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1 | .. SPDX-License-Identifier: GPL-2.0 |
2 | ||
3 | ================= | |
4 | Automount Support | |
5 | ================= | |
6 | ||
7 | ||
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8 | Support is available for filesystems that wish to do automounting |
9 | support (such as kAFS which can be found in fs/afs/ and NFS in | |
10 | fs/nfs/). This facility includes allowing in-kernel mounts to be | |
11 | performed and mountpoint degradation to be requested. The latter can | |
12 | also be requested by userspace. | |
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13 | |
14 | ||
41defb4d | 15 | In-Kernel Automounting |
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16 | ====================== |
17 | ||
a83aaf49 | 18 | See section "Mount Traps" of Documentation/filesystems/autofs.rst |
1da177e4 | 19 | |
41defb4d | 20 | Then from userspace, you can just do something like:: |
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21 | |
22 | [root@andromeda root]# mount -t afs \#root.afs. /afs | |
23 | [root@andromeda root]# ls /afs | |
24 | asd cambridge cambridge.redhat.com grand.central.org | |
25 | [root@andromeda root]# ls /afs/cambridge | |
26 | afsdoc | |
27 | [root@andromeda root]# ls /afs/cambridge/afsdoc/ | |
28 | ChangeLog html LICENSE pdf RELNOTES-1.2.2 | |
29 | ||
41defb4d | 30 | And then if you look in the mountpoint catalogue, you'll see something like:: |
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31 | |
32 | [root@andromeda root]# cat /proc/mounts | |
33 | ... | |
34 | #root.afs. /afs afs rw 0 0 | |
35 | #root.cell. /afs/cambridge.redhat.com afs rw 0 0 | |
36 | #afsdoc. /afs/cambridge.redhat.com/afsdoc afs rw 0 0 | |
37 | ||
38 | ||
41defb4d | 39 | Automatic Mountpoint Expiry |
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40 | =========================== |
41 | ||
42 | Automatic expiration of mountpoints is easy, provided you've mounted the | |
99ff6cf0 | 43 | mountpoint to be expired in the automounting procedure outlined separately. |
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44 | |
45 | To do expiration, you need to follow these steps: | |
46 | ||
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47 | (1) Create at least one list off which the vfsmounts to be expired can be |
48 | hung. | |
1da177e4 | 49 | |
99ff6cf0 | 50 | (2) When a new mountpoint is created in the ->d_automount method, add |
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51 | the mnt to the list using mnt_set_expiry():: |
52 | ||
99ff6cf0 | 53 | mnt_set_expiry(newmnt, &afs_vfsmounts); |
1da177e4 | 54 | |
99ff6cf0 | 55 | (3) When you want mountpoints to be expired, call mark_mounts_for_expiry() |
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56 | with a pointer to this list. This will process the list, marking every |
57 | vfsmount thereon for potential expiry on the next call. | |
58 | ||
59 | If a vfsmount was already flagged for expiry, and if its usage count is 1 | |
60 | (it's only referenced by its parent vfsmount), then it will be deleted | |
61 | from the namespace and thrown away (effectively unmounted). | |
62 | ||
63 | It may prove simplest to simply call this at regular intervals, using | |
64 | some sort of timed event to drive it. | |
65 | ||
66 | The expiration flag is cleared by calls to mntput. This means that expiration | |
67 | will only happen on the second expiration request after the last time the | |
68 | mountpoint was accessed. | |
69 | ||
70 | If a mountpoint is moved, it gets removed from the expiration list. If a bind | |
71 | mount is made on an expirable mount, the new vfsmount will not be on the | |
72 | expiration list and will not expire. | |
73 | ||
74 | If a namespace is copied, all mountpoints contained therein will be copied, | |
75 | and the copies of those that are on an expiration list will be added to the | |
76 | same expiration list. | |
77 | ||
78 | ||
41defb4d | 79 | Userspace Driven Expiry |
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80 | ======================= |
81 | ||
82 | As an alternative, it is possible for userspace to request expiry of any | |
83 | mountpoint (though some will be rejected - the current process's idea of the | |
84 | rootfs for example). It does this by passing the MNT_EXPIRE flag to | |
85 | umount(). This flag is considered incompatible with MNT_FORCE and MNT_DETACH. | |
86 | ||
87 | If the mountpoint in question is in referenced by something other than | |
88 | umount() or its parent mountpoint, an EBUSY error will be returned and the | |
89 | mountpoint will not be marked for expiration or unmounted. | |
90 | ||
91 | If the mountpoint was not already marked for expiry at that time, an EAGAIN | |
92 | error will be given and it won't be unmounted. | |
93 | ||
94 | Otherwise if it was already marked and it wasn't referenced, unmounting will | |
95 | take place as usual. | |
96 | ||
97 | Again, the expiration flag is cleared every time anything other than umount() | |
98 | looks at a mountpoint. |