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

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Message-Id: <EC18DC0D-45BB-40B4-B34F-0AD7DA09EC99@linux.dev>
Date: Sun, 11 Jan 2026 21:47:42 +0800
From: Yajun Deng <yajun.deng@...ux.dev>
To: Joshua Hahn <joshua.hahnjy@...il.com>
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()



> 2026年1月10日 00:31，Joshua Hahn <joshua.hahnjy@...il.com> 写道：
> 
> 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. 
> 
Hey Joshua,

Thank you for sharing your thoughts. 

I currently don’t have any performance data, I just noticed from looking at the code
that there may be room for optimization.
You’re right. The original code is indeed more straightforward. I think we can add some
comments in the  account_freepages_both to make it easier to understand.

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