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

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Message-ID: <5cda8821bd3ac432a06698fa7eebdab0aff77452@linux.dev>
Date: Tue, 13 Jan 2026 07:13:39 +0000
From: "Yajun Deng" <yajun.deng@...ux.dev>
To: "Johannes Weiner" <hannes@...xchg.org>
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()

January 12, 2026 at 10:50 PM, "Johannes Weiner" <hannes@...xchg.org mailto:hannes@...xchg.org?to=%22Johannes%20Weiner%22%20%3Channes%40cmpxchg.org%3E > wrote:


> 
> 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).
> 
How about V1? It will introduce account_freepages_both().
https://lore.kernel.org/all/20260109105121.328780-1-yajun.deng@linux.dev/

> 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.
> 
Not all cases are of the isolation type. There are indeed duplicate caculations.
move_freepages_block() will be called in __isolate_free_page().
Both old_mt and new_mt are mergeable type in this case.

> Simplicity & maintainability trumps here, IMO, unless you have hard
> data showing this is worth the pain.
>
I'm tring to do it, but I haven't yet.