[PATCH 5.15 757/779] xen-blkback: Apply feature_persistent parameter when connect

[Greg Kroah-Hartman]

From: Maximilian Heyne <mheyne@xxxxxxxxx>

commit e94c6101e151b019b8babc518ac2a6ada644a5a1 upstream.

In some use cases[1], the backend is created while the frontend doesn't
support the persistent grants feature, but later the frontend can be
changed to support the feature and reconnect.  In the past, 'blkback'
enabled the persistent grants feature since it unconditionally checked
if frontend supports the persistent grants feature for every connect
('connect_ring()') and decided whether it should use persistent grans or
not.

However, commit aac8a70db24b ("xen-blkback: add a parameter for
disabling of persistent grants") has mistakenly changed the behavior.
It made the frontend feature support check to not be repeated once it
shown the 'feature_persistent' as 'false', or the frontend doesn't
support persistent grants.

This commit changes the behavior of the parameter to make effect for
every connect, so that the previous workflow can work again as expected.

[1] https://lore.kernel.org/xen-devel/CAJwUmVB6H3iTs-C+U=v-pwJB7-_ZRHPxHzKRJZ22xEPW7z8a=g@xxxxxxxxxxxxxx/

Reported-by: Andrii Chepurnyi <andrii.chepurnyi82@xxxxxxxxx>
Fixes: aac8a70db24b ("xen-blkback: add a parameter for disabling of persistent grants")
Cc: <stable@xxxxxxxxxxxxxxx> # 5.10.x
Signed-off-by: Maximilian Heyne <mheyne@xxxxxxxxx>
Signed-off-by: SeongJae Park <sj@xxxxxxxxxx>
Reviewed-by: Maximilian Heyne <mheyne@xxxxxxxxx>
Reviewed-by: Juergen Gross <jgross@xxxxxxxx>
Link: https://lore.kernel.org/r/20220715225108.193398-3-sj@xxxxxxxxxx
Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 Documentation/ABI/testing/sysfs-driver-xen-blkback |    2 +-
 drivers/block/xen-blkback/xenbus.c                 |    9 +++------
 2 files changed, 4 insertions(+), 7 deletions(-)

--- a/Documentation/ABI/testing/sysfs-driver-xen-blkback
+++ b/Documentation/ABI/testing/sysfs-driver-xen-blkback
@@ -42,5 +42,5 @@ KernelVersion:  5.10
 Contact:        SeongJae Park <sjpark@xxxxxxxxx>
 Description:
                 Whether to enable the persistent grants feature or not.  Note
-                that this option only takes effect on newly created backends.
+                that this option only takes effect on newly connected backends.
                 The default is Y (enable).
--- a/drivers/block/xen-blkback/xenbus.c
+++ b/drivers/block/xen-blkback/xenbus.c
@@ -185,8 +185,6 @@ static struct xen_blkif *xen_blkif_alloc
 	__module_get(THIS_MODULE);
 	INIT_WORK(&blkif->free_work, xen_blkif_deferred_free);
 
-	blkif->vbd.feature_gnt_persistent = feature_persistent;
-
 	return blkif;
 }
 
@@ -1089,10 +1087,9 @@ static int connect_ring(struct backend_i
 		xenbus_dev_fatal(dev, err, "unknown fe protocol %s", protocol);
 		return -ENOSYS;
 	}
-	if (blkif->vbd.feature_gnt_persistent)
-		blkif->vbd.feature_gnt_persistent =
-			xenbus_read_unsigned(dev->otherend,
-					"feature-persistent", 0);
+
+	blkif->vbd.feature_gnt_persistent = feature_persistent &&
+		xenbus_read_unsigned(dev->otherend, "feature-persistent", 0);
 
 	blkif->vbd.overflow_max_grants = 0;