linux-kernel - Re: [Xen-devel] [PATCH] xen/p2m: fix extra memory regions accounting

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Date:	Fri, 4 Sep 2015 09:47:12 +0200
From:	Juergen Gross <jgross@...e.com>
To:	Roger Pau Monné <roger.pau@...rix.com>
Cc:	David Vrabel <david.vrabel@...rix.com>,
	linux-kernel@...r.kernel.org, xen-devel@...ts.xenproject.org,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>
Subject: Re: [Xen-devel] [PATCH] xen/p2m: fix extra memory regions accounting

On 09/04/2015 09:37 AM, Roger Pau Monné wrote:
> Hello,
>
> El 04/09/15 a les 7.07, Juergen Gross ha escrit:
>> Could you try the attached patch? It should do the job. It is booting
>> fine on my laptop, but I think you should try it on the machine with
>> the memory ranges not at page boundary.
>>
>>
>> Juergen
>>
>>
>> extramem.patch
>>
>>
>> commit 3d0f8aa4d1b4c9c16a81902a197b5d6a77e182a0
>> Author: Juergen Gross <jgross@...e.com>
>> Date:   Thu Sep 3 17:44:04 2015 +0200
>>
>>      xen: switch extra memory accounting to use pfns
>>
>>      Instead of using physical addresses for accounting of extra memory
>>      areas available for ballooning switch to pfns as this is much less
>>      error prone regarding partial pages.
>
> Thanks for taking care of this! I've tested it on the box that has
> non-page aligned memory ranges and it works fine, only a couple of
> comments below.
>
>>      Reported-by: Roger Pau MonnÃ© <roger.pau@...rix.com>
>>      Signed-off-by: Juergen Gross <jgross@...e.com>
>>
>> diff --git a/arch/x86/xen/setup.c b/arch/x86/xen/setup.c
>> index 7a5d566..aa58bc4 100644
>> --- a/arch/x86/xen/setup.c
>> +++ b/arch/x86/xen/setup.c
>> @@ -90,62 +90,65 @@ static void __init xen_parse_512gb(void)
>>   	xen_512gb_limit = val;
>>   }
>>
>> -static void __init xen_add_extra_mem(phys_addr_t start, phys_addr_t size)
>> +static void __init xen_add_extra_mem(unsigned long start_pfn,
>> +				     unsigned long n_pfns)
>
> Not very important, but for type consistency this should probably be
> xen_pfn_t instead of unsigned long here and below.

All of the p2m code is using unsigned long for pfns. I wouldn't mind
changing this to use xen_pfn_t instead, but this should be done in a
separate patch. I'll put it on my list.

>
>>   {
>>   	int i;
>>
>>   	for (i = 0; i < XEN_EXTRA_MEM_MAX_REGIONS; i++) {
>>   		/* Add new region. */
>> -		if (xen_extra_mem[i].size == 0) {
>> -			xen_extra_mem[i].start = start;
>> -			xen_extra_mem[i].size  = size;
>> +		if (xen_extra_mem[i].n_pfns == 0) {
>> +			xen_extra_mem[i].start_pfn = start_pfn;
>> +			xen_extra_mem[i].n_pfns = n_pfns;
>>   			break;
>>   		}
>>   		/* Append to existing region. */
>> -		if (xen_extra_mem[i].start + xen_extra_mem[i].size == start) {
>> -			xen_extra_mem[i].size += size;
>> +		if (xen_extra_mem[i].start_pfn + xen_extra_mem[i].n_pfns ==
>> +		    start_pfn) {
>> +			xen_extra_mem[i].n_pfns += n_pfns;
>>   			break;
>>   		}
>
> I also noticed this with the original code, why isn't there a case to
> prepend to an existing region:
>
> if (start_pfn + n_pfns == xen_extra_mem[i].start_pfn) {
>      xen_extra_mem[i].n_pfns += n_pfns;
>      xen_extra_mem[i].start_pfn = start_pfn;
> }

Processing of memory is done from low to high addresses. This case
should never happen. And even if it does, the only downside from
not handling this scenario is wasting an additional table entry.

>
>>   	}
>>   	if (i == XEN_EXTRA_MEM_MAX_REGIONS)
>>   		printk(KERN_WARNING "Warning: not enough extra memory regions\n");
>>
>> -	memblock_reserve(start, size);
>> +	memblock_reserve(PFN_PHYS(start_pfn), PFN_PHYS(n_pfns));
>>   }
>
> [...]
>
>> @@ -831,9 +833,11 @@ char * __init xen_memory_setup(void)
>>   				chunk_size = min(size, mem_end - addr);
>>   			} else if (extra_pages) {
>>   				chunk_size = min(size, PFN_PHYS(extra_pages));
>> -				extra_pages -= PFN_DOWN(chunk_size);
>> -				xen_add_extra_mem(addr, chunk_size);
>> -				xen_max_p2m_pfn = PFN_DOWN(addr + chunk_size);
>> +				pfn_s = PFN_UP(addr);
>> +				n_pfns = PFN_DOWN(addr + chunk_size) - pfn_s;
>
> Should xen_add_extra_mem check for empty ranges and bail out early, or
> should the caller make sure it doesn't try to add empty ranges?
>
> IMHO it's easier and cleaner to add the check to xen_add_extra_mem.

This isn't critical at all. Adding an empty range is a nop, as a table
entry is regarded to be not used when n_pfns is 0.


Juergen

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