Date:   Fri, 25 Sep 2020 19:42:51 +0300
From:   Amir Goldstein <amir73il@...il.com>
To:     Pavel Tikhomirov <ptikhomirov@...tuozzo.com>
Cc:     Miklos Szeredi <miklos@...redi.hu>,
        Vivek Goyal <vgoyal@...hat.com>,
        overlayfs <linux-unionfs@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 2/2] ovl: introduce new "uuid=off" option for inodes
 index feature

On Fri, Sep 25, 2020 at 11:35 AM Pavel Tikhomirov
<ptikhomirov@...tuozzo.com> 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@...il.com>
> CC: Vivek Goyal <vgoyal@...hat.com>
> CC: Miklos Szeredi <miklos@...redi.hu>
> CC: linux-unionfs@...r.kernel.org
> CC: linux-kernel@...r.kernel.org
> Signed-off-by: Pavel Tikhomirov <ptikhomirov@...tuozzo.com>
>

Apart from some typos, looks good to me.

You may add:
Reviewed-by: Amir Goldstein <amir73il@...il.com>

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.

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