Re: GET_ARRAY_INFO assumptions?

[Jes Sorensen <jes.sorensen@gmail.com>]

On 04/13/2017 05:06 PM, Jes Sorensen wrote:
> On 04/13/2017 04:37 PM, Shaohua Li wrote:
>> On Thu, Apr 13, 2017 at 01:50:06PM -0400, Jes Sorensen wrote:
>>> Hi Neil,
>>>
>>> Looking at trying to phase out the ioctl usage, I am trying to
>>> introduce a
>>> helper for the 'is the array valid' situation.
>>>
>>> Now looking at places like Incremental.c (around like 557 in my current
>>> tree):
>>>     /* 7b/ if yes, */
>>>     /* - if number of OK devices match expected, or -R and there */
>>>     /*             are enough, */
>>>     /*   + add any bitmap file  */
>>>     /*   + start the array (auto-readonly). */
>>>
>>>     if (md_get_array_info(mdfd, &ainf) == 0) {
>>>         if (c->export) {
>>>             printf("MD_STARTED=already\n");
>>>         } else if (c->verbose >= 0)
>>>             pr_err("%s attached to %s which is already active.\n",
>>>                    devname, chosen_name);
>>>         rv = 0;
>>>         goto out_unlock;
>>>     }
>>>
>>> I am wondering if there are any side effects/assumptions about
>>> GET_ARRAY_INFO that I am not considering? Basically I am making the
>>> assumption that if /sys/block/md<X>/md exists, the array is valid.
>>
>> what does 'valid' really mean? md<x>/md exists after a md device is
>> allocated,
>> the md device might not have any under layer disks bound yet.
>>
>>> The code in Incremental.c already deals with sysfs higher up in the
>>> code, so
>>> I guess the question is if the above test is even relevant anymore?
>>>
>>> Alternative, do we need export a new state in sysfs 'running'?
>>
>> I'd assume 'running' means the md device has a personality attached. See
>> array_state_show(), !running == 'clear' or 'inactive'.
>
> Good point, I guess what I am trying to figure out is what is assumed
> when ioctl(GET_ARRAY_INFO) returns 0 and how do we map it to sysfs?

Looking some more at this, it may be simpler than I thought. How about 
this approach (only compile tested):

int md_array_active(int fd)
{
	struct mdinfo *sra;
	struct mdu_array_info_s array;
	int ret;

	sra = sysfs_read(fd, NULL, GET_VERSION | GET_DISKS);
	if (sra) {
		if (!sra->array.raid_disks &&
		    !(sra->array.major_version == -1 &&
		      sra->array.minor_version == -2))
			ret = -ENODEV;
		else
			ret = 0;

		free(sra);
	} else {
		ret = ioctl(fd, GET_ARRAY_INFO, &array);
	}

	return !ret;
}

Note 'major = -1 && minor = -2' is sysfs_read's way of saying 'external'.

This pretty much mimics what the kernel does in the ioctl handler for 
GET_ARRAY_INFO:

	case GET_ARRAY_INFO:
		if (!mddev->raid_disks && !mddev->external)
			err = -ENODEV;
		else
			err = get_array_info(mddev, argp);
		goto out;

What do you think?

Cheers,
Jes