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Diffstat (limited to 'Documentation/filesystems/overlayfs.txt')
| -rw-r--r-- | Documentation/filesystems/overlayfs.txt | 35 |
1 files changed, 35 insertions, 0 deletions
diff --git a/Documentation/filesystems/overlayfs.txt b/Documentation/filesystems/overlayfs.txt index 213547cb6d36..7a05329844d2 100644 --- a/Documentation/filesystems/overlayfs.txt +++ b/Documentation/filesystems/overlayfs.txt @@ -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 --------- |