linux-kernel - Re: [PATCH v2] xen-blkfront: allow discard-* nodes to be optional

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Message-ID: <20210120144425.ccdkettzzr4hvwip@Air-de-Roger>
Date:   Wed, 20 Jan 2021 15:44:25 +0100
From:   Roger Pau Monné <roger.pau@...rix.com>
To:     Jan Beulich <jbeulich@...e.com>
CC:     <linux-kernel@...r.kernel.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Jens Axboe <axboe@...nel.dk>, <xen-devel@...ts.xenproject.org>,
        <linux-block@...r.kernel.org>,
        Arthur Borsboom <arthurborsboom@...il.com>
Subject: Re: [PATCH v2] xen-blkfront: allow discard-* nodes to be optional

On Wed, Jan 20, 2021 at 03:37:57PM +0100, Jan Beulich wrote:
> On 20.01.2021 15:35, Roger Pau Monné wrote:
> > On Wed, Jan 20, 2021 at 03:23:30PM +0100, Arthur Borsboom wrote:
> >> Hi Roger,
> >>
> >> I have set up a test environment based on Linux 5.11.0-rc4.
> >> The patch did not apply clean, so I copied/pasted the patch manually.
> >>
> >> Without the patch the call trace (as reported) is visible in dmesg.
> >> With the patch the call trace in dmesg is gone, but ... (there is always a
> >> but) ...
> >>
> >> Now the discard action returns the following.
> >>
> >> [arthur@...t-arch ~]$ sudo fstrim -v /
> >> fstrim: /: the discard operation is not supported
> >>
> >> It might be correct, but of course I was hoping the Xen VM guest would pass
> >> on the discard request to the block device in the Xen VM host, which is a
> >> disk partition.
> >> Any suggestions?
> > 
> > Hm, that's not what I did see on my testing, the operation worked OK,
> > and that's what I would expect to happen in your case also, since I
> > know the xenstore keys.
> 
> Does discard work on partitions (and not just on entire disks)?
> It's been a while since I last had anything to do with this code,
> so I may well misremember.

The command provided did work for me, ie:

# fstrim -v /
/: 19.8 GiB (21190717440 bytes) trimmed

AFAICT the blkif discard interface we provide operates at physical
block granularity, so it's possible to discard a single block, and
hence should work against partitions.

Roger.