Re: [Xen-devel] [PATCH] xen/balloon: don't alloc page while non-preemptible

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Thu, 19 Sep 2013, David Vrabel wrote:
> From: David Vrabel <david.vrabel@xxxxxxxxxx>
> 
> get_balloon_scratch_page() disables preemption so we cannot call
> alloc_page() in between get/put_balloon_scratch_page().  Shuffle bits
> around in decrease_reservation() to avoid this.
> 
> Signed-off-by: David Vrabel <david.vrabel@xxxxxxxxxx>
> ---
>  drivers/xen/balloon.c |   23 +++++++++++------------
>  1 files changed, 11 insertions(+), 12 deletions(-)
> 
> diff --git a/drivers/xen/balloon.c b/drivers/xen/balloon.c
> index a50c6e3..b232908 100644
> --- a/drivers/xen/balloon.c
> +++ b/drivers/xen/balloon.c
> @@ -398,8 +398,6 @@ static enum bp_state decrease_reservation(unsigned long 
> nr_pages, gfp_t gfp)
>       if (nr_pages > ARRAY_SIZE(frame_list))
>               nr_pages = ARRAY_SIZE(frame_list);
>  
> -     scratch_page = get_balloon_scratch_page();
> -
>       for (i = 0; i < nr_pages; i++) {
>               page = alloc_page(gfp);
>               if (page == NULL) {
> @@ -413,6 +411,12 @@ static enum bp_state decrease_reservation(unsigned long 
> nr_pages, gfp_t gfp)
>  
>               scrub_page(page);
>  
> +             /*
> +              * Ballooned out frames are effectively replaced with
> +              * a scratch frame.  Ensure direct mappings and the
> +              * p2m are consistent.
> +              */
> +             scratch_page = get_balloon_scratch_page();
>  #ifdef CONFIG_XEN_HAVE_PVMMU
>               if (xen_pv_domain() && !PageHighMem(page)) {
>                       ret = HYPERVISOR_update_va_mapping(
> @@ -422,24 +426,19 @@ static enum bp_state decrease_reservation(unsigned long 
> nr_pages, gfp_t gfp)
>                       BUG_ON(ret);
>               }
>  #endif
> -     }
> -
> -     /* Ensure that ballooned highmem pages don't have kmaps. */
> -     kmap_flush_unused();
> -     flush_tlb_all();
> -
> -     /* No more mappings: invalidate P2M and add to balloon. */
> -     for (i = 0; i < nr_pages; i++) {
> -             pfn = mfn_to_pfn(frame_list[i]);
>               if (!xen_feature(XENFEAT_auto_translated_physmap)) {
>                       unsigned long p;
>                       p = page_to_pfn(scratch_page);
>                       __set_phys_to_machine(pfn, pfn_to_mfn(p));
>               }
> +             put_balloon_scratch_page();
> +
>               balloon_append(pfn_to_page(pfn));
>       }
>  
> -     put_balloon_scratch_page();
> +     /* Ensure that ballooned highmem pages don't have kmaps. */
> +     kmap_flush_unused();
> +     flush_tlb_all();

The change with possible side effects in this patch is that
kmap_flush_unused and flush_tlb_all are now called after setting the p2m
and after calling balloon_append instead of before.

However they are still called before the XENMEM_decrease_reservation
hypercall, so I think it's OK.

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