linux-kernel - Re: [PATCH] xen/p2m: Don't call get_balloon_scratch_page() twice, keep interrupts disabled for multicalls

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Date:	Fri, 06 Sep 2013 12:06:25 -0400
From:	konrad wilk <konrad.wilk@...cle.com>
To:	Stefano Stabellini <stefano.stabellini@...citrix.com>
CC:	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	david.vrabel@...rix.com, xen-devel@...ts.xenproject.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] xen/p2m: Don't call get_balloon_scratch_page() twice,
 keep interrupts disabled for multicalls


On 9/6/2013 11:09 AM, Stefano Stabellini wrote:
> On Fri, 6 Sep 2013, Boris Ostrovsky wrote:
>> m2p_remove_override() calls get_balloon_scratch_page() in
>> MULTI_update_va_mapping() even though it already has pointer to this page from
>> the earlier call (in scratch_page). This second call doesn't have a matching
>> put_balloon_scratch_page() thus not restoring preempt count back. (Also, there
>> is no put_balloon_scratch_page() in the error path.)
>>
>> In addition, the second multicall uses __xen_mc_entry() which does not disable
>> interrupts. Rearrange xen_mc_* calls to keep interrupts off while performing
>> multicalls.
>>
>> Signed-off-by: Boris Ostrovsky <boris.ostrovsky@...cle.com>
You also might want to mention that this fixes the regression introduced 
by ..
commit id <"title"
> Reviewed-by: Stefano Stabellini <stefano.stabellini@...citrix.com>
>
>
>>   arch/x86/xen/p2m.c | 10 ++++++----
>>   1 file changed, 6 insertions(+), 4 deletions(-)
>>
>> diff --git a/arch/x86/xen/p2m.c b/arch/x86/xen/p2m.c
>> index 0d4ec35..8b901e8 100644
>> --- a/arch/x86/xen/p2m.c
>> +++ b/arch/x86/xen/p2m.c
>> @@ -990,10 +990,13 @@ int m2p_remove_override(struct page *page,
>>   				printk(KERN_WARNING "m2p_remove_override: "
>>   						"pfn %lx mfn %lx, failed to modify kernel mappings",
>>   						pfn, mfn);
>> +				put_balloon_scratch_page();
>>   				return -1;
>>   			}
>>   
>> -			mcs = xen_mc_entry(
>> +			xen_mc_batch();
>> +
>> +			mcs = __xen_mc_entry(
>>   					sizeof(struct gnttab_unmap_and_replace));
>>   			unmap_op = mcs.args;
>>   			unmap_op->host_addr = kmap_op->host_addr;
>> @@ -1003,12 +1006,11 @@ int m2p_remove_override(struct page *page,
>>   			MULTI_grant_table_op(mcs.mc,
>>   					GNTTABOP_unmap_and_replace, unmap_op, 1);
>>   
>> -			xen_mc_issue(PARAVIRT_LAZY_MMU);
>> -
>>   			mcs = __xen_mc_entry(0);
>>   			MULTI_update_va_mapping(mcs.mc, scratch_page_address,
>> -					pfn_pte(page_to_pfn(get_balloon_scratch_page()),
>> +					pfn_pte(page_to_pfn(scratch_page),
>>   					PAGE_KERNEL_RO), 0);
>> +
>>   			xen_mc_issue(PARAVIRT_LAZY_MMU);
>>   
>>   			kmap_op->host_addr = 0;
>> -- 
>> 1.8.3.1
>>

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