linux-kernel - Re: [PATCH v2] mm/page_alloc: Avoid duplicate NR_FREE_PAGES updates in move_to_free

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Message-ID: <aWUKKyBZzsCBELLi@cmpxchg.org>
Date: Mon, 12 Jan 2026 09:50:19 -0500
From: Johannes Weiner <hannes@...xchg.org>
To: Yajun Deng <yajun.deng@...ux.dev>
Cc: akpm@...ux-foundation.org, vbabka@...e.cz, surenb@...gle.com,
	mhocko@...e.com, jackmanb@...gle.com, ziy@...dia.com,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Joshua Hahn <joshua.hahnjy@...il.com>
Subject: Re: [PATCH v2] mm/page_alloc: Avoid duplicate NR_FREE_PAGES updates
 in move_to_free_list()

On Mon, Jan 12, 2026 at 08:16:14PM +0800, Yajun Deng 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 adds a condition that updates the NR_FREE_PAGES only if one of
> the two types is the isolate type. This avoids NR_FREE_PAGES being
> updates twice.
> 
> The optimization avoid duplicate NR_FREE_PAGES updates in
> move_to_free_list().
> 
> Signed-off-by: Yajun Deng <yajun.deng@...ux.dev>
> Suggested-by: Joshua Hahn <joshua.hahnjy@...il.com>

I'm not a fan of this.

The code ends up more complicated, more lines, and fragile because the
accounting decisions are now spread out over multiple places (again).

Is it worth it? move_to_free_list() is used in page isolation, which
has to do the accounting anyway; and migratetype fallbacks, which we
are trying to avoid as much as possible. So this path shouldn't be all
that hot to begin with.

Simplicity & maintainability trumps here, IMO, unless you have hard
data showing this is worth the pain.