linux-kernel - Re: [RFC 10/23] xen/biomerge: WORKAROUND always says the biovec are not mergeable

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Date:	Fri, 15 May 2015 11:54:59 -0400
From:	Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:	Julien Grall <julien.grall@...rix.com>,
	xen-devel@...ts.xenproject.org
CC:	linux-arm-kernel@...ts.infradead.org, ian.campbell@...rix.com,
	stefano.stabellini@...citrix.com, linux-kernel@...r.kernel.org,
	tim@....org, Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	David Vrabel <david.vrabel@...rix.com>
Subject: Re: [RFC 10/23] xen/biomerge: WORKAROUND always says the biovec are
 not mergeable

On 05/14/2015 01:00 PM, Julien Grall wrote:
> When Linux is using 64K page granularity, every page will be slipt in
> multiple non-contiguous 4K MFN.
>
> I'm not sure how to handle efficiently the check to know whether we can
> merge 2 biovec with a such case. So for now, always says that biovec are
> not mergeable.
>
> Signed-off-by: Julien Grall <julien.grall@...rix.com>
> Cc: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
> Cc: Boris Ostrovsky <boris.ostrovsky@...cle.com>
> Cc: David Vrabel <david.vrabel@...rix.com>
> ---
>   drivers/xen/biomerge.c | 3 +++
>   1 file changed, 3 insertions(+)
>
> diff --git a/drivers/xen/biomerge.c b/drivers/xen/biomerge.c
> index 0edb91c..20387c2 100644
> --- a/drivers/xen/biomerge.c
> +++ b/drivers/xen/biomerge.c
> @@ -9,6 +9,9 @@ bool xen_biovec_phys_mergeable(const struct bio_vec *vec1,
>   	unsigned long mfn1 = pfn_to_mfn(page_to_pfn(vec1->bv_page));
>   	unsigned long mfn2 = pfn_to_mfn(page_to_pfn(vec2->bv_page));
>   
> +	/* TODO: Implement it correctly */
> +	return 0;
> +
>   	return __BIOVEC_PHYS_MERGEABLE(vec1, vec2) &&
>   		((mfn1 == mfn2) || ((mfn1+1) == mfn2));
>   }


I think this is a bit too blunt.  Perhaps check first whether page sizes 
are different in the hypervisor and the guest?

(And I am not sure we need __BIOVEC_PHYS_MERGEABLE() test here as it is 
already checked by BIOVEC_PHYS_MERGEABLE() which appears to be the only 
user of xen_biovec_phys_mergeable())


-boris
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