We can now drop description of the ro/rw inconsistency from the documentation.
Also clarify, that now fully standard compliant behavior can be enabled with kernel/module/mount options. Signed-off-by: Miklos Szeredi <mszer...@redhat.com> --- Documentation/filesystems/overlayfs.txt | 64 ++++++++++++++++++++++----------- 1 file changed, 43 insertions(+), 21 deletions(-) diff --git a/Documentation/filesystems/overlayfs.txt b/Documentation/filesystems/overlayfs.txt index 961b287ef323..095186080d23 100644 --- a/Documentation/filesystems/overlayfs.txt +++ b/Documentation/filesystems/overlayfs.txt @@ -306,27 +306,49 @@ the copied layers will fail the verification of the lower root file handle. Non-standard behavior --------------------- -The copy_up operation essentially creates a new, identical file and -moves it over to the old name. Any open files referring to this inode -will access the old data. - -The new file may be on a different filesystem, so both st_dev and st_ino -of the real file may change. The values of st_dev and st_ino returned by -stat(2) on an overlay object are often not the same as the real file -stat(2) values to prevent the values from changing on copy_up. - -Unless "xino" feature is enabled, when overlay layers are not all on the -same underlying filesystem, the value of st_dev may be different for two -non-directory objects in the same overlay filesystem and the value of -st_ino for directory objects may be non persistent and could change even -while the overlay filesystem is still mounted. - -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. +Overlayfs can now act as a POSIX compliant filesystem with the following +features turned on: + +1) "redirect_dir" + +Enabled with the mount option or module option: "redirect_dir=on" or with +the kernel config option CONFIG_OVERLAY_FS_REDIRECT_DIR=y. + +If this feature is disabled, then rename(2) on a lower or merged directory +will fail with EXDEV ("Invalid cross-device link"). + +2) "inode index" + +Enabled with the mount option or module option "index=on" or with the +kernel config option CONFIG_OVERLAY_FS_INDEX=y. + +If this feature is disabled and 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. + +3) "xino" + +Enabled with the mount option "xino=auto" or "xino=on", with the module +option "xino_auto=on" or with the kernel config option +CONFIG_OVERLAY_FS_XINO_AUTO=y. Also implicitly enabled by using the same +underlying filesystem for all layers making up the overlay. + +If this feature is disabled or the underlying filesystem doesn't have +enough free bits in the inode number, then overlayfs will not be able to +guarantee that the values of st_ino and st_dev returned by stat(2) and the +value of d_ino returned by readdir(3) will act like on a normal filesystem. +E.g. the value of st_dev may be different for two objects in the same +overlay filesystem and the value of st_ino for directory objects may not be +persistent and could change even while the overlay filesystem is mounted. + +4) "copy_up_shared" + +Enabled with the mount option or module option "copy_up_shared=on" or with +the kernel config option CONFIG_OVERLAY_FS_COPY_UP_SHARED=y. + +If this feature is disabled, then a memory mapping created with MAP_SHARED +might contain stale data if the file has been copied up and modified in the +meantime. Changes to underlying filesystems -- 2.14.3