Based on kernel version 4.1. Page generated on 2015-06-28 12:12 EST.
1 Support is available for filesystems that wish to do automounting support (such 2 as kAFS which can be found in fs/afs/). This facility includes allowing 3 in-kernel mounts to be performed and mountpoint degradation to be 4 requested. The latter can also be requested by userspace. 5 6 7 ====================== 8 IN-KERNEL AUTOMOUNTING 9 ====================== 10 11 A filesystem can now mount another filesystem on one of its directories by the 12 following procedure: 13 14 (1) Give the directory a follow_link() operation. 15 16 When the directory is accessed, the follow_link op will be called, and 17 it will be provided with the location of the mountpoint in the nameidata 18 structure (vfsmount and dentry). 19 20 (2) Have the follow_link() op do the following steps: 21 22 (a) Call vfs_kern_mount() to call the appropriate filesystem to set up a 23 superblock and gain a vfsmount structure representing it. 24 25 (b) Copy the nameidata provided as an argument and substitute the dentry 26 argument into it the copy. 27 28 (c) Call do_add_mount() to install the new vfsmount into the namespace's 29 mountpoint tree, thus making it accessible to userspace. Use the 30 nameidata set up in (b) as the destination. 31 32 If the mountpoint will be automatically expired, then do_add_mount() 33 should also be given the location of an expiration list (see further 34 down). 35 36 (d) Release the path in the nameidata argument and substitute in the new 37 vfsmount and its root dentry. The ref counts on these will need 38 incrementing. 39 40 Then from userspace, you can just do something like: 41 42 [root@andromeda root]# mount -t afs \#root.afs. /afs 43 [root@andromeda root]# ls /afs 44 asd cambridge cambridge.redhat.com grand.central.org 45 [root@andromeda root]# ls /afs/cambridge 46 afsdoc 47 [root@andromeda root]# ls /afs/cambridge/afsdoc/ 48 ChangeLog html LICENSE pdf RELNOTES-1.2.2 49 50 And then if you look in the mountpoint catalogue, you'll see something like: 51 52 [root@andromeda root]# cat /proc/mounts 53 ... 54 #root.afs. /afs afs rw 0 0 55 #root.cell. /afs/cambridge.redhat.com afs rw 0 0 56 #afsdoc. /afs/cambridge.redhat.com/afsdoc afs rw 0 0 57 58 59 =========================== 60 AUTOMATIC MOUNTPOINT EXPIRY 61 =========================== 62 63 Automatic expiration of mountpoints is easy, provided you've mounted the 64 mountpoint to be expired in the automounting procedure outlined above. 65 66 To do expiration, you need to follow these steps: 67 68 (3) Create at least one list off which the vfsmounts to be expired can be 69 hung. Access to this list will be governed by the vfsmount_lock. 70 71 (4) In step (2c) above, the call to do_add_mount() should be provided with a 72 pointer to this list. It will hang the vfsmount off of it if it succeeds. 73 74 (5) When you want mountpoints to be expired, call mark_mounts_for_expiry() 75 with a pointer to this list. This will process the list, marking every 76 vfsmount thereon for potential expiry on the next call. 77 78 If a vfsmount was already flagged for expiry, and if its usage count is 1 79 (it's only referenced by its parent vfsmount), then it will be deleted 80 from the namespace and thrown away (effectively unmounted). 81 82 It may prove simplest to simply call this at regular intervals, using 83 some sort of timed event to drive it. 84 85 The expiration flag is cleared by calls to mntput. This means that expiration 86 will only happen on the second expiration request after the last time the 87 mountpoint was accessed. 88 89 If a mountpoint is moved, it gets removed from the expiration list. If a bind 90 mount is made on an expirable mount, the new vfsmount will not be on the 91 expiration list and will not expire. 92 93 If a namespace is copied, all mountpoints contained therein will be copied, 94 and the copies of those that are on an expiration list will be added to the 95 same expiration list. 96 97 98 ======================= 99 USERSPACE DRIVEN EXPIRY 100 ======================= 101 102 As an alternative, it is possible for userspace to request expiry of any 103 mountpoint (though some will be rejected - the current process's idea of the 104 rootfs for example). It does this by passing the MNT_EXPIRE flag to 105 umount(). This flag is considered incompatible with MNT_FORCE and MNT_DETACH. 106 107 If the mountpoint in question is in referenced by something other than 108 umount() or its parent mountpoint, an EBUSY error will be returned and the 109 mountpoint will not be marked for expiration or unmounted. 110 111 If the mountpoint was not already marked for expiry at that time, an EAGAIN 112 error will be given and it won't be unmounted. 113 114 Otherwise if it was already marked and it wasn't referenced, unmounting will 115 take place as usual. 116 117 Again, the expiration flag is cleared every time anything other than umount() 118 looks at a mountpoint.