linux-kernel - Re: [Xen-devel] [PATCH V3 2/8] xen: Delay remapping memory of pv-domain

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Date:	Tue, 11 Nov 2014 11:45:11 +0000
From:	Andrew Cooper <andrew.cooper3@...rix.com>
To:	Juergen Gross <jgross@...e.com>, <linux-kernel@...r.kernel.org>,
	<xen-devel@...ts.xensource.com>, <konrad.wilk@...cle.com>,
	<david.vrabel@...rix.com>, <boris.ostrovsky@...cle.com>,
	<x86@...nel.org>, <tglx@...utronix.de>, <mingo@...hat.com>,
	<hpa@...or.com>
Subject: Re: [Xen-devel] [PATCH V3 2/8] xen: Delay remapping memory of pv-domain

On 11/11/14 05:43, Juergen Gross wrote:
> diff --git a/arch/x86/xen/p2m.c b/arch/x86/xen/p2m.c
> index fa75842..f67f8cf 100644
> --- a/arch/x86/xen/p2m.c
> +++ b/arch/x86/xen/p2m.c
> @@ -268,6 +271,22 @@ static void p2m_init(unsigned long *p2m)
>  		p2m[i] = INVALID_P2M_ENTRY;
>  }
>  
> +static void * __ref alloc_p2m_page(void)
> +{
> +	if (unlikely(use_brk))
> +		return extend_brk(PAGE_SIZE, PAGE_SIZE);
> +
> +	if (unlikely(!slab_is_available()))
> +		return alloc_bootmem_align(PAGE_SIZE, PAGE_SIZE);
> +
> +	return (void *)__get_free_page(GFP_KERNEL | __GFP_REPEAT);
> +}
> +
> +static void free_p2m_page(void *p)
> +{
> +	free_page((unsigned long)p);
> +}
> +

What guarantees are there that free_p2m_page() is only called on p2m
pages allocated using __get_free_page() ?  I can see from this diff that
this is the case, but that doesn't help someone coming along in the future.

At the very least, a comment is warranted about the apparent mismatch
between {alloc,free}_p2m_page().

> @@ -420,6 +439,7 @@ unsigned long __init xen_revector_p2m_tree(void)
>  	unsigned long *mfn_list = NULL;
>  	unsigned long size;
>  
> +	use_brk = 0;
>  	va_start = xen_start_info->mfn_list;
>  	/*We copy in increments of P2M_PER_PAGE * sizeof(unsigned long),
>  	 * so make sure it is rounded up to that */
> @@ -484,6 +504,7 @@ unsigned long __init xen_revector_p2m_tree(void)
>  #else
>  unsigned long __init xen_revector_p2m_tree(void)
>  {
> +	use_brk = 0;
>  	return 0;
>  }
>  #endif

This appears to be a completely orphaned function.

It has a split definition based on CONFIG_X86_64, but the sole caller is
xen_pagetable_p2m_copy() which is X86_64 only.

How does use_brk get cleared for 32bit PV guests?

~Andrew
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