Subject: Re: [PATCH] Xen backend support for paged out grant targets.

From: Konrad Rzeszutek Wilk <[email-masked]>
Date: Wed, 12 Sep 2012 16:06:41 -0400

On Wed, Sep 12, 2012 at 03:45:53PM -0400, Andres Lagar-Cavilla wrote:
> Since Xen-4.2, hvm domains may have portions of their memory paged out. When a
> foreign domain (such as dom0) attempts to map these frames, the map will
> initially fail. The hypervisor returns a suitable errno, and kicks an
> asynchronous page-in operation carried out by a helper. The foreign domain is
> expected to retry the mapping operation until it eventually succeeds. The
> foreign domain is not put to sleep because itself could be the one running the
> pager assist (typical scenario for dom0).

Looks ok to me. You forgot to put on the CC LKML and netdev mailing list
so I did that for you.

> 
> This patch adds support for this mechanism for backend drivers using grant
> mapping and copying operations. Specifically, this covers the blkback and
> gntdev drivers (which map foregin grants), and the netback driver (which copies
> foreign grants).
> 
> * Add GNTST_eagain, already exposed by Xen, to the grant interface.
> * Add a retry method for grants that fail with GNTST_eagain (i.e. because the
>   target foregin frame is paged out).
> * Insert hooks with appropriate macro decorators in the aforementioned drivers.
> 
> The retry loop is only invoked if the grant operation status is GNTST_eagain.
> It guarantees to leave a new status code different from GNTST_eagain. Any other
> status code results in identical code execution as before.
> 
> The retry loop performs 256 attempts with increasing time intervals through a
> 32 second period. It uses msleep to yield while waiting for the next retry.
> 
> V2 after feedback from David Vrabel:
> * Explicit MAX_DELAY instead of wrap-around delay into zero
> * Abstract GNTST_eagain check into core grant table code for netback module.
> 
> Signed-off-by: Andres Lagar-Cavilla <[email-masked]>
> ---
>  drivers/net/xen-netback/netback.c   |   11 +++-------
>  drivers/xen/grant-table.c           |   26 ++++++++++++++++++++++++
>  drivers/xen/xenbus/xenbus_client.c  |    6 ++----
>  include/xen/grant_table.h           |   38 +++++++++++++++++++++++++++++++++++
>  include/xen/interface/grant_table.h |    2 ++
>  5 files changed, 71 insertions(+), 12 deletions(-)
> 
> diff --git a/drivers/net/xen-netback/netback.c b/drivers/net/xen-netback/netback.c
> index 682633b..fc40258 100644
> --- a/drivers/net/xen-netback/netback.c
> +++ b/drivers/net/xen-netback/netback.c
> @@ -635,9 +635,7 @@ static void xen_netbk_rx_action(struct xen_netbk *netbk)
>  		return;
>  
>  	BUG_ON(npo.copy_prod > ARRAY_SIZE(netbk->grant_copy_op));
> -	ret = HYPERVISOR_grant_table_op(GNTTABOP_copy, &netbk->grant_copy_op,
> -					npo.copy_prod);
> -	BUG_ON(ret != 0);
> +	gnttab_batch_copy_retry_eagain(netbk->grant_copy_op, npo.copy_prod);
>  
>  	while ((skb = __skb_dequeue(&rxq)) != NULL) {
>  		sco = (struct skb_cb_overlay *)skb->cb;
> @@ -1460,18 +1458,15 @@ static void xen_netbk_tx_submit(struct xen_netbk *netbk)
>  static void xen_netbk_tx_action(struct xen_netbk *netbk)
>  {
>  	unsigned nr_gops;
> -	int ret;
>  
>  	nr_gops = xen_netbk_tx_build_gops(netbk);
>  
>  	if (nr_gops == 0)
>  		return;
> -	ret = HYPERVISOR_grant_table_op(GNTTABOP_copy,
> -					netbk->tx_copy_ops, nr_gops);
> -	BUG_ON(ret);
>  
> -	xen_netbk_tx_submit(netbk);
> +	gnttab_batch_copy_retry_eagain(netbk->tx_copy_ops, nr_gops);
>  
> +	xen_netbk_tx_submit(netbk);
>  }
>  
>  static void xen_netbk_idx_release(struct xen_netbk *netbk, u16 pending_idx)
> diff --git a/drivers/xen/grant-table.c b/drivers/xen/grant-table.c
> index eea81cf..96543b2 100644
> --- a/drivers/xen/grant-table.c
> +++ b/drivers/xen/grant-table.c
> @@ -38,6 +38,7 @@
>  #include <linux/vmalloc.h>
>  #include <linux/uaccess.h>
>  #include <linux/io.h>
> +#include <linux/delay.h>
>  #include <linux/hardirq.h>
>  
>  #include <xen/xen.h>
> @@ -823,6 +824,26 @@ unsigned int gnttab_max_grant_frames(void)
>  }
>  EXPORT_SYMBOL_GPL(gnttab_max_grant_frames);
>  
> +#define MAX_DELAY 256
> +void
> +gnttab_retry_eagain_gop(unsigned int cmd, void *gop, int16_t *status,
> +						const char *func)
> +{
> +	unsigned delay = 1;
> +
> +	do {
> +		BUG_ON(HYPERVISOR_grant_table_op(cmd, gop, 1));
> +		if (*status == GNTST_eagain)
> +			msleep(delay++);
> +	} while ((*status == GNTST_eagain) && (delay < MAX_DELAY));
> +
> +	if (delay >= MAX_DELAY) {
> +		printk(KERN_ERR "%s: %s eagain grant\n", func, current->comm);
> +		*status = GNTST_bad_page;
> +	}
> +}
> +EXPORT_SYMBOL_GPL(gnttab_retry_eagain_gop);
> +
>  int gnttab_map_refs(struct gnttab_map_grant_ref *map_ops,
>  		    struct gnttab_map_grant_ref *kmap_ops,
>  		    struct page **pages, unsigned int count)
> @@ -836,6 +857,11 @@ int gnttab_map_refs(struct gnttab_map_grant_ref *map_ops,
>  	if (ret)
>  		return ret;
>  
> +	/* Retry eagain maps */
> +	for (i = 0; i < count; i++)
> +		if (map_ops[i].status == GNTST_eagain)
> +			gnttab_retry_eagain_map(map_ops + i);
> +
>  	if (xen_feature(XENFEAT_auto_translated_physmap))
>  		return ret;
>  
> diff --git a/drivers/xen/xenbus/xenbus_client.c b/drivers/xen/xenbus/xenbus_client.c
> index b3e146e..a6e07ea 100644
> --- a/drivers/xen/xenbus/xenbus_client.c
> +++ b/drivers/xen/xenbus/xenbus_client.c
> @@ -490,8 +490,7 @@ static int xenbus_map_ring_valloc_pv(struct xenbus_device *dev,
>  
>  	op.host_addr = arbitrary_virt_to_machine(pte).maddr;
>  
> -	if (HYPERVISOR_grant_table_op(GNTTABOP_map_grant_ref, &op, 1))
> -		BUG();
> +	gnttab_map_grant_retry_eagain(&op);
>  
>  	if (op.status != GNTST_okay) {
>  		free_vm_area(area);
> @@ -572,8 +571,7 @@ int xenbus_map_ring(struct xenbus_device *dev, int gnt_ref,
>  	gnttab_set_map_op(&op, (unsigned long)vaddr, GNTMAP_host_map, gnt_ref,
>  			  dev->otherend_id);
>  
> -	if (HYPERVISOR_grant_table_op(GNTTABOP_map_grant_ref, &op, 1))
> -		BUG();
> +	gnttab_map_grant_retry_eagain(&op);
>  
>  	if (op.status != GNTST_okay) {
>  		xenbus_dev_fatal(dev, op.status,
> diff --git a/include/xen/grant_table.h b/include/xen/grant_table.h
> index 11e27c3..c829c83 100644
> --- a/include/xen/grant_table.h
> +++ b/include/xen/grant_table.h
> @@ -43,6 +43,7 @@
>  #include <xen/interface/grant_table.h>
>  
>  #include <asm/xen/hypervisor.h>
> +#include <asm/xen/hypercall.h>
>  
>  #include <xen/features.h>
>  
> @@ -183,6 +184,43 @@ unsigned int gnttab_max_grant_frames(void);
>  
>  #define gnttab_map_vaddr(map) ((void *)(map.host_virt_addr))
>  
> +/* Retry a grant map/copy operation when the hypervisor returns GNTST_eagain.
> + * This is typically due to paged out target frames.
> + * Generic entry-point, use macro decorators below for specific grant
> + * operations.
> + * Will retry for 1, 2, ... 255 ms, i.e. 256 times during 32 seconds.
> + * Return value in *status guaranteed to no longer be GNTST_eagain. */
> +void gnttab_retry_eagain_gop(unsigned int cmd, void *gop, int16_t *status,
> +                             const char *func);
> +
> +#define gnttab_retry_eagain_map(_gop)                       \
> +    gnttab_retry_eagain_gop(GNTTABOP_map_grant_ref, (_gop), \
> +                            &(_gop)->status, __func__)
> +
> +#define gnttab_retry_eagain_copy(_gop)                  \
> +    gnttab_retry_eagain_gop(GNTTABOP_copy, (_gop),      \
> +                            &(_gop)->status, __func__)
> +
> +static inline void
> +gnttab_map_grant_retry_eagain(struct gnttab_map_grant_ref *op)
> +{
> +    BUG_ON((HYPERVISOR_grant_table_op(GNTTABOP_map_grant_ref, op, 1)));
> +    if (op->status == GNTST_eagain)
> +        gnttab_retry_eagain_map(op);
> +}
> +
> +static inline void
> +gnttab_batch_copy_retry_eagain(struct gnttab_copy *batch, unsigned count)
> +{
> +    unsigned i;
> +    struct gnttab_copy *op;
> +
> +    BUG_ON(HYPERVISOR_grant_table_op(GNTTABOP_copy, batch, count));
> +    for (i = 0, op = batch; i < count; i++, op++)
> +        if (op->status == GNTST_eagain)
> +            gnttab_retry_eagain_copy(op);
> +}
> +
>  int gnttab_map_refs(struct gnttab_map_grant_ref *map_ops,
>  		    struct gnttab_map_grant_ref *kmap_ops,
>  		    struct page **pages, unsigned int count);
> diff --git a/include/xen/interface/grant_table.h b/include/xen/interface/grant_table.h
> index 7da811b..66cb734 100644
> --- a/include/xen/interface/grant_table.h
> +++ b/include/xen/interface/grant_table.h
> @@ -520,6 +520,7 @@ DEFINE_GUEST_HANDLE_STRUCT(gnttab_get_version);
>  #define GNTST_permission_denied (-8) /* Not enough privilege for operation.  */
>  #define GNTST_bad_page         (-9) /* Specified page was invalid for op.    */
>  #define GNTST_bad_copy_arg    (-10) /* copy arguments cross page boundary */
> +#define GNTST_eagain          (-12) /* Retry.                                */
>  
>  #define GNTTABOP_error_msgs {                   \
>      "okay",                                     \
> @@ -533,6 +534,7 @@ DEFINE_GUEST_HANDLE_STRUCT(gnttab_get_version);
>      "permission denied",                        \
>      "bad page",                                 \
>      "copy arguments cross page boundary"        \
> +    "retry"                                     \
>  }
>  
>  #endif /* __XEN_PUBLIC_GRANT_TABLE_H__ */
> -- 
> 1.7.9.5