Re: [PATCH] mm/page_alloc: break on the first hit of mem range

[Wei Yang <richard.weiyang@gmail.com>]

On Tue, Mar 27, 2018 at 06:37:57PM -0700, Andrew Morton wrote:
>On Wed, 28 Mar 2018 08:51:42 +0800 Wei Yang <richard.weiyang@gmail.com> wrote:
>
>> On Tue, Mar 27, 2018 at 03:47:40PM -0700, Andrew Morton wrote:
>> >On Tue, 27 Mar 2018 11:57:07 +0800 Wei Yang <richard.weiyang@gmail.com> wrote:
>> >
>> >> find_min_pfn_for_node() iterate on pfn range to find the minimum pfn for a
>> >> node. The memblock_region in memblock_type are already ordered, which means
>> >> the first hit in iteration is the minimum pfn.
>> >> 
>> >> This patch returns the fist hit instead of iterating the whole regions.
>> >> 
>> >> ...
>> >>
>> >> --- a/mm/page_alloc.c
>> >> +++ b/mm/page_alloc.c
>> >> @@ -6365,14 +6365,14 @@ unsigned long __init node_map_pfn_alignment(void)
>> >>  /* Find the lowest pfn for a node */
>> >>  static unsigned long __init find_min_pfn_for_node(int nid)
>> >>  {
>> >> -	unsigned long min_pfn = ULONG_MAX;
>> >> -	unsigned long start_pfn;
>> >> +	unsigned long min_pfn;
>> >>  	int i;
>> >>  
>> >> -	for_each_mem_pfn_range(i, nid, &start_pfn, NULL, NULL)
>> >> -		min_pfn = min(min_pfn, start_pfn);
>> >> +	for_each_mem_pfn_range(i, nid, &min_pfn, NULL, NULL) {
>> >> +		break;
>> >> +	}
>> >
>> >That would be the weirdest-looking code snippet in mm/!
>> >
>> 
>> You mean the only break in a for_each loop? Hmm..., this is really not that
>> nice. Haven't noticed could get a "best" in this way :-)
>
>I guess we can make it nicer by adding a comment along the lines of
>
>	/*
>	 * Use for_each_mem_pfn_range() to locate the lowest valid pfn in the
>	 * range.  We only need to iterate a single time, as the pfn's are
>	 * sorted in ascending order.
>	 */
>
>Because adding a call to the obviously-internal __next_mem_pfn_range()
>isn't very nice either.

Yep, you are right.

>
>Anyway, please have a think, see what we can come up with.

My approach is to add a macro fist_mem_pfn() as a self-explain wrapper of
__next_mem_pfn_range().

Hope you would like this :-)

-- 
Wei Yang
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