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

[Richard Weinberger <richard@nod.at>]

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-)

Thanks,
//richard