linux-kernel - Re: [PATCH] mm/page_alloc: Avoid duplicate NR_FREE_PAGES updates in move_to_free

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Message-ID: <20260109163140.1605599-1-joshua.hahnjy@gmail.com>
Date: Fri,  9 Jan 2026 08:31:39 -0800
From: Joshua Hahn <joshua.hahnjy@...il.com>
To: Yajun Deng <yajun.deng@...ux.dev>
Cc: akpm@...ux-foundation.org,
	vbabka@...e.cz,
	surenb@...gle.com,
	mhocko@...e.com,
	jackmanb@...gle.com,
	hannes@...xchg.org,
	ziy@...dia.com,
	linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/page_alloc: Avoid duplicate NR_FREE_PAGES updates in move_to_free_list()

On Fri,  9 Jan 2026 18:51:21 +0800 Yajun Deng <yajun.deng@...ux.dev> wrote:

> In move_to_free_list(), when a page block changes its migration type,
> we need to update free page counts for both the old and new types.
> Originally, this was done by two calls to account_freepages(), which
> updates NR_FREE_PAGES and also type-specific counters. However, this
> causes NR_FREE_PAGES to be updated twice, while the net change is zero
> in most cases.
> 
> This patch introduces a new function account_freepages_both() that
> updates the statistics for both old and new migration types in one go.
> It avoids the double update of NR_FREE_PAGES by computing the net change
> only when the isolation status changes.
> 
> The optimization avoid duplicate NR_FREE_PAGES updates in
> move_to_free_list().

Hi Yajun,

I hope you are doing well, thank you for the patch! I was hoping to better
understand the motivation behind this patch.

>From my perspective, I believe that the current state of the code is
not optimal, but it is also not problematic. account_freepages seems like
a relatively cheap function (at the core, it's just some atomic operations).
Personally I also think that semantically, the code currently makes sense;
we are doing the accounting for the old mounttype, then for the new mounttype,
in a way that cancels out. And given that there is still some cases where
the work doesn't end up canceling out due to one of the mounttypes being
MIGRATE_ISOLATE, I think that there is enough purpose in making the two
calls to do the accounting twice.

On the other hand I think there is only one place in the codebase that
will use account_freepages_both, so it might make the burden to understand
the code a bit higher.

What do you think? I don't have a strong stance on whether the performance
effects are big here (if this change indeed has a big performance implication,
then we should definitely go forth with this!) but I do believe the current
code is quite semantically sound and more readable. 

Thank you again for the patch. I hope you have a great day!
Joshua