Re: [PATCH v4 06/10] memory: batch processing in acquire_resource()

["Roger Pau Monné" <roger.pau@citrix.com>]

On Tue, Jun 30, 2020 at 02:33:49PM +0200, Michał Leszczyński wrote:
> From: Michal Leszczynski <michal.leszczynski@cert.pl>
> 
> Allow to acquire large resources by allowing acquire_resource()
> to process items in batches, using hypercall continuation.

This patch should be the first of thew series IMO, since it can go in
independently of the rest, as it's a general improvement to
XENMEM_acquire_resource.

> Signed-off-by: Michal Leszczynski <michal.leszczynski@cert.pl>
> ---
>  xen/common/memory.c | 32 +++++++++++++++++++++++++++++---
>  1 file changed, 29 insertions(+), 3 deletions(-)
> 
> diff --git a/xen/common/memory.c b/xen/common/memory.c
> index 714077c1e5..3ab06581a2 100644
> --- a/xen/common/memory.c
> +++ b/xen/common/memory.c
> @@ -1046,10 +1046,12 @@ static int acquire_grant_table(struct domain *d, unsigned int id,
>  }
>  
>  static int acquire_resource(
> -    XEN_GUEST_HANDLE_PARAM(xen_mem_acquire_resource_t) arg)
> +    XEN_GUEST_HANDLE_PARAM(xen_mem_acquire_resource_t) arg,
> +    unsigned long *start_extent)
>  {
>      struct domain *d, *currd = current->domain;
>      xen_mem_acquire_resource_t xmar;
> +    uint32_t total_frames;
>      /*
>       * The mfn_list and gfn_list (below) arrays are ok on stack for the
>       * moment since they are small, but if they need to grow in future
> @@ -1077,8 +1079,17 @@ static int acquire_resource(
>          return 0;
>      }
>  
> +    total_frames = xmar.nr_frames;
> +
> +    if ( *start_extent )
> +    {
> +        xmar.frame += *start_extent;
> +        xmar.nr_frames -= *start_extent;
> +        guest_handle_add_offset(xmar.frame_list, *start_extent);
> +    }
> +
>      if ( xmar.nr_frames > ARRAY_SIZE(mfn_list) )
> -        return -E2BIG;
> +        xmar.nr_frames = ARRAY_SIZE(mfn_list);
>  
>      rc = rcu_lock_remote_domain_by_id(xmar.domid, &d);
>      if ( rc )
> @@ -1135,6 +1146,14 @@ static int acquire_resource(
>          }
>      }
>  
> +    if ( !rc )
> +    {
> +        *start_extent += xmar.nr_frames;
> +
> +        if ( *start_extent != total_frames )
> +            rc = -ERESTART;
> +    }

I think you should add some kind of loop here, processing just 32
frames and preempting might be too low. You generally want to loop
doing batches of 32 entries until hypercall_preempt_check() returns
true.

Thanks, Roger.