linux-kernel - Re: [PATCH] mm/page_alloc: Harmonize should_compact

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Message-ID: <a6571b18-a63a-4e9f-b911-7cc10c72a4c1@suse.cz>
Date: Thu, 28 Aug 2025 20:31:30 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: Brendan Jackman <jackmanb@...gle.com>,
 Andrew Morton <akpm@...ux-foundation.org>
Cc: Suren Baghdasaryan <surenb@...gle.com>, Michal Hocko <mhocko@...e.com>,
 Johannes Weiner <hannes@...xchg.org>, Zi Yan <ziy@...dia.com>,
 linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/page_alloc: Harmonize should_compact_retry() type

On 8/27/25 17:30, Brendan Jackman wrote:
> On Wed Aug 27, 2025 at 2:13 AM UTC, Andrew Morton wrote:
>> On Tue, 26 Aug 2025 14:06:54 +0000 Brendan Jackman <jackmanb@...gle.com> wrote:
>>
>>> Currently order is signed in one version of the function and unsigned in
>>> the other. Tidy that up.
>>> 
>>> In page_alloc.c, order is unsigned in the vast majority of cases. But,
>>> there is a cluster of exceptions in compaction-related code (probably
>>> stemming from the fact that compact_control.order is signed). So, prefer
>>> local consistency and make this one signed too.
>>> 
>>
>> grumble, pet peeve.  Negative orders make no sense.  Can we make
>> cc->order unsigned in order (heh) to make everything nice?
> 
> I think we can't "just" do that:

That part should be easy to convert to a compact_control flag.
Zi's point about going negative seems like more prone to overlook some case.
But worth trying and the cleanups I'd say.
> /*
>  * order == -1 is expected when compacting proactively via
>  * 1. /proc/sys/vm/compact_memory
>  * 2. /sys/devices/system/node/nodex/compact
>  * 3. /proc/sys/vm/compaction_proactiveness
>  */
> static inline bool is_via_compact_memory(int order)
> {
> 	return order == -1;
> }