Re: [Xen-devel] [PATCH v3] pvcalls-front: Avoid __get_free_pages(GFP_KERNEL) under spinlock

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 11/30/18 6:01 AM, Wen Yang wrote:
> The problem is that we call this with a spin lock held.
> The call tree is:
> pvcalls_front_accept() holds bedata->socket_lock.
>     -> create_active()
>         -> __get_free_pages() uses GFP_KERNEL
>
> The create_active() function is only called from pvcalls_front_accept()
> with a spin_lock held, The allocation is not allowed to sleep and
> GFP_KERNEL is not sufficient.
>
> This issue was detected by using the Coccinelle software.
>
> v2: Add a function doing the allocations which is called
>     outside the lock and passing the allocated data to
>     create_active().
> v3: Use the matching deallocators i.e., free_page()
>     and free_pages(), respectively.
>
> Suggested-by: Juergen Gross <jgross@xxxxxxxx>
> Signed-off-by: Wen Yang <wen.yang99@xxxxxxxxxx>
> CC: Julia Lawall <julia.lawall@xxxxxxx>
> CC: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
> CC: Juergen Gross <jgross@xxxxxxxx>
> CC: Stefano Stabellini <sstabellini@xxxxxxxxxx>
> CC: xen-devel@xxxxxxxxxxxxxxxxxxxx
> CC: linux-kernel@xxxxxxxxxxxxxxx
> ---
>  drivers/xen/pvcalls-front.c | 71 ++++++++++++++++++++++++++++++-------
>  1 file changed, 59 insertions(+), 12 deletions(-)
>
> diff --git a/drivers/xen/pvcalls-front.c b/drivers/xen/pvcalls-front.c
> index 2f11ca72a281..a26f416daf46 100644
> --- a/drivers/xen/pvcalls-front.c
> +++ b/drivers/xen/pvcalls-front.c
> @@ -335,7 +335,43 @@ int pvcalls_front_socket(struct socket *sock)
>       return ret;
>  }
>  
> -static int create_active(struct sock_mapping *map, int *evtchn)
> +struct sock_mapping_active_ring {
> +     struct pvcalls_data_intf *ring;
> +     RING_IDX ring_order;
> +     void *bytes;
> +};
> +
> +static int alloc_active_ring(struct sock_mapping_active_ring *active_ring)
> +{
> +     active_ring->ring = NULL;

This is not necessary.

> +     active_ring->bytes = NULL;
> +
> +     active_ring->ring = (struct pvcalls_data_intf *)
> +             __get_free_page(GFP_KERNEL | __GFP_ZERO);
> +     if (active_ring->ring == NULL)
> +             goto out_error;
> +     active_ring->ring_order = PVCALLS_RING_ORDER;
> +     active_ring->bytes = (void *)__get_free_pages(GFP_KERNEL | __GFP_ZERO,
> +                     PVCALLS_RING_ORDER);
> +     if (active_ring->bytes == NULL)
> +             goto out_error;
> +
> +     return 0;
> +
> +out_error:
> +     free_pages((unsigned long)active_ring->bytes, active_ring->ring_order);
> +     free_page((unsigned long)active_ring->ring);
> +     return -ENOMEM;
> +}
> +



> @@ -397,6 +427,7 @@ int pvcalls_front_connect(struct socket *sock, struct 
> sockaddr *addr,
>       struct sock_mapping *map = NULL;
>       struct xen_pvcalls_request *req;
>       int notify, req_id, ret, evtchn;
> +     struct sock_mapping_active_ring active_ring;
>  
>       if (addr->sa_family != AF_INET || sock->type != SOCK_STREAM)
>               return -EOPNOTSUPP;
> @@ -406,15 +437,21 @@ int pvcalls_front_connect(struct socket *sock, struct 
> sockaddr *addr,
>               return PTR_ERR(map);
>  
>       bedata = dev_get_drvdata(&pvcalls_front_dev->dev);
> +     ret = alloc_active_ring(&active_ring);

Why not just alloc_active_ring(map)?


-boris


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