linux-kernel - Re: [PATCH 1/2] mm: compaction: limit the suitable target page order to be less than cc->order

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Date: Mon, 12 Feb 2024 10:13:56 +0100
From: Vlastimil Babka <vbabka@...e.cz>
To: Baolin Wang <baolin.wang@...ux.alibaba.com>, akpm@...ux-foundation.org,
 Zi Yan <ziy@...dia.com>
Cc: mgorman@...hsingularity.net, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] mm: compaction: limit the suitable target page order
 to be less than cc->order

On 1/22/24 14:01, Baolin Wang wrote:
> It can not improve the fragmentation if we isolate the target free pages
> exceeding cc->order, especially when the cc->order is less than pageblock_order.
> For example, suppose the pageblock_order is MAX_ORDER (size is 4M) and cc->order
> is 2M THP size, we should not isolate other 2M free pages to be the migration
> target, which can not improve the fragmentation.
> 
> Moreover this is also applicable for large folio compaction.

So why not Cc: Zi Yan? (done)

> Signed-off-by: Baolin Wang <baolin.wang@...ux.alibaba.com>

I doubt this will make much difference, because if such a larger order free
page exists, we shouldn't have a reason to be compacting for a lower order
in the first place?

> ---
>  mm/compaction.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/compaction.c b/mm/compaction.c
> index 27ada42924d5..066b72b3471a 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -1346,12 +1346,14 @@ static bool suitable_migration_target(struct compact_control *cc,
>  {
>  	/* If the page is a large free page, then disallow migration */
>  	if (PageBuddy(page)) {
> +		int order = cc->order > 0 ? cc->order : pageblock_order;
> +
>  		/*
>  		 * We are checking page_order without zone->lock taken. But
>  		 * the only small danger is that we skip a potentially suitable
>  		 * pageblock, so it's not worth to check order for valid range.
>  		 */
> -		if (buddy_order_unsafe(page) >= pageblock_order)
> +		if (buddy_order_unsafe(page) >= order)
>  			return false;
>  	}
>