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ovl: redirect on rename-dir

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Current code returns EXDEV when a directory would need to be copied up to
move.  We could copy up the directory tree in this case, but there's
another, simpler solution: point to old lower directory from moved upper
directory.

This is achieved with a "trusted.overlay.redirect" xattr storing the path
relative to the root of the overlay.  After such attribute has been set,
the directory can be moved without further actions required.

This is a backward incompatible feature, old kernels won't be able to
correctly mount an overlay containing redirected directories.

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
This commit is contained in:
Miklos Szeredi
2016-12-16 11:02:56 +01:00
parent 02b69b284c
commit a6c6065511
7 changed files with 195 additions and 30 deletions
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21
Documentation/filesystems/overlayfs.txt
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@@ -130,6 +130,23 @@ directory.
Readdir on directories that are not merged is simply handled by the
underlying directory (upper or lower).
renaming directories
--------------------
When renaming a directory that is on the lower layer or merged (i.e. the
directory was not created on the upper layer to start with) overlayfs can
handle it in two different ways:
1) return EXDEV error: this error is returned by rename(2) when trying to
move a file or directory across filesystem boundaries. Hence
applications are usually prepared to hande this error (mv(1) for example
recursively copies the directory tree). This is the default behavior.
2) If the "redirect_dir" feature is enabled, then the directory will be
copied up (but not the contents). Then the "trusted.overlay.redirect"
extended attribute is set to the path of the original location from the
root of the overlay. Finally the directory is moved to the new
location.
Non-directories
---------------
@@ -189,8 +206,8 @@ 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.
Directory trees are not copied up. If rename(2) is performed on a directory
which is on the lower layer or is merged, then -EXDEV will be returned.
Unless "redirect_dir" feature is enabled, rename(2) on a lower or merged
directory will fail with EXDEV.
Changes to underlying filesystems
---------------------------------