ACK: [RESEND][SRU][Z][PATCH 0/2] LP#1728489: tar -x sometimes fails on overlayfs

[Stefan Bader]

On 16.11.2017 07:18, Daniel Axtens wrote:
> BugLink: https://bugs.launchpad.net/bugs/1728489
> 
> [SRU Justification]
> 
> [Impact]
> 
> A user is seeing failures from extracting tar archives on overlay
> filesystems on the 4.4 kernel in constrained environments. The error
> presents as:
> 
> `tar: ./deps/0/bin: Directory renamed before its status could be extracted`
> 
> Following this thread
> (http://www.spinics.net/lists/linux-unionfs/msg00856.html), it appears
> that this occurs when entries in the kernel's inode cache are
> reclaimed, and subsequent lookups return new inode numbers.
> 
> Further testing showed that when setting
> `/proc/sys/vm/vfs_cache_pressure` to 0 (don't allow the kernel to
> reclaim inode cache entries due to memory pressure) the error does not
> recur, supporting the hypothesis that cache entries are being
> evicted. However, this setting may lead to a kernel OOM so is not a
> reasonable workaround even temporarily.
> 
> The error cannot be reproduced on a 4.13 kernel, due to the series at
> https://www.spinics.net/lists/linux-fsdevel/msg110235.html. The
> particular relevant commit is
> b7a807dc2010334e62e0afd89d6f7a8913eb14ff, which needs a couple of
> dependencies.
> 
> [Fix]
> Backport the key patch and its dependency.
> 
> [Testcase]
> 
> # Testing this bug
> 
> The testcase for this particular bug is simple - create an overlay
> filesystem with all layers on the same underlying file system, and
> then see if the inode of a directory is constant across dropping the
> caches:
> 
> mkdir -p /upper/upper /upper/work /lower
> mount -t overlay none /mnt -o lowerdir=/lower,upperdir=/upper/upper,workdir=/upper/work
> cd /mnt
> mkdir a
> stat a # observe inode number
> echo 2 > /proc/sys/vm/drop_caches
> stat a # compare inode number
> 
> If the inode number is the same, the fix is successful.
> 
> # Regression testing
> 
> I have run the unionmount test suite from
> http://git.infradead.org/users/dhowells/unionmount-testsuite.git in
> overlay mode (./run --ov), and verified that it still passes.
> 
> (The series cover letter mentions a fork of the test suite at
> https://github.com/amir73il/unionmount-testsuite/commits/overlayfs-devel. I
> have *not* attempted to get this running: it assumes a range of
> changes that are not present in our kernels.)
> 
> [Regression Potential]
> 
> As this changes overlayfs, there is potential for regression in the
> form of unexpected breakages to overlaysfs behaviour.
> 
> I think this is adequately addressed by the regression testing.
> 
> Amir Goldstein (2):
>   ovl: check if all layers are on the same fs
>   ovl: persistent inode number for directories
> 
>  fs/overlayfs/dir.c       | 36 ++++++++++++++++++++++++++++++++----
>  fs/overlayfs/overlayfs.h |  1 +
>  fs/overlayfs/ovl_entry.h |  2 ++
>  fs/overlayfs/super.c     |  8 ++++++++
>  fs/overlayfs/util.c      |  7 +++++++
>  5 files changed, 50 insertions(+), 4 deletions(-)
> 

Looks sensible and positive testing.

Acked-by: Stefan Bader <stefan.bader at canonical.com>


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