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ovl: add support for "nfs_export" configuration

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Introduce the "nfs_export" config, module and mount options.

The NFS export feature depends on the "index" feature and enables two
implicit overlayfs features: "index_all" and "verify_lower".
The "index_all" feature creates an index on copy up of every file and
directory. The "verify_lower" feature uses the full index to detect
overlay filesystems inconsistencies on lookup, like redirect from
multiple upper dirs to the same lower dir.

NFS export can be enabled for non-upper mount with no index. However,
because lower layer redirects cannot be verified with the index, enabling
NFS export support on an overlay with no upper layer requires turning off
redirect follow (e.g. "redirect_dir=nofollow").

The full index may incur some overhead on mount time, especially when
verifying that lower directory file handles are not stale.

NFS export support, full index and consistency verification will be
implemented by following patches.

Signed-off-by: Amir Goldstein <[email protected]>
Signed-off-by: Miklos Szeredi <[email protected]>
This commit is contained in:
Amir Goldstein 2018-01-19 11:26:53 +02:00 • committed by Miklos Szeredi
commit f168f1098d
6 changed files with 120 additions and 5 deletions
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35
Documentation/filesystems/overlayfs.txt
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@ -190,6 +190,24 @@ 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").
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
---------------
@ -299,6 +317,23 @@ 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.
Testsuite
---------