Re: [PATCH] mm/page_alloc: Avoid duplicate NR_FREE_PAGES updates in move_to_free_list()

[Joshua Hahn <joshua.hahnjy@gmail.com>]

On Fri,  9 Jan 2026 18:51:21 +0800 Yajun Deng <yajun.deng@linux.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