Re: [PATCH v3 12/20] xen/balloon: Don't rely on the page granularity is the same for Xen and Linux
From: Stefano Stabellini
Date: Mon Aug 10 2015 - 07:19:52 EST
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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