Re: [PATCH v2 for v3.18] mm/compaction: skip the range until proper target pageblock is met

[Vlastimil Babka]

On 11/04/2014 03:37 AM, Joonsoo Kim wrote:
> commit 7d49d8868336 ("mm, compaction: reduce zone checking frequency in
> the migration scanner") makes side-effect that change iteration
> range calculation. Before change, block_end_pfn is calculated using
> start_pfn, but, now, blindly add pageblock_nr_pages to previous value.
>
> This cause the problem that isolation_start_pfn is larger than
> block_end_pfn when we isolate the page with more than pageblock order.
> In this case, isolation would be failed due to invalid range parameter.
>
> To prevent this, this patch recalculate the range to find valid target
> pageblock. Without this patch, CMA with more than pageblock order always
> fail, but, with this patch, it will succeed.
>
> Changes from v1:
> recalculate the range rather than just skipping to find valid one.
> add code comment.
>
> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>

Acked-by: Vlastimil Babka <vbabka@xxxxxxx>

(nitpick below)

> ---
>   mm/compaction.c |   10 ++++++++++
>   1 file changed, 10 insertions(+)
>
> diff --git a/mm/compaction.c b/mm/compaction.c
> index ec74cf0..4f0151c 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -479,6 +479,16 @@ isolate_freepages_range(struct compact_control *cc,
>
>   		block_end_pfn = min(block_end_pfn, end_pfn);
>
> +		/*
> +		 * pfn could pass the block_end_pfn if isolated freepage
> +		 * is more than pageblock order. In this case, we adjust
> +		 * scanning range to right one.
> +		 */
> +		if (pfn >= block_end_pfn) {
> +			block_end_pfn = ALIGN(pfn + 1, pageblock_nr_pages);
> +			block_end_pfn = min(block_end_pfn, end_pfn);
> +		}

If you moved this up, there could be just one min(block_end_pfn, 
end_pfn) instance in the code. If the first min() makes block_end_pfn == 
end_pfn and pfn >= block_end_pfn, then pfn >= end_pfn and the loop would 
be terminated already (assuming this was why you left the first min() 
before the new check). But I don't mind if you leave it like this.

> +
>   		if (!pageblock_pfn_to_page(pfn, block_end_pfn, cc->zone))
>   			break;

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