linux-kernel - Re: [PATCH] xen/balloon: Subtract from xen_released

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Date:	Tue, 29 May 2012 18:36:24 +0100
From:	David Vrabel <david.vrabel@...rix.com>
To:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
CC:	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] xen/balloon: Subtract from xen_released_pages the count
 that is populated.

On 29/05/12 17:58, Konrad Rzeszutek Wilk wrote:
> We did not take into account that xen_released_pages would be
> used outside the initial E820 parsing code. As such we would
> not subtract from xen_released_pages the count of pages
> that we had populated back. Instead we just did a simple
> extra_pages = released - populated calculation.
> 
> However the balloon worker uses xen_released_pages to figure
> out how many more pages it can balloon up to and not having
> the proper numbers we would balloon more than we should have.

I would probably rephrase this paragraph to be a bit clearer but it's
not a big deal.  Something like this perhaps:

"The balloon driver uses xen_released_pages to set the initial
current_pages count.  If this is wrong (too low) then when a new
(higher) target is set, the balloon driver will request too many pages
from Xen."

> This fixes errors such as:
> 
> (XEN) memory.c:133:d0 Could not allocate order=0 extent: id=0 memflags=0 (51 of 512)
> during bootup and
> free_memory            : 0
> 
> where the free_memory should be 128.
> 
> Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>

Acked-by: David Vrabel <david.vrabel@...rix.com>

> ---
>  arch/x86/xen/setup.c |    3 ++-
>  1 files changed, 2 insertions(+), 1 deletions(-)
> 
> diff --git a/arch/x86/xen/setup.c b/arch/x86/xen/setup.c
> index 3ebba07..a4790bf 100644
> --- a/arch/x86/xen/setup.c
> +++ b/arch/x86/xen/setup.c
> @@ -371,7 +371,8 @@ char * __init xen_memory_setup(void)
>  	populated = xen_populate_chunk(map, memmap.nr_entries,
>  			max_pfn, &last_pfn, xen_released_pages);
>  
> -	extra_pages += (xen_released_pages - populated);
> +	xen_released_pages -= populated;
> +	extra_pages += xen_released_pages;
>  
>  	if (last_pfn > max_pfn) {
>  		max_pfn = min(MAX_DOMAIN_PAGES, last_pfn);

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