Linux Kernel Architecture

Chapter 8: The Virtual Filesystem

If a slave — or a private or unbindable mount — has to be established, the kernel has to rearrange the
mount data structure such that thevfsmountinstance under consideration is turned into a slave mount.
This is done bydo_make_slave. The function proceeds in several steps:

1. A master for the mount itself and any possible slave mounts needs to be found. First,
   the kernel searches among the shared-mount peers; the first one with the same root
   dentry is taken as the new master. If no such peer exists, the first element in the peer list
   is used.


2. If a new master has been found, both the mount under consideration and all its slave mounts
   are made slaves of the new master.


3. If the kernel could not find a new master, all slave mounts of the mount under consideration
   are freed — they do not have a master anymore.

In any case, theMNT_SHAREDflag is also removed.

Afterdo_make_slavehas performed these rearrangements,change_mnt_propagationneeds some more
steps for unbindable and private mounts.^17 In both cases, the mount is deleted from a slave list if it should
be on one, and the master is set toNULL— neither mount type has a master. For unbindable mounts, the
MNT_UNBINDABLEflag is set to identify it as such.

Shared subtrees obviously also influence the kernel behavior when new mounts are added to the sys-
tem. The crucial steps are taken inattach_recursive_mnt. Recall that the function has already been
touched on before, but the presentation was simplified. This time, I also include the effects of shared
subtrees.^18 First of all, the function needs to check into which mounts the mount event is supposed to
propagate.

fs/namespace.c

static int attach_recursive_mnt(struct vfsmount *source_mnt,

struct nameidata *nd, struct nameidata *parent_nd)

{

LIST_HEAD(tree_list);

struct vfsmount *dest_mnt = nd->mnt;

struct dentry *dest_dentry = nd->detnry;

struct vfsmount *child, *p;

if (propagate_mnt(dest_mnt, dest_dentry, source_mnt, &tree_list))

return -EINVAL;

...

propagate_mntiterates over all slave and shared mounts of the mount destination and mounts the new
filesystem into them usingmnt_set_montpoint. All mount points that are affected by this are returned
intree_list.

(^17) Since the function has already returned to the caller in the case of shared mounts, only these mount types remain and can be dif-
ferent fromMS_SLAVEin theifconditional.
(^18) Note that we also perform a slight simplification this time since we only consider add mounts, but no move mounts where an
existing mount is shifted from one place in the filesystem hierarchy to another.