Merge branch 'overlayfs-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/mszeredi/vfs

Pull overlayfs updates from Miklos Szeredi:
 "This work from Amir adds NFS export capability to overlayfs. NFS
  exporting an overlay filesystem is a challange because we want to keep
  track of any copy-up of a file or directory between encoding the file
  handle and decoding it.

  This is achieved by indexing copied up objects by lower layer file
  handle. The index is already used for hard links, this patchset
  extends the use to NFS file handle decoding"

* 'overlayfs-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/mszeredi/vfs: (51 commits)
  ovl: check ERR_PTR() return value from ovl_encode_fh()
  ovl: fix regression in fsnotify of overlay merge dir
  ovl: wire up NFS export operations
  ovl: lookup indexed ancestor of lower dir
  ovl: lookup connected ancestor of dir in inode cache
  ovl: hash non-indexed dir by upper inode for NFS export
  ovl: decode pure lower dir file handles
  ovl: decode indexed dir file handles
  ovl: decode lower file handles of unlinked but open files
  ovl: decode indexed non-dir file handles
  ovl: decode lower non-dir file handles
  ovl: encode lower file handles
  ovl: copy up before encoding non-connectable dir file handle
  ovl: encode non-indexed upper file handles
  ovl: decode connected upper dir file handles
  ovl: decode pure upper file handles
  ovl: encode pure upper file handles
  ovl: document NFS export
  vfs: factor out helpers d_instantiate_anon() and d_alloc_anon()
  ovl: store 'has_upper' and 'opaque' as bit flags
  ...

Documentation/filesystems/overlayfs.txt

@@ -190,6 +190,20 @@ Mount options:
     Redirects are not created and not followed (equivalent to "redirect_dir=off"
     if "redirect_always_follow" feature is not enabled).
 
+When the NFS export feature is enabled, every copied up directory is
+indexed by the file handle of the lower inode and a file handle of the
+upper directory is stored in a "trusted.overlay.upper" extended attribute
+on the index entry.  On lookup of a merged directory, if the upper
+directory does not match the file handle stores in the index, that is an
+indication that multiple upper directories may be redirected to the same
+lower directory.  In that case, lookup returns an error and warns about
+a possible inconsistency.
+
+Because lower layer redirects cannot be verified with the index, enabling
+NFS export support on an overlay filesystem with no upper layer requires
+turning off redirect follow (e.g. "redirect_dir=nofollow").
+
+
 Non-directories
 ---------------
 
@@ -281,9 +295,9 @@ filesystem, so both st_dev and st_ino of the file may change.
 
 Any open files referring to this inode will access the old data.
 
-If a file with multiple hard links is copied up, then this will
-"break" the link.  Changes will not be propagated to other names
-referring to the same inode.
+Unless "inode index" feature is enabled, if a file with multiple hard
+links is copied up, then this will "break" the link.  Changes will not be
+propagated to other names referring to the same inode.
 
 Unless "redirect_dir" feature is enabled, rename(2) on a lower or merged
 directory will fail with EXDEV.
@@ -299,6 +313,92 @@ filesystem are not allowed.  If the underlying filesystem is changed,
 the behavior of the overlay is undefined, though it will not result in
 a crash or deadlock.
 
+When the overlay NFS export feature is enabled, overlay filesystems
+behavior on offline changes of the underlying lower layer is different
+than the behavior when NFS export is disabled.
+
+On every copy_up, an NFS file handle of the lower inode, along with the
+UUID of the lower filesystem, are encoded and stored in an extended
+attribute "trusted.overlay.origin" on the upper inode.
+
+When the NFS export feature is enabled, a lookup of a merged directory,
+that found a lower directory at the lookup path or at the path pointed
+to by the "trusted.overlay.redirect" extended attribute, will verify
+that the found lower directory file handle and lower filesystem UUID
+match the origin file handle that was stored at copy_up time.  If a
+found lower directory does not match the stored origin, that directory
+will not be merged with the upper directory.
+
+
+
+NFS export
+----------
+
+When the underlying filesystems supports NFS export and the "nfs_export"
+feature is enabled, an overlay filesystem may be exported to NFS.
+
+With the "nfs_export" feature, on copy_up of any lower object, an index
+entry is created under the index directory.  The index entry name is the
+hexadecimal representation of the copy up origin file handle.  For a
+non-directory object, the index entry is a hard link to the upper inode.
+For a directory object, the index entry has an extended attribute
+"trusted.overlay.upper" with an encoded file handle of the upper
+directory inode.
+
+When encoding a file handle from an overlay filesystem object, the
+following rules apply:
+
+1. For a non-upper object, encode a lower file handle from lower inode
+2. For an indexed object, encode a lower file handle from copy_up origin
+3. For a pure-upper object and for an existing non-indexed upper object,
+   encode an upper file handle from upper inode
+
+The encoded overlay file handle includes:
+ - Header including path type information (e.g. lower/upper)
+ - UUID of the underlying filesystem
+ - Underlying filesystem encoding of underlying inode
+
+This encoding format is identical to the encoding format file handles that
+are stored in extended attribute "trusted.overlay.origin".
+
+When decoding an overlay file handle, the following steps are followed:
+
+1. Find underlying layer by UUID and path type information.
+2. Decode the underlying filesystem file handle to underlying dentry.
+3. For a lower file handle, lookup the handle in index directory by name.
+4. If a whiteout is found in index, return ESTALE. This represents an
+   overlay object that was deleted after its file handle was encoded.
+5. For a non-directory, instantiate a disconnected overlay dentry from the
+   decoded underlying dentry, the path type and index inode, if found.
+6. For a directory, use the connected underlying decoded dentry, path type
+   and index, to lookup a connected overlay dentry.
+
+Decoding a non-directory file handle may return a disconnected dentry.
+copy_up of that disconnected dentry will create an upper index entry with
+no upper alias.
+
+When overlay filesystem has multiple lower layers, a middle layer
+directory may have a "redirect" to lower directory.  Because middle layer
+"redirects" are not indexed, a lower file handle that was encoded from the
+"redirect" origin directory, cannot be used to find the middle or upper
+layer directory.  Similarly, a lower file handle that was encoded from a
+descendant of the "redirect" origin directory, cannot be used to
+reconstruct a connected overlay path.  To mitigate the cases of
+directories that cannot be decoded from a lower file handle, these
+directories are copied up on encode and encoded as an upper file handle.
+On an overlay filesystem with no upper layer this mitigation cannot be
+used NFS export in this setup requires turning off redirect follow (e.g.
+"redirect_dir=nofollow").
+
+The overlay filesystem does not support non-directory connectable file
+handles, so exporting with the 'subtree_check' exportfs configuration will
+cause failures to lookup files over NFS.
+
+When the NFS export feature is enabled, all directory index entries are
+verified on mount time to check that upper file handles are not stale.
+This verification may cause significant overhead in some cases.
+
+
 Testsuite
 ---------