linux-kernel - Re: [PATCH] mm/page_alloc: optimize lowmem

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Message-ID: <912CFDB8-6374-4428-A0F2-29FB00E6ECE2@nvidia.com>
Date: Fri, 14 Nov 2025 11:12:16 -0500
From: Zi Yan <ziy@...dia.com>
To: Brendan Jackman <jackmanb@...gle.com>
Cc: fujunjie <fujunjie1@...com>, akpm@...ux-foundation.org, vbabka@...e.cz,
 surenb@...gle.com, mhocko@...e.com, hannes@...xchg.org, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/page_alloc: optimize lowmem_reserve max lookup using
 monotonicity

On 14 Nov 2025, at 7:36, Brendan Jackman wrote:

> On Fri Nov 14, 2025 at 10:40 AM UTC, fujunjie wrote:
>> Although this code is not on a hot path, the revised form is clearer
>
> Is it...?
>
> If people do think it is clearer, let's at least write the right comment
> in the right place. Instead of having one piece of code
> (calculate_totalreserve_pages()) describe at a distance the behaviour of
> another piece of code (setup_per_zone_lowmem_reserve()), let's describe
> an invariant of the data ("lowmem_reserve is monotonic, up to the first
> zero value"), at the site where the data is defined.

I agree.

>
> I know sometimes in code this complex we do need these
> "spooky-action-at-a-distance" comments but this doesn't seem like one of
> those places to me.

My concern on this change is that the correctness of
calculate_totalreserve_pages() now relies on the implementation of
setup_per_zone_lowmem_reserve(). How can we make sure in the future
this will not break when setup_per_zone_lowmem_reserve() is changed?
Hoping people read the comment and do the right thing?

Best Regards,
Yan, Zi