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-rw-r--r--Documentation/filesystems/overlayfs.txt73
1 files changed, 69 insertions, 4 deletions
diff --git a/Documentation/filesystems/overlayfs.txt b/Documentation/filesystems/overlayfs.txt
index 7a05329..6ea1e64 100644
--- a/Documentation/filesystems/overlayfs.txt
+++ b/Documentation/filesystems/overlayfs.txt
@@ -203,10 +203,6 @@ 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").
-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.
-
Non-directories
---------------
@@ -334,6 +330,75 @@ 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.
+
+
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---------
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