[PATCH v4 3/3] xen-blkfront: Apply 'feature_persistent' parameter when connect

From: SeongJae Park
Date: Fri Jul 15 2022 - 18:51:38 EST

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

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.

Similar behavioral change has made on 'blkfront' by commit 74a852479c68
("xen-blkfront: add a parameter for disabling of persistent grants").
This commit changes the behavior of the parameter to make effect for
every connect, so that the previous behavior of 'blkfront' can be

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

Fixes: 74a852479c68 ("xen-blkfront: add a parameter for disabling of persistent grants")
Cc: <stable@xxxxxxxxxxxxxxx> # 5.10.x
Signed-off-by: SeongJae Park <sj@xxxxxxxxxx>
Documentation/ABI/testing/sysfs-driver-xen-blkfront | 2 +-
drivers/block/xen-blkfront.c | 4 +---
2 files changed, 2 insertions(+), 4 deletions(-)

diff --git a/Documentation/ABI/testing/sysfs-driver-xen-blkfront b/Documentation/ABI/testing/sysfs-driver-xen-blkfront
index 7f646c58832e..4d36c5a10546 100644
--- a/Documentation/ABI/testing/sysfs-driver-xen-blkfront
+++ b/Documentation/ABI/testing/sysfs-driver-xen-blkfront
@@ -15,5 +15,5 @@ KernelVersion: 5.10
Contact: Maximilian Heyne <mheyne@xxxxxxxxx>
Whether to enable the persistent grants feature or not. Note
- that this option only takes effect on newly created frontends.
+ that this option only takes effect on newly connected frontends.
The default is Y (enable).
diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
index 3646c0cae672..4e763701b372 100644
--- a/drivers/block/xen-blkfront.c
+++ b/drivers/block/xen-blkfront.c
@@ -1988,8 +1988,6 @@ static int blkfront_probe(struct xenbus_device *dev,
info->vdevice = vdevice;

- info->feature_persistent = feature_persistent;
/* Front end dir is a number, which is used as the id. */
info->handle = simple_strtoul(strrchr(dev->nodename, '/')+1, NULL, 0);
dev_set_drvdata(&dev->dev, info);
@@ -2283,7 +2281,7 @@ static void blkfront_gather_backend_features(struct blkfront_info *info)
if (xenbus_read_unsigned(info->xbdev->otherend, "feature-discard", 0))

- if (info->feature_persistent)
+ if (feature_persistent)
info->feature_persistent =
"feature-persistent", 0);