Re: Too large badblocks sysfs file (was: [PATCH v3 0/7] badblocks improvement for multiple bad block ranges)

[Hannes Reinecke]

On 9/23/21 7:59 AM, Coly Li wrote:
> Hi all the kernel gurus, and folks in mailing lists,
> 
> This is a question about exporting 4KB+ text information via sysfs
> interface. I need advice on how to handle the problem.
> 
> Recently I work on the bad blocks API (block/badblocks.c) improvement,
> there is a sysfs file to export the bad block ranges for me raid. E.g
> for a md raid1 device, file
>     /sys/block/md0/md/rd0/bad_blocks
> may contain the following text content,
>     64 32
>    128 8
> The above lines mean there are two bad block ranges, one starts at LBA
> 64, length 32 sectors, another one starts at LBA 128 and length 8
> sectors. All the content is generated from the internal bad block
> records with 512 elements. In my testing the worst case only 185 from
> 512 records can be displayed via the sysfs file if the LBA string is
> very long, e.g.the following content,
>   17668164135030776 512
>   17668164135029776 512
>   17668164135028776 512
>   17668164135027776 512
>   ... ...
> The bad block ranges stored in internal bad blocks array are correct,
> but the output message is truncated. This is the problem I encountered.
> 
> I don't see sysfs has seq_file support (correct me if I am wrong), and I
> know it is improper to transfer 4KB+ text via sysfs interface, but the
> code is here already for long time.
> 
> There are 2 ideas to fix showing up in my brain,
> 1) Do not fix the problem
>     Normally it is rare that a storage media has 100+ bad block ranges,
> maybe in real world all the existing bad blocks information won't exceed
> the page size limitation of sysfs file.
> 2) Add seq_file support to sysfs interface if there is no
> 
> It is probably there is other better solution to fix. So I do want to
> get hint/advice from you.
> 
> Thanks in advance for any comment :-)
> 
> Coly Li
> 
> On 9/14/21 12:36 AM, Coly Li wrote:
>> This is the second effort to improve badblocks code APIs to handle
>> multiple ranges in bad block table.
>>
>> There are 2 changes from previous version,
>> - Fixes 2 bugs in front_overwrite() which are detected by the user
>>    space testing code.
>> - Provide the user space testing code in last patch.
>>
>> There is NO in-memory or on-disk format change in the whole series, all
>> existing API and data structures are consistent. This series just only
>> improve the code algorithm to handle more corner cases, the interfaces
>> are same and consistency to all existing callers (md raid and nvdimm
>> drivers).
>>
>> The original motivation of the change is from the requirement from our
>> customer, that current badblocks routines don't handle multiple ranges.
>> For example if the bad block setting range covers multiple ranges from
>> bad block table, only the first two bad block ranges merged and rested
>> ranges are intact. The expected behavior should be all the covered
>> ranges to be handled.
>>
>> All the patches are tested by modified user space code and the code
>> logic works as expected. The modified user space testing code is
>> provided in last patch. The testing code detects 2 defects in helper
>> front_overwrite() and fixed in this version.
>>
>> The whole change is divided into 6 patches to make the code review more
>> clear and easier. If people prefer, I'd like to post a single large
>> patch finally after the code review accomplished.
>>
>> This version is seriously tested, and so far no more defect observed.
>>
>>
>> Coly Li
>>
>> Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
>> Cc: Hannes Reinecke <hare@xxxxxxx>
>> Cc: Jens Axboe <axboe@xxxxxxxxx>
>> Cc: NeilBrown <neilb@xxxxxxx>
>> Cc: Richard Fan <richard.fan@xxxxxxxx>
>> Cc: Vishal L Verma <vishal.l.verma@xxxxxxxxx>
>> ---
>> Changelog:
>> v3: add tester Richard Fan <richard.fan@xxxxxxxx>
>> v2: the improved version, and with testing code.
>> v1: the first completed version.
>>
>>
>> Coly Li (6):
>>    badblocks: add more helper structure and routines in badblocks.h
>>    badblocks: add helper routines for badblock ranges handling
>>    badblocks: improvement badblocks_set() for multiple ranges handling
>>    badblocks: improve badblocks_clear() for multiple ranges handling
>>    badblocks: improve badblocks_check() for multiple ranges handling
>>    badblocks: switch to the improved badblock handling code
>> Coly Li (1):
>>    test: user space code to test badblocks APIs
>>
>>   block/badblocks.c         | 1599 ++++++++++++++++++++++++++++++-------
>>   include/linux/badblocks.h |   32 +
>>   2 files changed, 1340 insertions(+), 291 deletions(-)
>>
> 

Please have a look at the patchset 'start switching sysfs attributes to
expose the seq_file' from Christoph Hellwig on linux-block; that seems
to be the approach you are looking for.


Cheers,

Hannes
-- 
Dr. Hannes Reinecke		           Kernel Storage Architect
hare@xxxxxxx			                  +49 911 74053 688
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