linux-kernel - [PATCH 2/2] xen-blkfront: Advertise feature-persistent as user requested

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Date:   Thu, 25 Aug 2022 16:15:11 +0000
From:   SeongJae Park <sj@...nel.org>
To:     jgross@...e.com, roger.pau@...rix.com
Cc:     marmarek@...isiblethingslab.com, mheyne@...zon.de,
        xen-devel@...ts.xenproject.org, axboe@...nel.dk,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        SeongJae Park <sj@...nel.org>, stable@...r.kernel.org
Subject: [PATCH 2/2] xen-blkfront: Advertise feature-persistent as user requested

Commit e94c6101e151 ("xen-blkback: Apply 'feature_persistent' parameter
when connect") made blkback to advertise its support of the persistent
grants feature only if the user sets the 'feature_persistent' parameter
of the driver and the frontend advertised its support of the feature.
However, following commit 402c43ea6b34 ("xen-blkfront: Apply
'feature_persistent' parameter when connect") made the blkfront to work
in the same way.  That is, blkfront also advertises its support of the
persistent grants feature only if the user sets the 'feature_persistent'
parameter of the driver and the backend advertised its support of the
feature.

Hence blkback and blkfront will never advertise their support of the
feature but wait until the other advertises the support, even though
users set the 'feature_persistent' parameters of the drivers.  As a
result, the persistent grants feature is disabled always regardless of
the 'feature_persistent' values[1].

The problem comes from the misuse of the semantic of the advertisement
of the feature.  The advertisement of the feature should means only
availability of the feature not the decision for using the feature.
However, current behavior is working in the wrong way.

This commit fixes the issue by making the blkfront advertises its
support of the feature as user requested via 'feature_persistent'
parameter regardless of the otherend's support of the feature.

[1] https://lore.kernel.org/xen-devel/bd818aba-4857-bc07-dc8a-e9b2f8c5f7cd@suse.com/

Fixes: 402c43ea6b34 ("xen-blkfront: Apply 'feature_persistent' parameter when connect")
Cc: <stable@...r.kernel.org> # 5.10.x
Reported-by: Marek Marczykowski-Górecki <marmarek@...isiblethingslab.com>
Suggested-by: Juergen Gross <jgross@...e.com>
Signed-off-by: SeongJae Park <sj@...nel.org>
---
 drivers/block/xen-blkfront.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
index 8e56e69fb4c4..dfae08115450 100644
--- a/drivers/block/xen-blkfront.c
+++ b/drivers/block/xen-blkfront.c
@@ -213,6 +213,9 @@ struct blkfront_info
 	unsigned int feature_fua:1;
 	unsigned int feature_discard:1;
 	unsigned int feature_secdiscard:1;
+	/* Connect-time cached feature_persistent parameter */
+	unsigned int feature_persistent_parm:1;
+	/* Persistent grants feature negotiation result */
 	unsigned int feature_persistent:1;
 	unsigned int bounce:1;
 	unsigned int discard_granularity;
@@ -1848,7 +1851,7 @@ static int talk_to_blkback(struct xenbus_device *dev,
 		goto abort_transaction;
 	}
 	err = xenbus_printf(xbt, dev->nodename, "feature-persistent", "%u",
-			info->feature_persistent);
+			info->feature_persistent_parm);
 	if (err)
 		dev_warn(&dev->dev,
 			 "writing persistent grants feature to xenbus");
@@ -2281,7 +2284,8 @@ static void blkfront_gather_backend_features(struct blkfront_info *info)
 	if (xenbus_read_unsigned(info->xbdev->otherend, "feature-discard", 0))
 		blkfront_setup_discard(info);

-	if (feature_persistent)
+	info->feature_persistent_parm = feature_persistent;
+	if (info->feature_persistent_parm)
 		info->feature_persistent =
 			!!xenbus_read_unsigned(info->xbdev->otherend,
 					       "feature-persistent", 0);
-- 
2.25.1