On Thu, Dec 14, 2017 at 4:13 PM, Miklos Szeredi <miklos@xxxxxxxxxx> wrote: > On Thu, Dec 14, 2017 at 7:47 AM, zhangyi (F) <yi.zhang@xxxxxxxxxx> wrote: >> fsck.overlay >> ============ >> >> fsck.overlay is used to check and optionally repair underlying >> directories of overlay-filesystem. > > Thanks for working on this. > > >> >> Check the following points: >> >> Whiteouts >> --------- >> >> A whiteout is a character device with 0/0 device number. It is used to >> record the removed files or directories, When a whiteout is found in a >> directory, there should be at least one directory or file with the same >> name in any of the corresponding lower layers. If not exist, the whiteout >> will be treated as orphan whiteout and remove. > > Okay. > >> >> Opaque directories >> ------------------ >> >> An opaque directory is a directory with "trusted.overlay.opaque" xattr >> valued "y". There are two legal situations of making opaque directory: 1) >> create directory over whiteout 2) creat directory in merged directory. If an >> opaque directory is found, corresponding matching name in lower layers might >> exist or parent directory might merged, If not, the opaque xattr will be >> treated as invalid and remove. > > Current version of overlay fs doesn't bother with removing opaque > attribute. So not sure fsck.overlay should care. Or at least is > should be an optional thing and not worth warning about, since it's > perfectly normal. > > >> >> Redirect directories >> -------------------- >> >> An redirect directory is a directory with "trusted.overlay.redirect" >> xattr valued to the path of the original location from the root of the >> overlay. It is only used when renaming a directory and "redirect dir" >> feature is enabled. If an redirect directory is found, the following >> must be met: >> >> 1) The directory store in redirect xattr should exist in one of lower >> layers. > > Okay. > >> 2) The origin directory should be redirected only once in one layer, >> which mean there is only one redirect xattr point to this origin directory in >> the specified layer. >> 3) A whiteout or an opaque directory with the same name to origin should >> exist in the same directory as the redirect directory. >> >> If not, 1) The redirect xattr is invalid and need to remove 2) One of >> the redirect xattr is redundant but not sure which one is, ask user 3) >> Create a whiteout device or set opaque xattr to an existing directory if the >> parent directory was meregd or remove xattr if not. > > Hmm, in this case also should ask the user, as it's not clear that the > "new" copy resulting from removal of whiteout on upper is the wanted > one or the "old" renamed copy. > > >> >> Usage >> ===== >> >> 1. Ensure overlay filesystem is not mounted based on directories which >> need to check. >> >> 2. Run fsck.overlay program: >> Usage: >> fsck.overlay [-l lowerdir] [-u upperdir] [-w workdir] [-avhV] >> >> Options: >> -l, --lowerdir=LOWERDIR specify lower directories of overlayfs, >> multiple lower use ':' as separator. >> -u, --upperdir=UPPERDIR specify upper directory of overlayfs >> -w, --workdir=WORKDIR specify work directory of overlayfs > > Not sure what other fsck do, but I'd feel more logical to have the > same -olowerdir=..., etc. notation as for mount. > Other fsck do not have this problem. They only need blockdev as input. Which leads me to an idea I have been wondering about for the overlayfs utilities - a specification file, e.g.: # create dirs and write their path to a spec file: mkfs.overlay -ometacopy=on,lowerdir=... myovl.spec # mount overlay using mount.overlay helper: mount -t overlay myovl.spec /ovl # fsck with just one configuration that is consistent with mkfs and mount: fsck.overlay -n myovl.spec The specification file can also determine the backward incompatible features of the overlay, for example, if user sets -metacopy=on during mkfs mount.overlay helper will refuse to mount with kernel that does not support metacopy. The reason we CAN do that with spec file is because spec file determines that overlay was born with metacopy feature enabled. It is not the same an overlay that was once mounted with metacopy=on and then we don't allow it to mount with an old kernel. This method could delegate the entire feature compatibility to userspace mount helper. Another nice thing about mount.overlay helper is that it could abstarct away the move from mount() to mount2() when the time comes and then users can keep using existing tools but won't have the existing limit on number of lowerdirs. Thoughts? Amir. -- To unsubscribe from this list: send the line "unsubscribe linux-unionfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
