linux-kernel - Re: [PATCH 1/2] xen: prepare p2m list for memory hotplug

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Date:	Thu, 19 Mar 2015 17:14:40 +0100
From:	Daniel Kiper <daniel.kiper@...cle.com>
To:	Juergen Gross <jgross@...e.com>
Cc:	linux-kernel@...r.kernel.org, xen-devel@...ts.xensource.com,
	konrad.wilk@...cle.com, david.vrabel@...rix.com,
	boris.ostrovsky@...cle.com
Subject: Re: [PATCH 1/2] xen: prepare p2m list for memory hotplug

On Thu, Mar 19, 2015 at 03:31:01PM +0100, Juergen Gross wrote:
> Commit 054954eb051f35e74b75a566a96fe756015352c8 ("xen: switch to linear
> virtual mapped sparse p2m list") introduced a regression regarding to
> memory hotplug for a pv-domain: as the virtual space for the p2m list
> is allocated for the to be expected memory size of the domain only,
> hotplugged memory above that size will not be usable by the domain.
>
> Correct this by using a configurable size for the p2m list in case of
> memory hotplug enabled (default supported memory size is 512 GB for
> 64 bit domains and 4 GB for 32 bit domains).
>
> Signed-off-by: Juergen Gross <jgross@...e.com>
> ---
>  arch/x86/xen/p2m.c  | 13 ++++++++++++-
>  drivers/xen/Kconfig | 13 +++++++++++++
>  2 files changed, 25 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/xen/p2m.c b/arch/x86/xen/p2m.c
> index 9f93af5..30e84ae 100644
> --- a/arch/x86/xen/p2m.c
> +++ b/arch/x86/xen/p2m.c
> @@ -91,6 +91,17 @@ EXPORT_SYMBOL_GPL(xen_p2m_size);
>  unsigned long xen_max_p2m_pfn __read_mostly;
>  EXPORT_SYMBOL_GPL(xen_max_p2m_pfn);
>
> +#ifdef CONFIG_XEN_BALLOON_MEMORY_HOTPLUG_LIMIT
> +#ifdef CONFIG_X86_32
> +BUILD_BUG_ON_MSG(CONFIG_XEN_BALLOON_MEMORY_HOTPLUG_LIMIT > 64)
> +#endif
> +#define P2M_LIMIT max(xen_max_p2m_pfn,					\
> +	((unsigned long)((u64)CONFIG_XEN_BALLOON_MEMORY_HOTPLUG_LIMIT *	\
> +	1024 * 1024 * 1024 / PAGE_SIZE)))
> +#else
> +#define P2M_LIMIT xen_max_p2m_pfn
> +#endif
> +
>  static DEFINE_SPINLOCK(p2m_update_lock);
>
>  static unsigned long *p2m_mid_missing_mfn;
> @@ -387,7 +398,7 @@ void __init xen_vmalloc_p2m_tree(void)
>  	static struct vm_struct vm;
>
>  	vm.flags = VM_ALLOC;
> -	vm.size = ALIGN(sizeof(unsigned long) * xen_max_p2m_pfn,
> +	vm.size = ALIGN(sizeof(unsigned long) * P2M_LIMIT,
>  			PMD_SIZE * PMDS_PER_MID_PAGE);

What happens when somebody will allocate more memory for guest than
XEN_BALLOON_MEMORY_HOTPLUG_LIMIT? Additionally, I think that we do
not need allocate extra virtual address space if memory hotplug is
disabled.

>  	vm_area_register_early(&vm, PMD_SIZE * PMDS_PER_MID_PAGE);
>  	pr_notice("p2m virtual area at %p, size is %lx\n", vm.addr, vm.size);
> diff --git a/drivers/xen/Kconfig b/drivers/xen/Kconfig
> index b812462..0a61ddf 100644
> --- a/drivers/xen/Kconfig
> +++ b/drivers/xen/Kconfig
> @@ -55,6 +55,19 @@ config XEN_BALLOON_MEMORY_HOTPLUG
>
>  	  In that case step 3 should be omitted.
>
> +config XEN_BALLOON_MEMORY_HOTPLUG_LIMIT
> +	int
> +	default 512 if X86_64
> +	default 4 if X86_32
> +	depends on XEN_HAVE_PVMMU

Do we need dependency on XEN_HAVE_PVMMU?
If yes than maybe "depends on XEN_HAVE_PVMMU && XEN_BALLOON_MEMORY_HOTPLUG".

> +	depends on XEN_BALLOON_MEMORY_HOTPLUG

Should not this option be available even when memory hotplug is disabled?

Daniel
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