Date:   Thu, 23 Sep 2021 20:55:42 +0800
From:   Coly Li <colyli@...e.de>
To:     NeilBrown <neilb@...e.de>
Cc:     linux-kernel@...r.kernel.org, linux-block@...r.kernel.org,
        linux-raid@...r.kernel.org, nvdimm@...ts.linux.dev,
        antlists@...ngman.org.uk, Dan Williams <dan.j.williams@...el.com>,
        Hannes Reinecke <hare@...e.de>, Jens Axboe <axboe@...nel.dk>,
        Richard Fan <richard.fan@...e.com>,
        Vishal L Verma <vishal.l.verma@...el.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        rafael@...nel.org
Subject: Re: Too large badblocks sysfs file (was: [PATCH v3 0/7] badblocks
 improvement for multiple bad block ranges)

On 9/23/21 6:09 PM, NeilBrown wrote:
> On Thu, 23 Sep 2021, 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.

Hi Neil,

> Why do you think there is a problem?
> As documented in Documentation/admin-guide/md.rst, the truncation at 1
> page is expected and by design.

Oh, thanks for letting me know this. Yes this is as-designed, so I will 
not worry more about this.

>
> The "unacknowledge-bad-blocks" file is the important one that is needed
> for correct behaviour.  Being able to read a single block is sufficient,
> though being able to read more than one could provide better performance
> in some cases.
>
> The "bad-blocks" file primarily exist to provide visibility into the
> state of the system - useful during development.  It can be written to
> to add bad blocks.  I never *needs* to be read from.

Thanks for the hint.

>
> The authoritative source of information about the set of bad blocks is
> the on-disk data the can be and should be read directly...

The reply is informative. It is more clear for me.

Coly Li

>
> Except that mdadm does.  That was a mistake.  check_for_cleared_bb() is
> wrong.  I wonder why it was added.  The commit message doesn't give any
> justification.
>
> NeilBrown
>
>
>> 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@...el.com>
>>> Cc: Hannes Reinecke <hare@...e.de>
>>> Cc: Jens Axboe <axboe@...nel.dk>
>>> Cc: NeilBrown <neilb@...e.de>
>>> Cc: Richard Fan <richard.fan@...e.com>
>>> Cc: Vishal L Verma <vishal.l.verma@...el.com>
>>> ---
>>> Changelog:
>>> v3: add tester Richard Fan <richard.fan@...e.com>
>>> 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(-)
>>>
>>

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