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: Wed, 21 Feb 2024 23:22:43 +0100
From: Vlastimil Babka <vbabka@...e.cz>
To: Andrew Morton <akpm@...ux-foundation.org>,
 Baolin Wang <baolin.wang@...ux.alibaba.com>
Cc: Zi Yan <ziy@...dia.com>, 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 2/21/24 23:15, Andrew Morton wrote:
> On Mon, 19 Feb 2024 10:55:59 +0800 Baolin Wang <baolin.wang@...ux.alibaba.com> wrote:
>> 
>> >>> 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?
>> > 
>> > Unless kswapd gets us such a free block in the background right after
>> > get_page_from_freelist() and before compaction finishes in the allocation
>> > slow path.
>> > 
>> > If this happens often and cc->order is not -1, it might be better to stop
>> > compaction and get_page_from_freelist() to save cycles on unnecessary pfn
>> > scanning. For completeness, when cc->order == -1, the logic does not change.
>> 
>> Yes, this is one possible case. There are also some other concurrent 
>> scenarios, such as when compaction is running (after 
>> compaction_suitable()), at the same time, other applications release a 
>> large folio to the free list. In this case, the free large folio 
>> scanning should also be avoided.
> 
> This went quiet.
> 
> We have an ack from Mel.  Are people OK with sending this change
> upstream?

It's not wrong, so I'm OK.