Re: [PATCH] mm: walk the zone in pageblock_nr_pages steps

[Vlastimil Babka <vbabka@suse.cz>]

On 07/26/2016 05:08 AM, zhongjiang wrote:
> From: zhong jiang <zhongjiang@huawei.com>
>
> when walking the zone, we can happens to the holes. we should not
> align MAX_ORDER_NR_PAGES, so it can skip the normal memory.
>
> In addition, pagetypeinfo_showmixedcount_print reflect fragmentization.
> we hope to get more accurate data. therefore, I decide to fix it.

Can't say I'm happy with another random half-fix. What's the real 
granularity of holes for CONFIG_HOLES_IN_ZONE systems? I suspect it can 
be below pageblock_nr_pages. The pfn_valid_within() mechanism seems 
rather insufficient... it does prevent running unexpectedly into holes 
in the middle of pageblock/MAX_ORDER block, but together with the large 
skipping it doesn't guarantee that we cover all non-holes.

I think in a robust solution, functions such as these should use 
something like PAGE_HOLE_GRANULARITY which equals MAX_ORDER_NR_PAGES for 
!CONFIG_HOLES_IN_ZONE and some arch/config/system specific value for 
CONFIG_HOLES_IN_ZONE. This would then be used in the ALIGN() part.
It could be also used together with pfn_valid_within() in the inner loop 
to skip over holes more quickly (if it's worth).

Also I just learned there's also CONFIG_ARCH_HAS_HOLES_MEMORYMODEL that 
affects a function called memmap_valid_within(). But that one has only 
one caller - pagetypeinfo_showblockcount_print(). Why is it needed there 
and not in pagetypeinfo_showmixedcount_print() (or anywhere else?)

> Signed-off-by: zhong jiang <zhongjiang@huawei.com>
> ---
>  mm/vmstat.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/mm/vmstat.c b/mm/vmstat.c
> index cb2a67b..3508f74 100644
> --- a/mm/vmstat.c
> +++ b/mm/vmstat.c
> @@ -1033,7 +1033,7 @@ static void pagetypeinfo_showmixedcount_print(struct seq_file *m,
>  	 */
>  	for (; pfn < end_pfn; ) {
>  		if (!pfn_valid(pfn)) {
> -			pfn = ALIGN(pfn + 1, MAX_ORDER_NR_PAGES);
> +			pfn = ALIGN(pfn + 1, pageblock_nr_pages);
>  			continue;
>  		}
>
>

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