Fix persistent grants negotiation with a behavior change

The first patch of this patchset fixes 'feature_persistent' parameter handling in 'blkback' to respect the frontend's persistent grants support always. The fix makes a behavioral change, so the second patch makes the counterpart of 'blkfront' to consistently follow the behavior change.

Changes from v2
(https://lore.kernel.org/xen-devel/[email protected]/)
- Keep the behavioral change of v1
- Update blkfront's counterpart to follow the changed behavior
- Update documents for the changed behavior

Changes from v1
(https://lore.kernel.org/xen-devel/[email protected]/)
- Avoid the behavioral change
(https://lore.kernel.org/xen-devel/[email protected]/)
- Rebase on latest xen/tip/linux-next
- Re-work by SeongJae Park <[email protected]>
- Cc stable@

Maximilian Heyne (1):
xen, blkback: fix persistent grants negotiation

SeongJae Park (1):
xen-blkfront: Apply 'feature_persistent' parameter when connect

 Documentation/ABI/testing/sysfs-driver-xen-blkback  | 2 +-
 Documentation/ABI/testing/sysfs-driver-xen-blkfront | 2 +-
 drivers/block/xen-blkback/xenbus.c                  | 9 +++------
 drivers/block/xen-blkfront.c                        | 4 +---
 4 files changed, 6 insertions(+), 11 deletions(-)

2.25.1

From: Maximilian Heyne <[email protected]>

Given dom0 supports persistent grants but the guest does not. Then, when attaching a block device during runtime of the guest, dom0 will enable persistent grants for this newly attached block device:

  $ xenstore-ls -f | grep 20674 | grep persistent
  /local/domain/0/backend/vbd/20674/768/feature-persistent = "0"
  /local/domain/0/backend/vbd/20674/51792/feature-persistent = "1"

Here disk 768 was attached during guest creation while 51792 was attached at runtime. If the guest would have advertised the persistent grant feature, there would be a xenstore entry like:

/local/domain/20674/device/vbd/51792/feature-persistent = "1"

Persistent grants are also used when the guest tries to access the disk which can be seen when enabling log stats:

  $ echo 1 > /sys/module/xen_blkback/parameters/log_stats
  $ dmesg
  xen-blkback: (20674.xvdf-0): oo   0  |  rd    0  |  wr    0  |  f    0 |  ds    0 | pg:    1/1056

The "pg: 1/1056" shows that one persistent grant is used.

Before commit aac8a70db24b ("xen-blkback: add a parameter for disabling of persistent grants") vbd->feature_gnt_persistent was set in connect_ring. After the commit it was intended to be initialized in xen_vbd_create and then set according to the guest feature availability in connect_ring. However, with a running guest, connect_ring might be called before xen_vbd_create and vbd->feature_gnt_persistent will be incorrectly initialized. xen_vbd_create will overwrite it with the value of feature_persistent regardless whether the guest actually supports persistent grants.

With this commit, vbd->feature_gnt_persistent is set only in connect_ring and this is the only use of the module parameter feature_persistent. This avoids races when the module parameter changes during the block attachment process.

Note that vbd->feature_gnt_persistent doesn't need to be initialized in xen_vbd_create. It's next use is in connect which can only be called once connect_ring has initialized the rings. xen_update_blkif_status is checking for this.

Please also note that this commit makes a behavioral change of the parameter. That is, the parameter made effect on only newly created backends before this commit, but it makes the effect for every new connection after this commit. Therefore, this commit also updates the document.

---
 Documentation/ABI/testing/sysfs-driver-xen-blkback | 2 +-
 drivers/block/xen-blkback/xenbus.c                 | 9 +++------
 2 files changed, 4 insertions(+), 7 deletions(-)

index 7faf719af165..fac0f429a869 100644
@@ -42,5 +42,5 @@ KernelVersion:  5.10
 Contact:        Maximilian Heyne <[email protected]>
 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).

index 97de13b14175..874b846fb622 100644
@@ -520,8 +520,6 @@ static int xen_vbd_create(struct xen_blkif *blkif, blkif_vdev_t handle,
 	if (bdev_max_secure_erase_sectors(bdev))
 		vbd->discard_secure = true;
 
-	vbd->feature_gnt_persistent = feature_persistent;
-
 	pr_debug("Successful creation of handle=%04x (dom=%u)\n",
 		handle, blkif->domid);
 	return 0;

@@ -1087,10 +1085,9 @@ static int connect_ring(struct backend_info *be)
 		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;
 

2.25.1

Previous commit made xen-blkback's 'feature_persistent' parameter to make effect for not only newly created backends but also for every reconnected backends. This commit makes xen-blkfront's counterpart parameter to works in same manner and update the document to avoid any confusion due to inconsistent behavior of same-named parameters.

---
 Documentation/ABI/testing/sysfs-driver-xen-blkfront | 2 +-
 drivers/block/xen-blkfront.c                        | 4 +---
 2 files changed, 2 insertions(+), 4 deletions(-)

index 7f646c58832e..4d36c5a10546 100644
@@ -15,5 +15,5 @@ KernelVersion:  5.10
 Contact:        Maximilian Heyne <[email protected]>
 Description:
                 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).

index 3646c0cae672..4e763701b372 100644
@@ -1988,8 +1988,6 @@ static int blkfront_probe(struct xenbus_device *dev,
 	info->vdevice = vdevice;
 	info->connected = BLKIF_STATE_DISCONNECTED;
 
-	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))
 		blkfront_setup_discard(info);
 
-	if (info->feature_persistent)
+	if (feature_persistent)
 		info->feature_persistent =
 			!!xenbus_read_unsigned(info->xbdev->otherend,
 					       "feature-persistent", 0);

2.25.1

Hi all,

I made the behavior change as requested by Andrii[1]. I therefore made similar behavior change to blkfront and Cc-ed stable for the second change, too.

To make the change history clear and reduce the stable side overhead, however, it might be better to apply the v2, which don't make behavior change but only fix the issue, Cc stable@ for it, make the behavior change commits for both blkback and blkfront, update the documents, and don't Cc stable@ for the behavior change and documents update commits.

One downside of that would be that it will make a behavioral difference in pre-5.19.x and post-5.19.x.

I think both downsides are not critical, so posted this patchset in this shape. If anyone prefer some changes, please let me know, though.

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

Thanks,
SJ

Hi all,

Now I realize that commit aac8a70db24b ("xen-blkback: add a parameter for disabling of persistent grants") introduced two issues. One is what Max reported with his patch, and the second one is unintended behavioral change that broke Andrii's use case.

That is, Andrii's use case should had no problem at all before the introduction of 'feature_persistent', as at that time 'blkback' checked if the frontend support the persistent grants for every 'reconnect()' and enables it if so. However, introduction of the parameter made it behaves differently.

Yes, we intended to make the prameter to make effects to newly created devices. But, as it breaks user workflows, this should be fixed. Same for the 'blkfront' side 'feature_persistent'.

I'd say having one patch for each issue would be the right way to go, and all fixes should Cc stable@.

The unintended behavioral fix should also be considered fix and therefore should be merged into stable@, so above concern is not valid.

I will send the next spin soon.

Thanks,
SJ

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