Re: [RFC/PATCH 1/5] mtd: ubi: Read disturb infrastructure

[Tanya Brokhman <tlinder@codeaurora.org>]

On 10/2/2014 4:36 PM, Richard Weinberger wrote:
> Am 02.10.2014 14:50, schrieb Tanya Brokhman:
>> Hi Richard,
>>
>> Sorry it took me some time to answer, got per-occupied with some urgent staff.
>>
>> On 9/28/2014 1:54 PM, Richard Weinberger wrote:
>>> Am 28.09.2014 12:46, schrieb Tanya Brokhman:
>>>> On 9/28/2014 11:54 AM, Richard Weinberger wrote:
>>>>> Am 28.09.2014 10:48, schrieb Tanya Brokhman:
>>>>>>>> @@ -424,6 +440,8 @@ struct ubi_fm_sb {
>>>>>>>>          __be32 used_blocks;
>>>>>>>>          __be32 block_loc[UBI_FM_MAX_BLOCKS];
>>>>>>>>          __be32 block_ec[UBI_FM_MAX_BLOCKS];
>>>>>>>> +    __be32 block_rc[UBI_FM_MAX_BLOCKS];
>>>>>>>> +    __be64 block_let[UBI_FM_MAX_BLOCKS];
>>>>>>>
>>>>>>> Doesn't this break the fastmap on-disk layout?
>>>>>>
>>>>>> What do you mean "break"? I verified fastmap feature is working. the whole read-disturb depends on it so I tested this thoroughly.
>>>>>
>>>>> Did you write a fastmap with your changes applied and then an attach using a fastmap implementation *without*
>>>>> you changes?
>>>>> I bet it will not work because the disk layout is now different.
>>>>
>>>> you're right, it wont work. I did a set of attach/detach tests to verify fastmap, but of course with my changes.
>>>>
>>>>> Linux is not the only user of fastmap. We need to be very careful here.
>>>>
>>>> Could you please elaborate here? I'm not sure I understand the use case you're referring to.
>>>
>>> Consider the case where you have a board with a fastmap enabled bootloader and a Linux OS.
>>> The bootloader does a fastmap attach and boots the kernel from UBI and the kernel it self has the rootfs
>>> on UBI too. If you install a new kernel with your changes applied it will write the fastmap in a different
>>> format and the bootloader will fail badly. In worst case the board bricks, in best case the bootloader can fall back
>>> to scanning mode but it will be slow and the customer unhappy.
>>>
>>
>> Ok, I understand the problem now. I wanted to discuss a possible solution before implementing it:
>> We have a "fastmap version" in fm_sb. At the moment UBI_FM_FMT_VERSION = 1 and any other is not supported. We can use that; Add another fm version (UBI_FM_FMT_VERSION_RD = 2) and
>> then decide according to it. Meaning, if during attach process we find fm superblock we check it's version, if it's != UBI_FM_FMT_VERSION_RD, we fall back to full scan. The next
>> fastmap will be written with the new layout (and new version number) so second boot will attach from fastmap without any issues.
>
> BTW: I think I've found a way such that your change will not break anything.
> Keep UBI_FM_FMT_VERSION=1, but claim one field in ubi_fm_sb to indicate a fastmap subversion or extension.
> Create new data structures which carry all the information you need and place them at the end of the fastmap.
>
> An old implementation will not evaluate ubi_fm_sb->extension and therefore will not use the additional info
> you've placed at the end of the fastmap.
>
> A new implementation will evaluate ubi_fm_sb->extension and notice that this fastmap carries the "read disturb infrastructure"
> extension info at it's end and can use it...
>
> Not nice, not perfect but could work. 8-)

Agree, it will work, but seems a bit ugly to me.... You really think it 
will be better than add a new fm_version? I agree that breaking fm 
layout is dangerous but it seems to me like the correct way to implement 
this requirement. Saving all read-disturb data in "extensions" feels 
like a hack.
That said, you're have much more experienced with ubi&fm then I do, so 
I'll do as you see best.

>
> Thanks,
> //richard
>
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