Re: Can we deprecate ioctl(RAID_VERSION)?

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

On 04/05/2017 06:32 PM, NeilBrown wrote:
> On Thu, Apr 06 2017, jes.sorensen@gmail.com wrote:
>
>> jes.sorensen@gmail.com writes:
>>> Hi Neil,
>>>
>>> Looking through the code in mdadm, I noticed a number of cases calling
>>> ioctl(RAID_VERSION). At first I had it confused with metadata version,
>>> but it looks like RAID_VERSION will always return 90000 if it's a valid
>>> raid device.
>>>
>>> In the cases we want to confirm the fd is a valid raid array,
>>> ioctl(GET_ARRAY_INFO) should do, or sysfs_read(GET_VERSION).
>>>
>>> Am I missing something obvious here, or do you see any reason for
>>> leaving this around?
>>
>> Sorry the above is wrong, it will always return 900, not 90000. Some of
>> the code that stood out is in util.c:
>>
>> int md_get_version(int fd)
>> {
>>         struct stat stb;
>>         mdu_version_t vers;
>>
>>         if (fstat(fd, &stb)<0)
>>                 return -1;
>>         if ((S_IFMT&stb.st_mode) != S_IFBLK)
>>                 return -1;
>>
>>         if (ioctl(fd, RAID_VERSION, &vers) == 0)
>>                 return  (vers.major*10000) + (vers.minor*100) + vers.patchlevel;
>>         if (errno == EACCES)
>>                 return -1;
>>         if (major(stb.st_rdev) == MD_MAJOR)
>>                 return (3600);
>>         return -1;
>> }
>>
>> ....
>>
>> int set_array_info(int mdfd, struct supertype *st, struct mdinfo *info)
>> {
>>         /* Initialise kernel's knowledge of array.
>>          * This varies between externally managed arrays
>>          * and older kernels
>>          */
>>         int vers = md_get_version(mdfd);
>>         int rv;
>>
>> #ifndef MDASSEMBLE
>>         if (st->ss->external)
>>                 rv = sysfs_set_array(info, vers);
>>         else
>> #endif
>>                 if ((vers % 100) >= 1) { /* can use different versions */
>>                 mdu_array_info_t inf;
>>                 memset(&inf, 0, sizeof(inf));
>>                 inf.major_version = info->array.major_version;
>>                 inf.minor_version = info->array.minor_version;
>>                 rv = ioctl(mdfd, SET_ARRAY_INFO, &inf);
>>         } else
>>                 rv = ioctl(mdfd, SET_ARRAY_INFO, NULL);
>>         return rv;
>> }
>>
>> This has been around since at least 2008, the current code came in
>> f35f25259279573c6274e2783536c0b0a399bdd4, but it looks like even the
>> prior code made the same assumptions.
>>
>> In either case, the above 'if ((vers % 100) >= 1)' will always trigger
>> since the kernel does #define MD_PATCHLEVEL_VERSION 3
>>
>> It's not like we have been updating MD_PATCHLEVEL_VERSION for a
>> while. Was the code meant to be looking at the superblock minor version?
>> I've been staring at this for a while now, so please beat me over the
>> head if I missed something blatantly obvious.
>>
>> Jes
>
> It is hard to get versioning right...
>
> The version returned by the RAID_VERSION ioctl is meant to reflect the
> capabilities of the implementation.  We could use the kernel version
> number for that (and sometimes do), but as distro's often backport
> features, that isn't always reliable.
>
> I've incremented the MD_PATCHLEVEL_VERSION when a change is made that
> cannot easily be detected from user-space.  As you note, we are up to
> three.  The last change was in 2.6.15.
> I've never contemplated changing the other two numbers that RAID_VERSION
> return.  They don't seem to mean anything useful.
>
> What exactly do you mean by "deprecate" the ioctl?
> If you remove the code in mdadm that calls it, mdadm will not work
> correctly on kernels older than 2.6.15, and it will be harder to
> and an future capability that is not easily visible from user space.
> If you remove the code in the kernel that handles it, you'll break
> mdadm.

Neil,

I see, thanks for explaining.

The goal is to eventually get out of the ioctl() business and get to a 
state where we can do everything via sysfs/configfs. Right now we have a 
big mix between ioctl and sysfs where neither interface does everything. 
The recent issues with PPL (I think it was) showed that we had to add 
more ioctl support because the interfaces needed to do it for sysfs 
weren't quite there. My long term goal is to get that situation improved 
so we can avoid adding anymore ioctl interfaces and eventually allow for 
distros to build mdadm with ioctl support disabled. We had a discussion 
at LSF/MM in Boston about this (Hannes, Shaohua, Song, and myself).

Reading the code I found it confusing that it was so tied to the patch 
level, but didn't do anything with the version numbers. At least 
intuitively if I bumped the version number, I would reset the patchlevel 
which would break things.

I think it's fair to draw a line in the sand and say that mdadm-4.1+ 
will not support kernels older than 2.6.15. I am open to the kernel 
version we pick here, but I would like to start deprecating some of the 
really old code. I have patches that does this in my tree, but I need to 
add a check for kernel version > 2.6.15. I am not aware what SuSE's 
enterprise kernel versions look like, but checking RHEL/CentOS RHEL5 was 
2.6.18, while RHEL4 was 2.6.9 - and RHEL4 has been unsupported for quite 
a while. At least for RHEL/CentOS 2.6.15 as the line in the sand seems fine.

For the kernel to expose features to userland in the future, I would 
prefer to go with a feature-flag style interface exposed via sysfs. That 
way a distro could enable one feature, but not the other in their kernel 
without having to worry about actual version numbers.

Cheers,
Jes