On Wed, 22 Mar 2023, Xiao Ni wrote:

On Tue, Mar 14, 2023 at 8:08 AM NeilBrown [off-list ref] wrote:

quoted

mdopen() will use create_named_array() to ask the kernel to create the
given md array, but only if it is given a number or name.
If it is NOT given a name and is required to choose one itself using
find_free_devnm() it does NOT use create_named_array().

On kernels with CONFIG_BLOCK_LEGACY_AUTOLOAD not set, this can result in
failure to assemble an array.  This can particularly seen when the
"name" of the array begins with a host name different to the name of the
host running the command.

So add the missing call to create_named_array().

Link: https://bugzilla.kernel.org/show_bug.cgi?id=217074

Hi Neil

I have two questions, hope you can help to understand the function
create_mddev better.

Frist, from the comment7 of the bug you mentioned:

There are two different sorts names.  Note that you almost
acknowledged this by writing "name for my md device node" while the
documentation only talks about names for "md devices", not for "md
device nodes".

There are
1/ there are names in /dev or /dev/md/ (device nodes)
2/ there are names that appear in /proc/mdstat and in /sys/block/ (devices)

Thanks for the clarification. But it looks like it doesn't work like
what you said.
For example:
mdadm -CR /dev/md/root -l0 -n2 /dev/sda /dev/sdc --name=test
cat /proc/mdstat
Personalities : [raid0]
md127 : active raid0 sdc[1] sda[0]
      3906764800 blocks super 1.2 512k chunks
cd /sys/block/md127/md/

In /proc/mdstat and /sys/block, they all use md127 rather than the name(root)

Try again with "CREATE names=yes" in /etc/mdadm.conf.

mdadm generally tries to keep:
  - the names in /dev/
  - the names in /dev/md/
  - the names in /proc/mdstat
  - the names stored in the metadata

in sync.  It can only do this when:
 - you enabled "names=yes"
 - you don't confuse it by specifying a device name (/dev/md/root) that
   is different from the metadata names "test".

If you don't have "names=yes" then the name in /proc/mdstat and the name
in /dev/md* will be numeric.  The name in /dev/md/ and the name in the
metadata can be different and will usually be the same.

If you explicitly give a different name with --name= than the device
name then obviously they will be different.  If you then stop the array
and restart with "mdadm -As" or "mdadm -I /dev/sda; mdadm -I /dev/sdb"
then mdadm will create a name in /dev/md/ that matches the name in the
metadata.

Before this patch,  it creates a symbol link with the name root rather than test
ll /dev/md/root
lrwxrwxrwx. 1 root root 8 Mar 21 22:35 /dev/md/root -> ../md127

That is what you asked it to do.

So "test" which is specified by --name looks like it has little usage.

It is stored in the metadata.  You can see it in --examine output.  If
you reassemble the array without specifying a device name, it will use
the name "test".

By the way, after this patch, the symbol link /dev/md/root can't be
created anymore.
Is it a regression problem?

I cannot reproduce any problem like that.  Please provide a sequence of
steps so that I can try to duplicate it.

Second, are there possibilities that the arguments "dev" and "name" of
function create_mddev
are null at the same time?

No.  For Build or Create, dev is never NULL.  For Assemble and
Incremental, name is never NULL.

After some tests, I found dev can't be null when creating a raid
device. It can be checked before
calling create_mddev. And we must get a name after creating a raid
device. So when assembling
a raid device, the name must not be null. So the dev and name can't be
null at the same time, right?

Correct.

NeilBrown

Best Regards
Xiao

quoted

Signed-off-by: NeilBrown <redacted>
---
 mdopen.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/mdopen.c b/mdopen.c
index d18c931996d2..810f79a3d19a 100644
--- a/mdopen.c
+++ b/mdopen.c

@@ -370,6 +370,7 @@ int create_mddev(char *dev, char *name, int autof, int trustworthy,
                }
                if (block_udev)
                        udev_block(devnm);
+               create_named_array(devnm);
        }

        sprintf(devname, "/dev/%s", devnm);
--

2.39.2