Re: [PATCH] Tight check of pfn_valid on sparsemem - v4

[Minchan Kim <minchan.kim@gmail.com>]

On Fri, Jul 30, 2010 at 5:55 AM, Dave Hansen <dave@linux.vnet.ibm.com> wrote:
> On Thu, 2010-07-29 at 19:33 +0100, Russell King - ARM Linux wrote:
>> And no, setting the sparse section size to 512kB doesn't work - memory is
>> offset by 256MB already, so you need a sparsemem section array of 1024
>> entries just to cover that - with the full 256MB populated, that's 512
>> unused entries followed by 512 used entries.  That too is going to waste
>> memory like nobodies business.
>
> Sparsemem could use some work in the case where memory doesn't start at
> 0x0.  But, it doesn't seem like it would be _too_ oppressive to add.
> It's literally just adding an offset to all of the places where a
> physical address is stuck into the system.  It'll make a few of the
> calculations longer, of course, but it should be manageable.
>
> Could you give some full examples of how the memory is laid out on these
> systems?  I'm having a bit of a hard time visualizing it.
>
> As Christoph mentioned, SPARSEMEM_EXTREME might be viable here, too.
>
> If you free up parts of the mem_map[] array, how does the buddy
> allocator still work?  I thought we required at 'struct page's to be
> contiguous and present for at least 2^MAX_ORDER-1 pages in one go.

I think in that case, arch should define CONFIG_HOLES_IN_ZONE to prevent
crash. But I am not sure hole architectures on ARM have been used it well.
Kujkin's problem happens not buddy but walking whole pfn to echo
min_free_kbytes.

>
> -- Dave
>
>



-- 
Kind regards,
Minchan Kim

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