linux-kernel - Re: [PATCH v3 1/6] block: add disk sequence number

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Date:   Wed, 23 Jun 2021 16:21:13 +0200
From:   Hannes Reinecke <hare@...e.de>
To:     Luca Boccassi <bluca@...ian.org>,
        Lennart Poettering <mzxreary@...inter.de>,
        Matteo Croce <mcroce@...ux.microsoft.com>
Cc:     Christoph Hellwig <hch@...radead.org>, linux-block@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, Jens Axboe <axboe@...nel.dk>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Damien Le Moal <damien.lemoal@....com>,
        Tejun Heo <tj@...nel.org>,
        Javier Gonz??lez <javier@...igon.com>,
        Niklas Cassel <niklas.cassel@....com>,
        Johannes Thumshirn <johannes.thumshirn@....com>,
        Matthew Wilcox <willy@...radead.org>,
        JeffleXu <jefflexu@...ux.alibaba.com>
Subject: Re: [PATCH v3 1/6] block: add disk sequence number

On 6/23/21 4:07 PM, Luca Boccassi wrote:
> On Wed, 2021-06-23 at 16:01 +0200, Hannes Reinecke wrote:
>> On 6/23/21 3:51 PM, Lennart Poettering wrote:
>>> On Mi, 23.06.21 15:10, Matteo Croce (mcroce@...ux.microsoft.com) wrote:
>>>
>>>> On Wed, Jun 23, 2021 at 1:49 PM Christoph Hellwig <hch@...radead.org> wrote:
>>>>> On Wed, Jun 23, 2021 at 12:58:53PM +0200, Matteo Croce wrote:
>>>>>> +void inc_diskseq(struct gendisk *disk)
>>>>>> +{
>>>>>> +     static atomic64_t diskseq;
>>>>>
>>>>> Please don't hide file scope variables in functions.
>>>>>
>>>>
>>>> I just didn't want to clobber that file namespace, as that is the only
>>>> point where it's used.
>>>>
>>>>> Can you explain a little more why we need a global sequence count vs
>>>>> a per-disk one here?
>>>>
>>>> The point of the whole series is to have an unique sequence number for
>>>> all the disks.
>>>> Events can arrive to the userspace delayed or out-of-order, so this
>>>> helps to correlate events to the disk.
>>>> It might seem strange, but there isn't a way to do this yet, so I come
>>>> up with a global, monotonically incrementing number.
>>>
>>> To extend on this and given an example why the *global* sequence number
>>> matters:
>>>
>>> Consider you plug in a USB storage key, and it gets named
>>> /dev/sda. You unplug it, the kernel structures for that device all
>>> disappear. Then you plug in a different USB storage key, and since
>>> it's the only one it will too be called /dev/sda.
>>>
>>> With the global sequence number we can still distinguish these two
>>> devices even though otherwise they can look pretty much identical. If
>>> we had per-device counters then this would fall flat because the
>>> counter would be flushed out when the device disappears and when a device
>>> reappears under the same generic name we couldn't assign it a
>>> different sequence number than before.
>>>
>>> Thus: a global instead of local sequence number counter is absolutely
>>> *key* for the problem this is supposed to solve
>>>
>> Well ... except that you'll need to keep track of the numbers (otherwise
>> you wouldn't know if the numbers changed, right?).
>> And if you keep track of the numbers you probably will have to implement
>> an uevent listener to get the events in time.
>> But if you have an uevent listener you will also get the add/remove
>> events for these devices.
>> And if you get add and remove events you can as well implement sequence
>> numbers in your application, seeing that you have all information
>> allowing you to do so.
>> So why burden the kernel with it?
>>
>> Cheers,
>>
>> Hannes
> 
> Hi,
> 
> We need this so that we can reliably correlate events to instances of a
> device. Events alone cannot solve this problem, because events _are_
> the problem.
> 
In which sense?
Yes, events can be delayed (if you list to uevents), but if you listen 
to kernel events there shouldn't be a delay, right?

Cheers,

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