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

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Message-ID: <555B45A1.4070604@citrix.com>
Date:	Tue, 19 May 2015 15:16:01 +0100
From:	Julien Grall <julien.grall@...rix.com>
To:	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	Julien Grall <julien.grall@...rix.com>,
	<xen-devel@...ts.xenproject.org>
CC:	<ian.campbell@...rix.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	<stefano.stabellini@...citrix.com>, <tim@....org>,
	<linux-kernel@...r.kernel.org>,
	David Vrabel <david.vrabel@...rix.com>,
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [RFC 10/23] xen/biomerge: WORKAROUND always says the biovec are
 not mergeable

Hi Boris,

On 15/05/15 16:54, Boris Ostrovsky wrote:
> 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?

Sounds good.

> 
> (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())

I can send a patch to drop __BIOVEC_PHYS_MERGEABLE.

Regards,

-- 
Julien Grall
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