	unsigned int s_mount_flags;
 	ext4_fsblk_t s_sb_block;
 	uid_t s_resuid;
 	gid_t s_resgid;

fs/ext4/file.c

@@ -21,6 +21,8 @@
 #include <linux/time.h>
 #include <linux/fs.h>
 #include <linux/jbd2.h>
+#include <linux/mount.h>
+#include <linux/path.h>
 #include "ext4.h"
 #include "ext4_jbd2.h"
 #include "xattr.h"
@@ -145,6 +147,38 @@ static int ext4_file_mmap(struct file *file, struct vm_area_struct *vma)
 	return 0;
 }
 
+static int ext4_file_open(struct inode * inode, struct file * filp)
+{
+	struct super_block *sb = inode->i_sb;
+	struct ext4_sb_info *sbi = EXT4_SB(inode->i_sb);
+	struct vfsmount *mnt = filp->f_path.mnt;
+	struct path path;
+	char buf[64], *cp;
+
+	if (unlikely(!(sbi->s_mount_flags & EXT4_MF_MNTDIR_SAMPLED) &&
+		     !(sb->s_flags & MS_RDONLY))) {
+		sbi->s_mount_flags |= EXT4_MF_MNTDIR_SAMPLED;
+		/*
+		 * Sample where the filesystem has been mounted and
+		 * store it in the superblock for sysadmin convenience
+		 * when trying to sort through large numbers of block
+		 * devices or filesystem images.
+		 */
+		memset(buf, 0, sizeof(buf));
+		path.mnt = mnt->mnt_parent;
+		path.dentry = mnt->mnt_mountpoint;
+		path_get(&path);
+		cp = d_path(&path, buf, sizeof(buf));
+		path_put(&path);
+		if (!IS_ERR(cp)) {
+			memcpy(sbi->s_es->s_last_mounted, cp,
+			       sizeof(sbi->s_es->s_last_mounted));
+			sb->s_dirt = 1;
+		}
+	}
+	return generic_file_open(inode, filp);
+}
+
 const struct file_operations ext4_file_operations = {
 	.llseek		= generic_file_llseek,
 	.read		= do_sync_read,
@@ -156,7 +190,7 @@ const struct file_operations ext4_file_operations = {
 	.compat_ioctl	= ext4_compat_ioctl,
 #endif
 	.mmap		= ext4_file_mmap,
-	.open		= generic_file_open,
+	.open		= ext4_file_open,
 	.release	= ext4_release_file,
 	.fsync		= ext4_sync_file,
 	.splice_read	= generic_file_splice_read,