Re: [PATCH mdadm] mdadm: Don't open md device for CREATE and ASSEMBLE

[Guoqing Jiang <guoqing.jiang@linux.dev>]

On 7/15/22 6:37 AM, Logan Gunthorpe wrote:
> The mdadm command tries to open the md device for most modes, first
> thing, no matter what. When running to create or assemble an array,
> in most cases, the md device will not exist, the open call will fail
> and everything will proceed correctly.

I guess the BUILD mode doesn't need to create md as well since commit
7f91af49ad09 ("Delay creation of array devices for assemble/build/create").

> However, when running tests, a create or assembly command may be run
> shortly after stopping an array and the old md device file may still
> be around. Then, if create_on_open is set in the kernel, a new md
> device will be created when mdadm does its initial open.
>
> When mdadm gets around to creating the new device with the new_array
> parameter it issues this error:
>
>     mdadm: Fail to create md0 when using
>     /sys/module/md_mod/parameters/new_array, fallback to creation via node
>
> This is because an mddev was already created by the kernel with the
> earlier open() call and thus the new one being created will fail with
> EEXIST. The mdadm command will still successfully be created due to
> falling back to the node creation method. However, the error message
> itself will fail any test that's running it.
>
> This issue is a race condition that is very rare, but a recent change
> in the kernel caused this to happen more frequently: about 1 in 50
> times.
>
> To fix this, don't bother trying to open the md device for CREATE and
> ASSEMBLE commands, as the file descriptor will never be used anyway
> even if it is successfully openned.
>
> Side note: it would be nice to disable create_on_open as well to help
> solve this, but it seems the work for this was never finished. By default,
> mdadm will create using the old node interface when a name is specified
> unless the user specifically puts names=yes in a config file, which
> doesn't seem to be vcreate_on_openery common yet.

Hmm, 'create_on_open' is default to true, not sure if there is any side 
effect
after change to false.

> Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
> ---
>   mdadm.c | 3 +--
>   1 file changed, 1 insertion(+), 2 deletions(-)
>
> diff --git a/mdadm.c b/mdadm.c
> index 56722ed997a2..3b886b5c0639 100644
> --- a/mdadm.c
> +++ b/mdadm.c
> @@ -1347,8 +1347,7 @@ int main(int argc, char *argv[])
>   	 * an md device.  We check that here and open it.
>   	 */
>   
> -	if (mode == MANAGE || mode == BUILD || mode == CREATE ||
> -	    mode == GROW || (mode == ASSEMBLE && ! c.scan)) {
> +	if (mode == MANAGE || mode == BUILD || mode == GROW) {
>   		if (devs_found < 1) {
>   			pr_err("an md device must be given in this mode\n");
>   			exit(2);

I think it is a reasonable change.

Thanks,
Guoqing