linux-kernel - Re: [PATCH] nbd: provide a way for userspace processes to identify device backends

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Message-ID: <CANwsLLEtOws-+X=3CCGzO88poP9NF8uNN8aW9wHOJ+QA=SFH3A@mail.gmail.com>
Date:   Thu, 27 May 2021 10:52:50 +0530
From:   Prasanna Kalever <pkalever@...hat.com>
To:     Matteo Croce <mcroce@...ux.microsoft.com>
Cc:     Ming Lei <ming.lei@...hat.com>,
        Prasanna Kumar Kalever <prasanna.kalever@...hat.com>,
        linux-kernel@...r.kernel.org, linux-block@...r.kernel.org,
        nbd@...er.debian.org, Josef Bacik <josef@...icpanda.com>,
        Jens Axboe <axboe@...nel.dk>,
        Ilya Dryomov <idryomov@...hat.com>,
        Xiubo Li <xiubli@...hat.com>,
        Lennart Poettering <lennart@...ttering.net>,
        Luca Boccassi <bluca@...ian.org>
Subject: Re: [PATCH] nbd: provide a way for userspace processes to identify
 device backends

On Wed, May 26, 2021 at 7:44 PM Matteo Croce <mcroce@...ux.microsoft.com> wrote:
>
> On Wed, May 19, 2021 at 4:33 PM Matteo Croce <mcroce@...ux.microsoft.com> wrote:
> >
> > On Tue, May 18, 2021 at 9:52 AM Prasanna Kalever <pkalever@...hat.com> wrote:
> > > > BTW, loop has similar issue, and patch of 'block: add a sequence number to disks'
> > > > is added for addressing this issue, what do you think of that generic
> > > > approach wrt. this nbd's issue? such as used the exposed sysfs sequence number
> > > > for addressing this issue?
> > > >
> > > > https://lore.kernel.org/linux-block/YH81n34d2G3C4Re+@gardel-login/#r
> > >
> > > If I understand the changes and the background of the fix correctly, I
> > > think with that fix author is trying to monotonically increase the seq
> > > number and add it to the disk on every single device map/attach and
> > > expose it through the sysfs, which will help the userspace processes
> > > further to correlate events for particular and specific devices that
> > > reuse the same loop device.
> > >
> >
> > Yes, but nothing prevents to use diskseq in nbd, and increase it on reconfigure.
> > That would allow to detect if the device has been reconfigured.
> >
> > Regards,
> > --
> > per aspera ad upstream
>
> FYI: I just sent a v2 of the diskseq change
>
> https://lore.kernel.org/linux-block/20210520135622.44625-1-mcroce@linux.microsoft.com/

Thanks, Matteo, I will take a look.

Just to set the expectation here, I don't have any thoughts on
leverage the diskseq number for nbd as part of this patch. This patch
is trying to solve a different problem which is more severe for us
than helping to identify the reconfigured events.

That all said, once diskseq patches are merged, I will surely open a
new patch with the required changes in nbd, to leverage diskseq
number.

Best Regards,
--
Prasanna

>
> --
> per aspera ad upstream
>