On Fri, Sep 25, 2020 at 11:35 AM Pavel Tikhomirov
<ptikhomirov@xxxxxxxxxxxxx> wrote:
>
> This replaces uuid with null in overelayfs file handles and thus relaxes
typo: overelayfs
> uuid checks for overlay index feature. It is only possible in case there
> is only one filesystem for all the work/upper/lower directories and bare
> file handles from this backing filesystem are uniq. In other case when
> we have multiple filesystems lets just fallback to "uuid=on" which is
> and equivalent of how it worked before with all uuid checks.
>
typo: uniq
> This is needed when overlayfs is/was mounted in a container with index
> enabled (e.g.: to be able to resolve inotify watch file handles on it to
> paths in CRIU), and this container is copied and started alongside with
> the original one. This way the "copy" container can't have the same uuid
> on the superblock and mounting the overlayfs from it later would fail.
>
> Note: In our (Virtuozzo) use case users inside a container can create
> "regular" overlayfs mounts without any "index=" option, but we still
> want to migrate this containers with CRIU so we set "index=on" as kernel
this container
> default so that all the container overlayfs mounts get support of file
> handles automatically. With "uuid=off" we want the same thing (to be
> able to "copy" container with uuid change) - we would set kernel default
> so that all the container overlayfs mounts get "uuid=off" automatically.
>
> That is an example of the problem on top of loop+ext4:
>
> dd if=/dev/zero of=loopbackfile.img bs=100M count=10
> losetup -fP loopbackfile.img
> losetup -a
> #/dev/loop0: [64768]:35 (/loop-test/loopbackfile.img)
> mkfs.ext4 loopbackfile.img
> mkdir loop-mp
> mount -o loop /dev/loop0 loop-mp
> mkdir loop-mp/{lower,upper,work,merged}
> mount -t overlay overlay -oindex=on,lowerdir=loop-mp/lower,\
> upperdir=loop-mp/upper,workdir=loop-mp/work loop-mp/merged
> umount loop-mp/merged
> umount loop-mp
> e2fsck -f /dev/loop0
> tune2fs -U random /dev/loop0
>
> mount -o loop /dev/loop0 loop-mp
> mount -t overlay overlay -oindex=on,lowerdir=loop-mp/lower,\
> upperdir=loop-mp/upper,workdir=loop-mp/work loop-mp/merged
> #mount: /loop-test/loop-mp/merged:
> #mount(2) system call failed: Stale file handle.
>
> If you just change the uuid of the backing filesystem, overlay is not
> mounting any more. In Virtuozzo we copy container disks (ploops) when
> crate the copy of container and we require fs uuid to be uniq for a new
typos: crat, uniq
> container.
>
> CC: Amir Goldstein <amir73il@xxxxxxxxx>
> CC: Vivek Goyal <vgoyal@xxxxxxxxxx>
> CC: Miklos Szeredi <miklos@xxxxxxxxxx>
> CC: linux-unionfs@xxxxxxxxxxxxxxx
> CC: linux-kernel@xxxxxxxxxxxxxxx
> Signed-off-by: Pavel Tikhomirov <ptikhomirov@xxxxxxxxxxxxx>
>
Apart from some typos, looks good to me.
You may add:
Reviewed-by: Amir Goldstein <amir73il@xxxxxxxxx>
Please do not post v5. you should wait for more feedback from others.
> ---
> v2: in v1 I missed actual uuid check skip
> v3: rebase to overlayfs-next, replace uuid with null in file handles,
> split ovl_fs propagation to function arguments to separate patch, add
> separate bool "uuid=on/off" option, move numfs check up, add doc note.
> v4: get rid of double negatives, remove nouuid leftower comment, fix
> missprint in kernel config name
>
> Documentation/filesystems/overlayfs.rst | 6 ++++++
> fs/overlayfs/Kconfig | 19 +++++++++++++++++++
> fs/overlayfs/copy_up.c | 3 ++-
> fs/overlayfs/namei.c | 4 +++-
> fs/overlayfs/ovl_entry.h | 1 +
> fs/overlayfs/super.c | 25 +++++++++++++++++++++++++
> 6 files changed, 56 insertions(+), 2 deletions(-)
>
> diff --git a/Documentation/filesystems/overlayfs.rst b/Documentation/filesystems/overlayfs.rst
> index 580ab9a0fe31..4f9cc20f255c 100644
> --- a/Documentation/filesystems/overlayfs.rst
> +++ b/Documentation/filesystems/overlayfs.rst
> @@ -563,6 +563,12 @@ This verification may cause significant overhead in some cases.
> Note: the mount options index=off,nfs_export=on are conflicting for a
> read-write mount and will result in an error.
>
> +Note: the mount option uuid=off (or corresponding module param, or kernel
> +config) can be used to replace UUID of the underlying filesystem in file
> +handles with null, and effectively disable UUID checks. This can be useful in
> +case the underlying disk is copied and the UUID of this copy is changed. This
> +is only applicable if all lower/upper/work directories are on the same
> +filesystem, otherwise it will fallback to normal behaviour.
>
> Volatile mount
> --------------
> diff --git a/fs/overlayfs/Kconfig b/fs/overlayfs/Kconfig
> index dd188c7996b3..c21abdb43206 100644
> --- a/fs/overlayfs/Kconfig
> +++ b/fs/overlayfs/Kconfig
> @@ -61,6 +61,25 @@ config OVERLAY_FS_INDEX
>
> If unsure, say N.
>
> +config OVERLAY_FS_INDEX_UUID
> + bool "Overlayfs: export uuid in file handles"
> + default y
> + depends on OVERLAY_FS
> + help
> + If this config option is disabled then overlay will replace uuid with
> + null in overlayfs file handles, effectively disabling uuid checks for
> + them. This affects overlayfs mounted with "index=on". This only can be
> + done if all upper and lower directories are on the same filesystem
> + where basic fhandles are uniq. In case the latter is not true
There are some typos in it, but I think this phrase can be dropped:
"where basic fhandles are uniq"
Thanks,
Amir.
