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: <[email protected]> # 5.10.x
Signed-off-by: Maximilian Heyne <[email protected]>
Signed-off-by: SeongJae Park <[email protected]>
---
Changes from v1[1]
- Avoid the behavioral change[2]
- Rebase on latest xen/tip/linux-next
- Re-work by SeongJae Park <[email protected]>
- Cc stable@
[1] https://lore.kernel.org/xen-devel/[email protected]/
[2] https://lore.kernel.org/xen-devel/[email protected]/
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
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: <[email protected]> # 5.10.x
> Signed-off-by: Maximilian Heyne <[email protected]>
> Signed-off-by: SeongJae Park <[email protected]>
> ---
>
> Changes from v1[1]
> - Avoid the behavioral change[2]
> - Rebase on latest xen/tip/linux-next
> - Re-work by SeongJae Park <[email protected]>
> - Cc stable@
>
> [1] https://lore.kernel.org/xen-devel/[email protected]/
> [2] https://lore.kernel.org/xen-devel/[email protected]/
>
> 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
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 <[email protected]> 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/[email protected]/
Thanks,
SJ
>
> [1]
> https://lore.kernel.org/xen-devel/[email protected]/
> [2] https://lore.kernel.org/xen-devel/[email protected]/
>
> Best regards,
> Andrii
>
> On Fri, Jul 15, 2022 at 1:15 PM Oleksandr <[email protected]> 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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