** Changed in: linux (Ubuntu Xenial)
Importance: Undecided => Medium
** Changed in: linux (Ubuntu Xenial)
Status: New => Fix Committed
** Changed in: linux (Ubuntu Zesty)
Importance: Undecided => Medium
** Changed in: linux (Ubuntu Zesty)
Status: New => Fix Committed
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https://bugs.launchpad.net/bugs/1728489
Title:
tar -x sometimes fails on overlayfs
Status in linux package in Ubuntu:
Confirmed
Status in linux source package in Xenial:
Fix Committed
Status in linux source package in Zesty:
Fix Committed
Bug description:
[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]
For Zesty, backport the entire series.
For Xenial, where a full backport is not feasible, backport the key commit
and the short list of dependencies.
[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.
One option to reduce the regression potential on Zesty is to reduce
the set of patches applied - rather than including the whole series we
could include just the patches to solve this bug, which are much
easier to inspect for correctness.
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