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Date: Wed, 21 Feb 2024 14:15:04 -0800
From: Andrew Morton <akpm@linux-foundation.org>
To: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Zi Yan <ziy@nvidia.com>, Vlastimil Babka <vbabka@suse.cz>,
 mgorman@techsingularity.net, linux-mm@kvack.org,
 linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/2] mm: compaction: limit the suitable target page
 order to be less than cc->order
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On Mon, 19 Feb 2024 10:55:59 +0800 Baolin Wang <baolin.wang@linux.alibaba.com> wrote:

> 
> 
> On 2024/2/12 23:00, Zi Yan wrote:
> > On 12 Feb 2024, at 4:13, Vlastimil Babka wrote:
> > 
> >> 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)
> >>
> > 
> > Thanks.
> > 
> > Hi Baolin,
> > 
> > How often do you see this happening?
> 
> This is theoretically analyzed from the code inspection.
> 
> >>> Signed-off-by: Baolin Wang <baolin.wang@linux.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?