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

["Roger Pau Monné" <roger.pau@citrix.com>]

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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@test-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.

I think it's possible your email client has mangled the patch, I'm
attaching the same patch to this email, could you try to apply it
again and report back? (this time it should apply cleanly)

Thanks, Roger.

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From f09acedb2e40fa84f31cfe4c960dea2037d06e3f Mon Sep 17 00:00:00 2001
From: Roger Pau Monne <roger.pau@citrix.com>
Date: Mon, 18 Jan 2021 11:47:05 +0100
Subject: [PATCH v2] xen-blkfront: allow discard-* nodes to be optional
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

This is inline with the specification described in blkif.h:

 * discard-granularity: should be set to the physical block size if
   node is not present.
 * discard-alignment, discard-secure: should be set to 0 if node not
   present.

This was detected as QEMU would only create the discard-granularity
node but not discard-alignment, and thus the setup done in
blkfront_setup_discard would fail.

Fix blkfront_setup_discard to not fail on missing nodes, and also fix
blkif_set_queue_limits to set the discard granularity to the physical
block size if none is specified in xenbus.

Fixes: ed30bf317c5ce ('xen-blkfront: Handle discard requests.')
Reported-by: Arthur Borsboom <arthurborsboom@gmail.com>
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
---
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: Juergen Gross <jgross@suse.com>
Cc: Stefano Stabellini <sstabellini@kernel.org>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: "Roger Pau Monné" <roger.pau@citrix.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: xen-devel@lists.xenproject.org
Cc: linux-block@vger.kernel.org
Cc: Arthur Borsboom <arthurborsboom@gmail.com>
---
Changes since v2:
 - Allow all discard-* nodes to be optional.
---
 drivers/block/xen-blkfront.c | 20 +++++++-------------
 1 file changed, 7 insertions(+), 13 deletions(-)

diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
index 5265975b3fba..e1c6798889f4 100644
--- a/drivers/block/xen-blkfront.c
+++ b/drivers/block/xen-blkfront.c
@@ -945,7 +945,8 @@ static void blkif_set_queue_limits(struct blkfront_info *info)
 	if (info->feature_discard) {
 		blk_queue_flag_set(QUEUE_FLAG_DISCARD, rq);
 		blk_queue_max_discard_sectors(rq, get_capacity(gd));
-		rq->limits.discard_granularity = info->discard_granularity;
+		rq->limits.discard_granularity = info->discard_granularity ?:
+						 info->physical_sector_size;
 		rq->limits.discard_alignment = info->discard_alignment;
 		if (info->feature_secdiscard)
 			blk_queue_flag_set(QUEUE_FLAG_SECERASE, rq);
@@ -2179,19 +2180,12 @@ 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;
-
 	info->feature_discard = 1;
-	err = xenbus_gather(XBT_NIL, info->xbdev->otherend,
-		"discard-granularity", "%u", &discard_granularity,
-		"discard-alignment", "%u", &discard_alignment,
-		NULL);
-	if (!err) {
-		info->discard_granularity = discard_granularity;
-		info->discard_alignment = discard_alignment;
-	}
+	info->discard_granularity = xenbus_read_unsigned(info->xbdev->otherend,
+							 "discard-granularity",
+							 0);
+	info->discard_alignment = xenbus_read_unsigned(info->xbdev->otherend,
+						       "discard-alignment", 0);
 	info->feature_secdiscard =
 		!!xenbus_read_unsigned(info->xbdev->otherend, "discard-secure",
 				       0);
-- 
2.29.2