Re: [Xen-devel] [PATCH v3 12/20] xen/balloon: Don't rely on the page granularity is the same for Xen and Linux

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Fri, 7 Aug 2015, Julien Grall wrote:
> For ARM64 guests, Linux is able to support either 64K or 4K page
> granularity. Although, the hypercall interface is always based on 4K
> page granularity.
> 
> With 64K page granularity, a single page will be spread over multiple
> Xen frame.
> 
> To avoid splitting the page into 4K frame, take advantage of the
> extent_order field to directly allocate/free chunk of the Linux page
> size.
> 
> Note that PVMMU is only used for PV guest (which is x86) and the page
> granularity is always 4KB. Some BUILD_BUG_ON has been added to ensure
> that because the code has not been modified.
> 
> Signed-off-by: Julien Grall <julien.grall@xxxxxxxxxx>

This is much better than the previous version. Good idea using the
extent_order field.

I only have a minor comment below.


> ---
> Cc: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
> Cc: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
> Cc: David Vrabel <david.vrabel@xxxxxxxxxx>
> Cc: Wei Liu <wei.liu2@xxxxxxxxxx>
> 
>     Changes in v3:
>         - Fix errors reported by checkpatch.pl
>         - s/mfn/gfn/ based on the new naming
>         - Rather than splitting the page into 4KB chunk, use the
>         extent_order field to allocate directly a Linux page size. This
>         is avoid lots of code for no benefits.
> 
>     Changes in v2:
>         - Use xen_apply_to_page to split a page in 4K chunk
>         - It's not necessary to have a smaller frame list. Re-use
>         PAGE_SIZE
>         - Convert reserve_additional_memory to use XEN_... macro
> ---
>  drivers/xen/balloon.c | 47 ++++++++++++++++++++++++++++++++++++-----------
>  1 file changed, 36 insertions(+), 11 deletions(-)
> 
> diff --git a/drivers/xen/balloon.c b/drivers/xen/balloon.c
> index 9734649..c8739c8 100644
> --- a/drivers/xen/balloon.c
> +++ b/drivers/xen/balloon.c
> @@ -70,6 +70,9 @@
>  #include <xen/features.h>
>  #include <xen/page.h>
>  
> +/* Use one extent per PAGE_SIZE to avoid break down into multiple frame */
> +#define EXTENT_ORDER (fls(XEN_PFN_PER_PAGE) - 1)
> +
>  /*
>   * balloon_process() state:
>   *
> @@ -230,6 +233,11 @@ static enum bp_state reserve_additional_memory(long 
> credit)
>       nid = memory_add_physaddr_to_nid(hotplug_start_paddr);
>  
>  #ifdef CONFIG_XEN_HAVE_PVMMU
> +     /* We don't support PV MMU when Linux and Xen is using
> +      * different page granularity.
> +      */
> +     BUILD_BUG_ON(XEN_PAGE_SIZE != PAGE_SIZE);
> +
>          /*
>           * add_memory() will build page tables for the new memory so
>           * the p2m must contain invalid entries so the correct
> @@ -326,11 +334,11 @@ static enum bp_state reserve_additional_memory(long 
> credit)
>  static enum bp_state increase_reservation(unsigned long nr_pages)
>  {
>       int rc;
> -     unsigned long  pfn, i;
> +     unsigned long i;
>       struct page   *page;
>       struct xen_memory_reservation reservation = {
>               .address_bits = 0,
> -             .extent_order = 0,
> +             .extent_order = EXTENT_ORDER,
>               .domid        = DOMID_SELF
>       };
>  
> @@ -352,7 +360,11 @@ static enum bp_state increase_reservation(unsigned long 
> nr_pages)
>                       nr_pages = i;
>                       break;
>               }
> -             frame_list[i] = page_to_pfn(page);
> +
> +             /* XENMEM_populate_physmap requires a PFN based on Xen
> +              * granularity.
> +              */
> +             frame_list[i] = xen_page_to_pfn(page);
>               page = balloon_next_page(page);
>       }
>  
> @@ -366,10 +378,15 @@ static enum bp_state increase_reservation(unsigned long 
> nr_pages)
>               page = balloon_retrieve(false);
>               BUG_ON(page == NULL);
>  
> -             pfn = page_to_pfn(page);
> -
>  #ifdef CONFIG_XEN_HAVE_PVMMU
> +             /* We don't support PV MMU when Linux and Xen is using
> +              * different page granularity.
> +              */
> +             BUILD_BUG_ON(XEN_PAGE_SIZE != PAGE_SIZE);
> +
>               if (!xen_feature(XENFEAT_auto_translated_physmap)) {
> +                     unsigned long pfn = page_to_pfn(page);
> +
>                       set_phys_to_machine(pfn, frame_list[i]);
>  
>                       /* Link back into the page tables if not highmem. */
> @@ -396,14 +413,15 @@ static enum bp_state increase_reservation(unsigned long 
> nr_pages)
>  static enum bp_state decrease_reservation(unsigned long nr_pages, gfp_t gfp)
>  {
>       enum bp_state state = BP_DONE;
> -     unsigned long  pfn, i;
> +     unsigned long i;
>       struct page   *page;
>       int ret;
>       struct xen_memory_reservation reservation = {
>               .address_bits = 0,
> -             .extent_order = 0,
> +             .extent_order = EXTENT_ORDER,
>               .domid        = DOMID_SELF
>       };
> +     static struct page *pages[ARRAY_SIZE(frame_list)];

This array can be rather large: I would try to avoid it, see below.


>  #ifdef CONFIG_XEN_BALLOON_MEMORY_HOTPLUG
>       if (balloon_stats.hotplug_pages) {
> @@ -426,7 +444,9 @@ static enum bp_state decrease_reservation(unsigned long 
> nr_pages, gfp_t gfp)
>               }
>               scrub_page(page);
>  
> -             frame_list[i] = page_to_pfn(page);
> +             /* XENMEM_decrease_reservation requires a GFN */
> +             frame_list[i] = xen_page_to_gfn(page);
> +             pages[i] = page;
>       }
>  
>       /*
> @@ -440,12 +460,17 @@ static enum bp_state decrease_reservation(unsigned long 
> nr_pages, gfp_t gfp)
>  
>       /* Update direct mapping, invalidate P2M, and add to balloon. */
>       for (i = 0; i < nr_pages; i++) {
> -             pfn = frame_list[i];
> -             frame_list[i] = pfn_to_gfn(pfn);
> -             page = pfn_to_page(pfn);
> +             page = pages[i];
>  
>  #ifdef CONFIG_XEN_HAVE_PVMMU
> +             /* We don't support PV MMU when Linux and Xen is using
> +              * different page granularity.
> +              */
> +             BUILD_BUG_ON(XEN_PAGE_SIZE != PAGE_SIZE);
> +
>               if (!xen_feature(XENFEAT_auto_translated_physmap)) {
> +                     unsigned long pfn = page_to_pfn(page);

I would simply and avoid introducing a new array:
    pfn = (frame_list[i] << XEN_PAGE_SHIFT) >> PAGE_SHIFT;
    page = pfn_to_page(pfn);
    
    
    
>                       if (!PageHighMem(page)) {
>                               ret = HYPERVISOR_update_va_mapping(
>                                               (unsigned long)__va(pfn << 
> PAGE_SHIFT),
> -- 
> 2.1.4
> 

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