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

[Oleksandr <olekstysh@gmail.com>]

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