On Tue, 2017-02-07 at 17:54 -0800, Josh Triplett wrote: > On Tue, Feb 07, 2017 at 11:49:33AM -0800, Christoph Hellwig wrote: > > On Tue, Feb 07, 2017 at 11:02:03AM -0800, James Bottomley wrote: > > > > Another option would be to require something like a project > > > > as used > > > > for project quotas as the root. This would also be conveniant > > > > as it > > > > could storge the used remapping tables. > > > > > > So this would be like the current project quota except set on a > > > subtree? I could see it being done that way but I don't see what > > > advantage it has over using flags in the subtree itself (the > > > mapping is > > > known based on the mount namespace, so there's really only a > > > single bit > > > of information to store). > > > > projects (which are the underling concept for project quotas) are > > per-subtree in practice - the flag is set on an inode and then > > all directories and files underneath inherit the project ID, > > hardlinking outside a project is prohinited. > > I'm interested in having a VFS-level way to do more than just a > shift; I'd like to be able to arbitrarily remap IDs between what's on > disk and the system IDs. OK, so the shift is effectively an arbitrary remap because it allows multiple ranges to be mapped (althought the userns currently imposes a maximum number of five extents but that limit is a bit arbitrary just to try to limit the amount of space the parametrisation takes). See kernel/user_namespace.c:map_id_up/down() > If we're talking about developing a VFS-level solution for this, > I'd like to avoid limiting it to just a shift. (A shift/range > would definitely be the simplest solution for many common container > cases, but not all.) I assume the above satisfies you on this point, but raises the question: do you want an arbitrary shift not parametrised by a user namespace? If so how many such shifts do you want ... giving some details of the use case would be helpful. James
