Re: [PATCH] hibernate / memory hotplug: always use for_each_populated_zone()

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

On Wed, 22 Jul 2009 09:25:35 +0900
KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> wrote:

> See ia64's ia64_pfn_valid(). It uses get_user() very effectively.
> (I think this cost cost is small in any arch...)
> 
>  523 ia64_pfn_valid (unsigned long pfn)
>  524 {
>  525         char byte;
>  526         struct page *pg = pfn_to_page(pfn);
>  527 
>  528         return     (__get_user(byte, (char __user *) pg) == 0)
>  529                 && ((((u64)pg & PAGE_MASK) == (((u64)(pg + 1) - 1) & PAGE_MASK))
>  530                         || (__get_user(byte, (char __user *) (pg + 1) - 1) == 0));
>  531 }
> 
Just an explanation. This code is for checking "there is memmap or not" for 
CONFIG_VIRTUAL_MEMMAP+CONFIG_DISCONTIGMEM, which allocates memmap in virtually
contiguous area. Because ia64 tends to have very sparse memory map,
memmap cannot be allocated in continuous area and memmap has holes.

This code checkes first byte and last byte of "struct page" is valid.

Thanks,
-Kame

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