On Thu, Jan 13, 2011 at 09:53:59PM +0000, David Howells wrote: > (1) struct vfsmount *(*d_automount)(struct path *path); > > This is used by follow_automount() in fs/namei.c to ask the filesystem > that owns the dentry at the current path point to mount something on > @path. > > It is called if DCACHE_NEED_AUTOMOUNT (which is propagated from the > S_AUTOMOUNT inode flag) is flagged on a dentry and if that dentry has > nothing mounted on it in the current namespace when someone attempts to > use that dentry. > > No locks will be held when this is called. > > d_op->d_automount() may return one of: > > (a) The vfsmount mounted upon that dentry, in which case pathwalk will > move to the root dentry of that vfsmount. ->d_automount() must have > in some manner mounted this before returning. > > (b) NULL if something was already mounted there, in which case pathwalk > will loop around and recheck the mountings. That makes very little sense as-is. Look: * autofs4 never does (a) * everybody else could replace (a) with (b) just fine - we do (a) only when we'd just mounted new vfsmount on top of path. So (b) would lead to follow_managed() looping over, finding DCACHE_MOUNTED and cheerfully transiting into the root of that vfsmount. Note that it's not subtle at all - *all* cases doing (a) lack ->d_manage(), so behaviour of follow_automount() is as simple as it gets. Now, I'd like to have (a) and (b) distinct, but not in that fashion. Namely, let's take do_add_mount() et.al. into follow_automount(). Leave autofs4 as in your series; it'll be completely unaffected. But switch all (b) in nfs/cifs/afs over to modified (a). That is, * have vfsmount created as it's done in your series * grab extra reference and put it on chosen list. That'd be done by helper in fs/namespace.c under namespace_sem. Extra ref would make sure that nobody walking the list would decide that it's expirable. * schedule whatever expiry activity we currently do. * return vfsmount In follow_automount() we'd see that we have non-NULL and non-ERR_PTR. Then we'd attempt do_add_mount(), without bothering to pass it expiry list. And do the same checks for return value, etc. we currently do in the method instances; just remember that we have an extra vfsmount reference that will need to be dropped and that we'll need to take the sucker off the expiry list in case we decide we don't need it (again, namespace.c helper). As the result, we stop abusing do_add_mount() in there. Moreover, with pending mnt_devname nfs rework we will be able to get rid of passing vfsmount to ->d_automount(), AFAICT, which would be nice... -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
