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

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Date:   Thu, 23 Sep 2021 08:47:28 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Coly Li <colyli@...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>,
        NeilBrown <neilb@...e.de>, Richard Fan <richard.fan@...e.com>,
        Vishal L Verma <vishal.l.verma@...el.com>, rafael@...nel.org
Subject: Re: Too large badblocks sysfs file (was: [PATCH v3 0/7] badblocks
 improvement for multiple bad block ranges)

On Thu, Sep 23, 2021 at 02:14:12PM +0800, Coly Li wrote:
> On 9/23/21 2:08 PM, Greg Kroah-Hartman wrote:
> > On Thu, Sep 23, 2021 at 01:59:28PM +0800, 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 Greg,
> 
> This is the code in mainline kernel for quite long time.

{sigh}

What tools rely on this?  If none, just don't add new stuff to the file
and work to create a new api instead.

> > Please do not do that.  Seriously, that is not what sysfs is for, and is
> > an abuse of it.
> > 
> > sysfs is for "one value per file" and should never even get close to a
> > 4kb limit.  If it does, you are doing something really really wrong and
> > should just remove that sysfs file from the system and redesign your
> > api.
> 
> I understand this. And what I addressed is the problem I need to fix.
> 
> The code is there for almost 10 years, I just find it during my work on bad
> blocks API fixing.
> 
> 
> > > 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
> > Ick, again, that's not ok at all.  sysfs files should never have to be
> > parsed like this.
> 
> I cannot agree more with you. What I am asking for was ---- how to fix it ?

Best solution, come up with a new api.

Worst solution, you are stuck with the existing file and I can show you
the "way out" of dealing with files larger than 4kb in sysfs that a
number of other apis are being forced to do as they grow over time.

But ideally, just drop ths api and make a new one please.

thanks,

greg k-h