linux-kernel - Re: [PATCH 1/4] compcache: xvmalloc memory allocator

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Date:	Tue, 25 Aug 2009 02:46:03 +0530
From:	Nitin Gupta <ngupta@...are.org>
To:	Hugh Dickins <hugh.dickins@...cali.co.uk>
CC:	Pekka Enberg <penberg@...helsinki.fi>, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	linux-mm-cc@...top.org
Subject: Re: [PATCH 1/4] compcache: xvmalloc memory allocator

On 08/25/2009 02:09 AM, Hugh Dickins wrote:
> On Tue, 25 Aug 2009, Nitin Gupta wrote:
>> On 08/24/2009 11:03 PM, Pekka Enberg wrote:
>>>
>>> What's the purpose of passing PFNs around? There's quite a lot of PFN
>>> to struct page conversion going on because of it. Wouldn't it make
>>> more sense to return (and pass) a pointer to struct page instead?
>>
>> PFNs are 32-bit on all archs
>
> Are you sure?  If it happens to be so for all machines built today,
> I think it can easily change tomorrow.  We consistently use unsigned long
> for pfn (there, now I've said that, I bet you'll find somewhere we don't!)
>
> x86_64 says MAX_PHYSMEM_BITS 46 and ia64 says MAX_PHYSMEM_BITS 50 and
> mm/sparse.c says
> unsigned long max_sparsemem_pfn = 1UL<<  (MAX_PHYSMEM_BITS-PAGE_SHIFT);
>

For PFN to exceed 32-bit we need to have physical memory > 16TB (2^32 * 4KB).
So, maybe I can simply add a check in ramzswap module load to make sure that
RAM is indeed < 16TB and then safely use 32-bit for PFN?

Thanks,
Nitin
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