ceph-mds infrastructure for fscrypt

tl;dr: we need to change the MDS infrastructure for fscrypt (again), and I want to do it in a way that would clean up some existing mess and more easily allow for future changes. The design is a bit odd though... Sorry for the long email here, but I needed communicate this design, and the rationale for the changes I'm proposing. First, the rationale: I've been (intermittently) working on the fscrypt implementation for cephfs, and have posted a few different draft proposals for the first part of it [1], which rely on a couple of changes in the MDS: - the alternate_names feature [2]. This is needed to handle extra-long filenames without allowing unprintable characters in the filename. - setting an "fscrypted" flag if the inode has an fscrypt context blob in encryption.ctx xattr [3]. With the filenames part more or less done, the next steps are to plumb in content encryption. Because the MDS handles truncates, we have to teach it to align those on fscrypt block boundaries. Rather than foist those details onto the MDS, the current idea is to add an opaque blob to the inode that would get updated along with size changes. The client would be responsible for filling out that field with the actual i_size, and would always round the existing size field up to the end of the last crypto block. That keeps the real size opaque to the MDS and the existing size handling logic should "just work". Regardless, that means we need another inode field for the size. Storing the context in an xattr is also proving to be problematic [4]. There are some situations where we can end up with an inode that is flagged as encrypted but doesn't have the caps to trust its xattrs. We could just treat "encryption.ctx" as special and not require Xs caps to read whatever cached value we have, and that might fix that issue, but I'm not fully convinced that's foolproof. We might end up with no cached context on a directory that is actually encrypted in some cases and not have a context. At this point, I'm thinking it might be best to unify all of the  per-inode info into a single field that the MDS would treat as opaque. Note that the alternate_names feature would remain more or less untouched since it's associated more with dentries than inodes. The initial version of this field would look something like this: struct ceph_fscrypt_context { u8 version; // == 1 struct fscrypt_context_v2 fscrypt_ctx; // 40 bytes __le32 blocksize // 4k for now __le64 size; // "real" i_size }; The MDS would send this along with any size updates (InodeStat, and MClientCaps replies). The client would need to send this in cap flushes/updates, and we'd also need to extend the SETATTR op too, so the client can update this field in truncates (at least). I don't look forward to having to plumb this into all of the different client ops that can create inodes though. What I'm thinking we might want to do is expose this field as the "ceph.fscrypt" vxattr. The client can stuff that into the xattr blob when creating a new inode, and the MDS can scrape it out of that and move the data into the correct field in the inode. A setxattr on this field would update the new field too. It's an ugly interface, but shouldn't be too bad to handle and we have some precedent for this sort of thing. The rules for handling the new field in the client would be a bit weird though. We'll need to allow it to reading the fscrypt_ctx part without any caps (since that should be static once it's set), but the size handling needs to be under the same caps as the traditional size field (Is that Fsx? The rules for this are never quite clear to me.) Would it be better to have two different fields here -- fscrypt_auth and fscrypt_file? Or maybe, fscrypt_static/_dynamic? We don't necessarily need to keep all of this info together, but it seemed neater that way. Thoughts? Opinions? Is this a horrible idea? What would be better? Thanks, -- Jeff Layton <jlayton(a)redhat.com> [1]: latest draft was posted here: https://lore.kernel.org/ceph-devel/53d5bebb28c1e0cd354a336a56bf103d5e3a6344… [2]: https://github.com/ceph/ceph/pull/37297 [3]: https://github.com/ceph/ceph/commit/7fe1c57846a42443f0258fd877d7166f33fd596f [4]: https://lore.kernel.org/ceph-devel/53d5bebb28c1e0cd354a336a56bf103d5e3a6344…