[PATCH v2] xen-blkback: fix persistent grants negotiation

[PATCH v2] xen-blkback: fix persistent grants negotiation

[SeongJae Park]

Persistent grants feature can be used only when both backend and the
frontend supports the feature.  The feature was always supported by
'blkback', but commit aac8a70db24b ("xen-blkback: add a parameter for
disabling of persistent grants") has introduced a parameter for
disabling it runtime.

To avoid the parameter be updated while being used by 'blkback', the
commit caches the parameter into 'vbd->feature_gnt_persistent' in
'xen_vbd_create()', and then check if the guest also supports the
feature and finally updates the field in 'connect_ring()'.

However, 'connect_ring()' could be called before 'xen_vbd_create()', so
later execution of 'xen_vbd_create()' can wrongly overwrite 'true' to
'vbd->feature_gnt_persistent'.  As a result, 'blkback' could try to use
'persistent grants' feature even if the guest doesn't support the
feature.

This commit fixes the issue by moving the parameter value caching to
'xen_blkif_alloc()', which allocates the 'blkif'.  Because the struct
embeds 'vbd' object, which will be used by 'connect_ring()' later, this
should be called before 'connect_ring()' and therefore this should be
the right and safe place to do the caching.

Fixes: aac8a70db24b ("xen-blkback: add a parameter for disabling of persistent grants")
Cc: <stable@vger.kernel.org> # 5.10.x
Signed-off-by: Maximilian Heyne <mheyne@amazon.de>
Signed-off-by: SeongJae Park <sj@kernel.org>
---

Changes from v1[1]
- Avoid the behavioral change[2]
- Rebase on latest xen/tip/linux-next
- Re-work by SeongJae Park <sj@kernel.org>
- Cc stable@

[1] https://lore.kernel.org/xen-devel/20220106091013.126076-1-mheyne@amazon.de/
[2] https://lore.kernel.org/xen-devel/20220121102309.27802-1-sj@kernel.org/

 drivers/block/xen-blkback/xenbus.c | 15 +++++++--------
 1 file changed, 7 insertions(+), 8 deletions(-)

diff --git a/drivers/block/xen-blkback/xenbus.c b/drivers/block/xen-blkback/xenbus.c
index 97de13b14175..16c6785d260c 100644
--- a/drivers/block/xen-blkback/xenbus.c
+++ b/drivers/block/xen-blkback/xenbus.c
@@ -157,6 +157,11 @@ static int xen_blkif_alloc_rings(struct xen_blkif *blkif)
 	return 0;
 }
 
+/* Enable the persistent grants feature. */
+static bool feature_persistent = true;
+module_param(feature_persistent, bool, 0644);
+MODULE_PARM_DESC(feature_persistent, "Enables the persistent grants feature");
+
 static struct xen_blkif *xen_blkif_alloc(domid_t domid)
 {
 	struct xen_blkif *blkif;
@@ -181,6 +186,8 @@ static struct xen_blkif *xen_blkif_alloc(domid_t domid)
 	__module_get(THIS_MODULE);
 	INIT_WORK(&blkif->free_work, xen_blkif_deferred_free);
 
+	blkif->vbd.feature_gnt_persistent = feature_persistent;
+
 	return blkif;
 }
 
@@ -472,12 +479,6 @@ static void xen_vbd_free(struct xen_vbd *vbd)
 	vbd->bdev = NULL;
 }
 
-/* Enable the persistent grants feature. */
-static bool feature_persistent = true;
-module_param(feature_persistent, bool, 0644);
-MODULE_PARM_DESC(feature_persistent,
-		"Enables the persistent grants feature");
-
 static int xen_vbd_create(struct xen_blkif *blkif, blkif_vdev_t handle,
 			  unsigned major, unsigned minor, int readonly,
 			  int cdrom)
@@ -520,8 +521,6 @@ 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;
-- 
2.25.1

Re: [PATCH v2] xen-blkback: fix persistent grants negotiation

[Oleksandr]

On 15.07.22 01:44, SeongJae Park wrote:


Hello all.

Adding Andrii Chepurnyi to CC who have played with the use-case which 
required reconnect recently and faced some issues with 
feature_persistent handling.




> Persistent grants feature can be used only when both backend and the
> frontend supports the feature.  The feature was always supported by
> 'blkback', but commit aac8a70db24b ("xen-blkback: add a parameter for
> disabling of persistent grants") has introduced a parameter for
> disabling it runtime.
>
> To avoid the parameter be updated while being used by 'blkback', the
> commit caches the parameter into 'vbd->feature_gnt_persistent' in
> 'xen_vbd_create()', and then check if the guest also supports the
> feature and finally updates the field in 'connect_ring()'.
>
> However, 'connect_ring()' could be called before 'xen_vbd_create()', so
> later execution of 'xen_vbd_create()' can wrongly overwrite 'true' to
> 'vbd->feature_gnt_persistent'.  As a result, 'blkback' could try to use
> 'persistent grants' feature even if the guest doesn't support the
> feature.
>
> This commit fixes the issue by moving the parameter value caching to
> 'xen_blkif_alloc()', which allocates the 'blkif'.  Because the struct
> embeds 'vbd' object, which will be used by 'connect_ring()' later, this
> should be called before 'connect_ring()' and therefore this should be
> the right and safe place to do the caching.
>
> Fixes: aac8a70db24b ("xen-blkback: add a parameter for disabling of persistent grants")
> Cc: <stable@vger.kernel.org> # 5.10.x
> Signed-off-by: Maximilian Heyne <mheyne@amazon.de>
> Signed-off-by: SeongJae Park <sj@kernel.org>
> ---
>
> Changes from v1[1]
> - Avoid the behavioral change[2]
> - Rebase on latest xen/tip/linux-next
> - Re-work by SeongJae Park <sj@kernel.org>
> - Cc stable@
>
> [1] https://lore.kernel.org/xen-devel/20220106091013.126076-1-mheyne@amazon.de/
> [2] https://lore.kernel.org/xen-devel/20220121102309.27802-1-sj@kernel.org/
>
>   drivers/block/xen-blkback/xenbus.c | 15 +++++++--------
>   1 file changed, 7 insertions(+), 8 deletions(-)
>
> diff --git a/drivers/block/xen-blkback/xenbus.c b/drivers/block/xen-blkback/xenbus.c
> index 97de13b14175..16c6785d260c 100644
> --- a/drivers/block/xen-blkback/xenbus.c
> +++ b/drivers/block/xen-blkback/xenbus.c
> @@ -157,6 +157,11 @@ static int xen_blkif_alloc_rings(struct xen_blkif *blkif)
>   	return 0;
>   }
>   
> +/* Enable the persistent grants feature. */
> +static bool feature_persistent = true;
> +module_param(feature_persistent, bool, 0644);
> +MODULE_PARM_DESC(feature_persistent, "Enables the persistent grants feature");
> +
>   static struct xen_blkif *xen_blkif_alloc(domid_t domid)
>   {
>   	struct xen_blkif *blkif;
> @@ -181,6 +186,8 @@ static struct xen_blkif *xen_blkif_alloc(domid_t domid)
>   	__module_get(THIS_MODULE);
>   	INIT_WORK(&blkif->free_work, xen_blkif_deferred_free);
>   
> +	blkif->vbd.feature_gnt_persistent = feature_persistent;
> +
>   	return blkif;
>   }
>   
> @@ -472,12 +479,6 @@ static void xen_vbd_free(struct xen_vbd *vbd)
>   	vbd->bdev = NULL;
>   }
>   
> -/* Enable the persistent grants feature. */
> -static bool feature_persistent = true;
> -module_param(feature_persistent, bool, 0644);
> -MODULE_PARM_DESC(feature_persistent,
> -		"Enables the persistent grants feature");
> -
>   static int xen_vbd_create(struct xen_blkif *blkif, blkif_vdev_t handle,
>   			  unsigned major, unsigned minor, int readonly,
>   			  int cdrom)
> @@ -520,8 +521,6 @@ 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;

-- 
Regards,

Oleksandr Tyshchenko

Re: [PATCH v2] xen-blkback: fix persistent grants negotiation

[SeongJae Park]

Hello,


Oleksandr, thank you for Cc-ing Andrii.  Andrii, thank you for the comment!

On Fri, 15 Jul 2022 15:00:10 +0300 Andrii Chepurnyi <andrii.chepurnyi82@gmail.com> wrote:

> [-- Attachment #1: Type: text/plain, Size: 5237 bytes --]
> 
> Hello All,
> 
> I faced the mentioned issue recently and just to bring more context here is
> our setup:
> We use pvblock backend for Android guest. It starts using u-boot with
> pvblock support(which frontend doesn't support the persistent grants
> feature), later it loads and starts the Linux kernel(which frontend
> supports the persistent grants feature). So in total, we have sequent two
> different frontends reconnection, the first of which doesn't support
> persistent grants.
> So the original patch [1] perfectly solves the original issue and provides
> the ability to use persistent grants after the reconnection when Linux
> frontend which supports persistent grants comes into play.
> At the same time [2] will disable the persistent grants feature for the
> first and second frontend.

Thank you for this great explanation of your situation.

> Is it possible to keep [1]  as is?

Yes, my concerns about Max's original patch[1] are conflicting behavior
description in the document[1] and different behavior on blkfront-side
'feature_persistent' parameter.  I will post Max's patch again with patches for
blkfront behavior change and Documents updates.

[1] https://lore.kernel.org/xen-devel/20220121102309.27802-1-sj@kernel.org/


Thanks,
SJ

> 
> [1]
> https://lore.kernel.org/xen-devel/20220106091013.126076-1-mheyne@amazon.de/
> [2] https://lore.kernel.org/xen-devel/20220714224410.51147-1-sj@kernel.org/
> 
> Best regards,
> Andrii
> 
> On Fri, Jul 15, 2022 at 1:15 PM Oleksandr <olekstysh@gmail.com> wrote:
> 
> >
> > On 15.07.22 01:44, SeongJae Park wrote:
> >
> >
> > Hello all.
> >
> > Adding Andrii Chepurnyi to CC who have played with the use-case which
> > required reconnect recently and faced some issues with
> > feature_persistent handling.
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