Re: Partitioned arrays initially missing from /proc/partitions

[David Greaves <david@dgreaves.com>]

Hi Neil

Just wondering what the status is here - do you need any more from me or is it
on your stack?

The patch helped but didn't cure.
After a clean boot it mounted correctly first try.
Then I unmounted, stopped and re-assembled the array.
The next mount failed.
The subsequent mount succeeded.

How do other block devices initialise their partitions on 'discovery'?

David

David Greaves wrote:
> Neil Brown wrote:
>> On Tuesday April 24, david@dgreaves.com wrote:
>>> Neil, isn't it easy to just do this after an assemble?
>> Yes, but it should not be needed, and I'd like to understand why it
>> is.
>> One of the last things do_md_run does is
>>    mddev->changed = 1;
>>
>> When you next open /dev/md_d0, md_open is called which calls
>> check_disk_change().
>> This will call into md_fops->md_media_changed which will return the
>> value of mddev->changed, which will be '1'.
>> So check_disk_change will then call md_fops->revalidate_disk which
>> will set mddev->changed to 0, and will then set bd_invalidated to 1
>> (as bd_disk->minors > 1 (being 64)).
>>
>> md_open will then return into do_open (in fs/block_dev.c) and because
>> bd_invalidated is true, it will call rescan_partitions and the
>> partitions will appear.
>>
>> Hmmm... there is room for a race there.  If some other process opens
>> /dev/md_d0 before mdadm gets to close it, it will call
>> rescan_partitions before first calling  bd_set_size to update the size
>> of the bdev.  So when we try to read the partition table, it will
>> appear to be reading past the EOF, and will not actually read
>> anything..
>>
>> I guess udev must be opening the block device at exactly the wrong
>> time. 
>>
>> I can simulate this by holding /dev/md_d0 open while assembling the
>> array.  If I do that, the partitions don't get created.
>> Yuck.
>>
>> Maybe I could call bd_set_size in md_open before calling
>> check_disk_change..
>>
>> Yep, this patch seems to fix it.  Could you confirm?
> almost...
> 
> teak:~# mdadm --assemble /dev/md_d0 --auto=parts /dev/sd[bcdef]1
> mdadm: /dev/md_d0 has been started with 5 drives.
> teak:~# mount /media
> teak:~# umount /media
> teak:~# mdadm --stop /dev/md_d0
> mdadm: stopped /dev/md_d0
> teak:~# mdadm --assemble /dev/md_d0 --auto=parts /dev/sd[bcdef]1
> mdadm: /dev/md_d0 has been started with 5 drives.
> teak:~# mount /media
> mount: No such file or directory
> teak:~# mount /media
> teak:~#
> (second mount succeeds second time around)
> 
> 
> 
> md: md_d0 stopped.
> md: bind<sdc1>
> md: bind<sdd1>
> md: bind<sdb1>
> md: bind<sdf1>
> md: bind<sde1>
> raid5: device sde1 operational as raid disk 0
> raid5: device sdf1 operational as raid disk 4
> raid5: device sdb1 operational as raid disk 3
> raid5: device sdd1 operational as raid disk 2
> raid5: device sdc1 operational as raid disk 1
> raid5: allocated 5236kB for md_d0
> raid5: raid level 5 set md_d0 active with 5 out of 5 devices, algorithm 2
> RAID5 conf printout:
>  --- rd:5 wd:5
>  disk 0, o:1, dev:sde1
>  disk 1, o:1, dev:sdc1
>  disk 2, o:1, dev:sdd1
>  disk 3, o:1, dev:sdb1
>  disk 4, o:1, dev:sdf1
> md_d0: bitmap initialized from disk: read 1/1 pages, set 0 bits, status: 0
> created bitmap (10 pages) for device md_d0
>  md_d0: p1 p2
> Filesystem "md_d0p1": Disabling barriers, not supported with external log device
> XFS mounting filesystem md_d0p1
> Ending clean XFS mount for filesystem: md_d0p1
> md: md_d0 stopped.
> md: unbind<sde1>
> md: export_rdev(sde1)
> md: unbind<sdf1>
> md: export_rdev(sdf1)
> md: unbind<sdb1>
> md: export_rdev(sdb1)
> md: unbind<sdd1>
> md: export_rdev(sdd1)
> md: unbind<sdc1>
> md: export_rdev(sdc1)
> md: md_d0 stopped.
> md: bind<sdc1>
> md: bind<sdd1>
> md: bind<sdb1>
> md: bind<sdf1>
> md: bind<sde1>
> raid5: device sde1 operational as raid disk 0
> raid5: device sdf1 operational as raid disk 4
> raid5: device sdb1 operational as raid disk 3
> raid5: device sdd1 operational as raid disk 2
> raid5: device sdc1 operational as raid disk 1
> raid5: allocated 5236kB for md_d0
> raid5: raid level 5 set md_d0 active with 5 out of 5 devices, algorithm 2
> RAID5 conf printout:
>  --- rd:5 wd:5
>  disk 0, o:1, dev:sde1
>  disk 1, o:1, dev:sdc1
>  disk 2, o:1, dev:sdd1
>  disk 3, o:1, dev:sdb1
>  disk 4, o:1, dev:sdf1
> md_d0: bitmap initialized from disk: read 1/1 pages, set 0 bits, status: 0
> created bitmap (10 pages) for device md_d0
>  md_d0: p1 p2
> XFS: Invalid device [/dev/md_d0p2], error=-2
> Filesystem "md_d0p1": Disabling barriers, not supported with external log device
> XFS mounting filesystem md_d0p1
> Ending clean XFS mount for filesystem: md_d0p1
> 
> 
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