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

[Oleksandr <olekstysh@xxxxxxxxx>]

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@xxxxxxxxxxxxxxx> # 5.10.x
> Signed-off-by: Maximilian Heyne <mheyne@xxxxxxxxx>
> Signed-off-by: SeongJae Park <sj@xxxxxxxxxx>
> ---
>
> Changes from v1[1]
> - Avoid the behavioral change[2]
> - Rebase on latest xen/tip/linux-next
> - Re-work by SeongJae Park <sj@xxxxxxxxxx>
> - Cc stable@
>
> [1] https://lore.kernel.org/xen-devel/20220106091013.126076-1-mheyne@xxxxxxxxx/
> [2] https://lore.kernel.org/xen-devel/20220121102309.27802-1-sj@xxxxxxxxxx/
>
>   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