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

From: Lennart Poettering <mzxreary@0pointer.de>
To: Matteo Croce <mcroce@linux.microsoft.com>
Cc: Christoph Hellwig <hch@infradead.org>,
	linux-block@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	Jens Axboe <axboe@kernel.dk>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Luca Boccassi <bluca@debian.org>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	Damien Le Moal <damien.lemoal@wdc.com>, Tejun Heo <tj@kernel.org>,
	Javier Gonz??lez <javier@javigon.com>,
	Niklas Cassel <niklas.cassel@wdc.com>,
	Johannes Thumshirn <johannes.thumshirn@wdc.com>,
	Hannes Reinecke <hare@suse.de>,
	Matthew Wilcox <willy@infradead.org>,
	JeffleXu <jefflexu@linux.alibaba.com>
Subject: Re: [PATCH v3 1/6] block: add disk sequence number
Date: Wed, 23 Jun 2021 15:51:11 +0200	[thread overview]
Message-ID: <YNM8T44v5FTViVWM@gardel-login> (raw)
In-Reply-To: <CAFnufp1gdag0rGQ8K4_2oB6_aC+EZgfgwd2eL4-AxpG0mK+_qQ@mail.gmail.com>

On Mi, 23.06.21 15:10, Matteo Croce (mcroce@linux.microsoft.com) wrote:

> On Wed, Jun 23, 2021 at 1:49 PM Christoph Hellwig <hch@infradead.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

Lennart

--
Lennart Poettering, Berlin