linux-kernel - Re: [PATCH] xen-blkfront: don't make discard-alignment mandatory

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Date:   Tue, 19 Jan 2021 11:21:39 +0100
From:   Roger Pau Monné <roger.pau@...rix.com>
To:     Jürgen Groß <jgross@...e.com>
CC:     <linux-kernel@...r.kernel.org>,
        Arthur Borsboom <arthurborsboom@...il.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.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>
Subject: Re: [PATCH] xen-blkfront: don't make discard-alignment mandatory

On Tue, Jan 19, 2021 at 11:11:26AM +0100, Jürgen Groß wrote:
> On 19.01.21 11:06, Roger Pau Monné wrote:
> > On Tue, Jan 19, 2021 at 08:43:01AM +0100, Jürgen Groß wrote:
> > > On 18.01.21 16:15, Roger Pau Monne wrote:
> > > > Don't require the discard-alignment xenstore node to be present in
> > > > order to correctly setup the feature. This can happen with versions of
> > > > QEMU that only write the discard-granularity but not the
> > > > discard-alignment node.
> > > > 
> > > > Assume discard-alignment is 0 if not present. While there also fix the
> > > > logic to not enable the discard feature if discard-granularity is not
> > > > present.
> > > > 
> > > > Reported-by: Arthur Borsboom <arthurborsboom@...il.com>
> > > > Signed-off-by: Roger Pau Monné <roger.pau@...rix.com>
> > > > ---
> > > > Cc: Boris Ostrovsky <boris.ostrovsky@...cle.com>
> > > > Cc: Juergen Gross <jgross@...e.com>
> > > > Cc: Stefano Stabellini <sstabellini@...nel.org>
> > > > Cc: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
> > > > Cc: "Roger Pau Monné" <roger.pau@...rix.com>
> > > > Cc: Jens Axboe <axboe@...nel.dk>
> > > > Cc: xen-devel@...ts.xenproject.org
> > > > Cc: linux-block@...r.kernel.org
> > > > Cc: Arthur Borsboom <arthurborsboom@...il.com>
> > > > ---
> > > >    drivers/block/xen-blkfront.c | 25 +++++++++++++------------
> > > >    1 file changed, 13 insertions(+), 12 deletions(-)
> > > > 
> > > > diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
> > > > index 5265975b3fba..5a93f7cc2939 100644
> > > > --- a/drivers/block/xen-blkfront.c
> > > > +++ b/drivers/block/xen-blkfront.c
> > > > @@ -2179,22 +2179,23 @@ static void blkfront_closing(struct blkfront_info *info)
> > > >    static void blkfront_setup_discard(struct blkfront_info *info)
> > > >    {
> > > > -	int err;
> > > > -	unsigned int discard_granularity;
> > > > -	unsigned int discard_alignment;
> > > > +	unsigned int discard_granularity = 0;
> > > > +	unsigned int discard_alignment = 0;
> > > > +	unsigned int discard_secure = 0;
> > > > -	info->feature_discard = 1;
> > > > -	err = xenbus_gather(XBT_NIL, info->xbdev->otherend,
> > > > +	xenbus_gather(XBT_NIL, info->xbdev->otherend,
> > > >    		"discard-granularity", "%u", &discard_granularity,
> > > >    		"discard-alignment", "%u", &discard_alignment,
> > > > +		"discard-secure", "%u", &discard_secure,
> > > >    		NULL);
> > > 
> > > This would mean that "discard-secure" will be evaluated only if the
> > > other two items are set in Xenstore. From blkif.h I can't see this is
> > > required, and your patch is modifying today's behavior in this regard.
> > > 
> > > You might want to have three xenbus_read_unsigned() calls instead.
> > 
> > You are right, discard-secure should be fetched regardless of whether
> > discard-alignment exists.
> > 
> > I can fetch discard-granularity and discard-alignment using
> > xenbus_gather and keep discard-secure using xenbus_read_unsigned. Let
> > me send a new version.
> 
> I'm still not convinced this is correct. blkif.h doesn't mention that
> discard-alignment will be valid only with discard-granularity being
> present.

No, in fact I think I need to rework this a little further. Just
having feature-discard = 1 should enable the discard functionality, by
setting discard-granularity = physical block size and
discard-alignment = 0.

Roger.