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SubjectRe: [patch 3/6] vfs: mountinfo stable peer group id
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On Sat, 2008-03-22 at 16:27 +0000, Al Viro wrote:
> On Wed, Mar 19, 2008 at 07:37:51PM +0100, Miklos Szeredi wrote:
> > set_mnt_shared() is called from namespace.c as well, without
> > vfsmount_lock. But agreed, that's not the real issue.
>
> How about the following: let's separate set_mnt_shared() and inventing
> group ids. All we need is this:
> invent_group_ids(mnt) /* call under namespace_sem */
> for all vfsmounts p in subtree rooted at mnt
> if p->mnt_share is non-empty
> continue
> get ID for p
> if allocation fails
> goto cleanup
> return 0
> cleanup:
> for all vfsmounts q in subtree rooted at mnt
> if q == p
> break
> if q->mnt_share is non-empty
> continue
> release ID of q
> return -ENOMEM
>
> Now here's what we do:
> * in do_change_type(), outside of vfsmount_lock, do invent_group_ids()
> If it fails - bugger off, if not - proceed as now.

Has it to be done outside vfsmount_lock? AFAICT, invent_group_ids()
does not sleep, nor does change_mnt_propagation().

> * in attach_recursive_mnt() if IS_MNT_SHARED(dest_mnt) do
> invent_group_ids() on the dest_mnt immediately and if it fails do

I think you meant, invent_group_ids() on the source_mnt. But again
applying invent_group_ids() on the source_mnt has to be done carefully,
because, source_mnt may have been shared to begin with.

right?
RP

> umount_tree(dest_mnt, 0, ) under vfsmount_lock, then release_mounts()
> and bugger off (FWIW, we might want to lift the last part to caller
> and do the same to release_mounts() in propagate_mnt()). If it hadn't
> failed, we proceed as now.
> * in clone_mnt() do
> int new_group = group ID of old;
> int free_group = 0;
> if (flag & (CL_SLAVE | CL_PRIVATE))
> new_group = 0; /* not a peer of original */
> if ((flag & CL_MAKE_SHARED) && !new_group)
> new_group = allocate new ID
> if failed
> return 0;
> free_group = 1;
> }
> mnt = alloc_vfsmount();
> if (mnt) {
> set group ID of mnt to new_group;
> free_group = 0;
> /* as in mainline */
> }
> if (free_group)
> release group ID found in new_group;
> return mnt;
>
> then (after allocating new vfsmount) set its group ID to new_group if
> alloc_vfsmount() succeeds. Otherwise release group ID if needed and
> bugger off as usual.
>
> No need to mess with any additional exclusion for idr protection or with
> any kind of retries; allocation failure is allocation failure.
>
> Releasing group ID should be done from do_make_slave(), along with clearing
> group ID in vfsmount.
>
> Care to do that using mountinfo-base in vfs-2.6.git as base tree?



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