On 12/9/19 12:48 PM, dann frazier wrote:
> [ + Jes ]
>
> Hi Song, Neil,
> Now that the merge window has closed, I wanted to check in on the
> status of this. fyi, this still applies cleanly to 5.5-rc1.
>
> Jes: note that this patch makes an assumption that the next version of
> mdadm would be >= 4.2, so seeking your ACK as well.
The last release was 4.1, so I think it's fair to assume it'll be 4.2
next time.
Cheers,
Jes
> -dann
>
> On Tue, Nov 12, 2019 at 03:21:05PM -0800, dann frazier wrote:
>> Helping an administrator understand this issue and how to deal with it
>> requires more text than achievable in a kernel error message. Let's
>> clarify the issue in the admin guide, and have the kernel emit a link
>> to it.
>>
>> v2:
>> - Add info about setting layout w/ mdadm, using presumed-next-mdadm
>> version.
>> - Add comment to doc to help prevent future changes from breaking
>> the link emitted by raid0.
>>
>> Fixes: c84a1372df92 ("md/raid0: avoid RAID0 data corruption due to layout confusion.")
>> Cc: stable@xxxxxxxxxxxxxxx (3.14+)
>> Signed-off-by: dann frazier <dann.frazier@xxxxxxxxxxxxx>
>> ---
>> Documentation/admin-guide/md.rst | 48 ++++++++++++++++++++++++++++++++
>> drivers/md/raid0.c | 2 ++
>> 2 files changed, 50 insertions(+)
>>
>> diff --git a/Documentation/admin-guide/md.rst b/Documentation/admin-guide/md.rst
>> index 3c51084ffd379..a736e3b4117fc 100644
>> --- a/Documentation/admin-guide/md.rst
>> +++ b/Documentation/admin-guide/md.rst
>> @@ -759,3 +759,51 @@ These currently include:
>>
>> ppl_write_hint
>> NVMe stream ID to be set for each PPL write request.
>> +
>> +Multi-Zone RAID0 Layout Migration
>> +---------------------------------
>> +.. Note: a public URL to this section is emitted in an error message from
>> + the raid0 driver, so please take care to not make changes that would
>> + cause that link to break.
>> +An unintentional RAID0 layout change was introduced in the v3.14 kernel.
>> +This effectively means there are 2 different layouts Linux will use to
>> +write data to RAID0 arrays in the wild - the "pre-3.14" way and the
>> +"3.14 and later" way. Mixing these layouts by writing to an array while
>> +booted on these different kernel versions can lead to corruption.
>> +
>> +Note that this only impacts RAID0 arrays that include devices of different
>> +sizes. If your devices are all the same size, both layouts are equivalent,
>> +and your array is not at risk of corruption due to this issue.
>> +
>> +Unfortunately, the kernel cannot detect which layout was used for writes
>> +to pre-existing arrays, and therefore requires input from the
>> +administrator. This input can be provided via the kernel command line
>> +with the ``raid0.default_layout=<N>`` parameter, or by setting the
>> +``default_layout`` module parameter when loading the ``raid0`` module.
>> +With a new enough version of mdadm (>= 4.2, or equivalent distro backports),
>> +you can set the layout version when assembling a stopped array. For example::
>> +
>> + mdadm --stop /dev/md0
>> + mdadm --assemble -U layout-alternate /dev/md0 /dev/sda1 /dev/sda2
>> +
>> +See the mdadm manpage for more details. Once set in this manner, the layout
>> +will be recorded in the array and will not need to be explicitly specified
>> +in the future.
>> +
>> +Which layout version should I use?
>> +++++++++++++++++++++++++++++++++++
>> +If your RAID array has only been written to by a 3.14 or later kernel, then
>> +you should specify default_layout=2, or set ``layout-alternate`` in mdadm.
>> +If your kernel has only been written to by a < 3.14 kernel, then you should
>> +specify default_layout=1 or set ``layout-original`` in mdadm. If the array
>> +may have already been written to by both kernels < 3.14 and >= 3.14, then it
>> +is possible that your data has already suffered corruption. Note that
>> +``mdadm --detail`` will show you when an array was created, which may be
>> +useful in helping determine the kernel version that was in-use at the time.
>> +
>> +When determining the scope of corruption, it may also be useful to know
>> +that the area susceptible to this corruption is limited to the area of the
>> +array after "MIN_DEVICE_SIZE * NUM DEVICES".
>> +
>> +For new arrays you may choose either layout version. Neither version is
>> +inherently better than the other.
>> diff --git a/drivers/md/raid0.c b/drivers/md/raid0.c
>> index 1e772287b1c8e..e01cd52d71aa4 100644
>> --- a/drivers/md/raid0.c
>> +++ b/drivers/md/raid0.c
>> @@ -155,6 +155,8 @@ static int create_strip_zones(struct mddev *mddev, struct r0conf **private_conf)
>> pr_err("md/raid0:%s: cannot assemble multi-zone RAID0 with default_layout setting\n",
>> mdname(mddev));
>> pr_err("md/raid0: please set raid0.default_layout to 1 or 2\n");
>> + pr_err("Read the following page for more information:\n");
>> + pr_err("https://www.kernel.org/doc/html/latest/admin-guide/md.html#multi-zone-raid0-layout-migration\n";);
>> err = -ENOTSUPP;
>> goto abort;
>> }
