Public bug reported:
TLDR: commit 72ecad22d9f198aafee64218512e02ffa7818671 (in v4.10)
introduced silent data corruption for O_DIRECT uses, it's fixed in
17d51b10d7773e4618bcac64648f30f12d4078fb (in v4.18)
A silent data corruption was introduced in v4.10-rc1 with commit
72ecad22d9f198aafee64218512e02ffa7818671 and was fixed in v4.18-rc7 with
commit 17d51b10d7773e4618bcac64648f30f12d4078fb. It affects users of
O_DIRECT, in our case a KVM virtual machine with drives which use qemu's
"cache=none" option.
This is the commit which fixes the issue:
---------------------
commit 17d51b10d7773e4618bcac64648f30f12d4078fb
Author: Martin Wilck <mwi...@suse.com>
Date: Wed Jul 25 23:15:09 2018 +0200
block: bio_iov_iter_get_pages: pin more pages for multi-segment IOs
bio_iov_iter_get_pages() currently only adds pages for the next non-zero
segment from the iov_iter to the bio. That's suboptimal for callers,
which typically try to pin as many pages as fit into the bio. This patch
converts the current bio_iov_iter_get_pages() into a static helper, and
introduces a new helper that allocates as many pages as
1) fit into the bio,
2) are present in the iov_iter,
3) and can be pinned by MM.
Error is returned only if zero pages could be pinned. Because of 3), a
zero return value doesn't necessarily mean all pages have been pinned.
Callers that have to pin every page in the iov_iter must still call this
function in a loop (this is currently the case).
This change matters most for __blkdev_direct_IO_simple(), which calls
bio_iov_iter_get_pages() only once. If it obtains less pages than
requested, it returns a "short write" or "short read", and
__generic_file_write_iter() falls back to buffered writes, which may
lead to data corruption.
Fixes: 72ecad22d9f1 ("block: support a full bio worth of IO for simplified
bdev direct-io")
Reviewed-by: Christoph Hellwig <h...@lst.de>
Signed-off-by: Martin Wilck <mwi...@suse.com>
Signed-off-by: Jens Axboe <ax...@kernel.dk>
-------------------------
Since there were a lot of components involved in the initial report to
us (xfs, guest kernel, guest virtio drivers, qemu, host kernel, storage
system), we had to isolate it. This is the commit which fixes the data
corruption bug. We created a reliable reproduction and tested with the
patch and without the patch. We also created a version of the kernel
which prints when the data-corrupting path in the kernel is triggered.
> 1) The release of Ubuntu you are using, via 'lsb_release -rd' or
System -> About Ubuntu
# lsb_release -rd
Description: Ubuntu 18.04.1 LTS
Release: 18.04
> 2) The version of the package you are using, via 'apt-cache policy
pkgname' or by checking in Software Center
# apt-cache policy linux-image-4.15.0-36-generic
linux-image-4.15.0-36-generic:
Installed: 4.15.0-36.39
Candidate: 4.15.0-36.39
Version table:
*** 4.15.0-36.39 500
500 http://archive.ubuntu.com/ubuntu bionic-security/main amd64 Packages
500 http://archive.ubuntu.com/ubuntu bionic-updates/main amd64 Packages
100 /var/lib/dpkg/status
> 3) What you expected to happen
We ran a fio random write workload over 8x 512MB files over XFS in guest OS,
over qemu/kvm, over kernel 4.15.0-36.39-generic.
qemu-system was configured with cache=none, which means Direct IO. This is a
very common configuration.
qemu-system was with aio=threads -- the default.
We were expecting no data corruption.
> 4) What happened instead
The guest filesystem was corrupted.
** Affects: linux-signed (Ubuntu)
Importance: Undecided
Status: New
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https://bugs.launchpad.net/bugs/1796542
Title:
Silent corruption in Linux kernel 4.15
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