linux-kernel - Re: [PATCH v2 4/6] mm/mm_init.c: remove meaningless calculation of zone->managed_pages in free_area_init

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Date: Wed, 27 Mar 2024 17:40:38 +0200
From: Mike Rapoport <rppt@...nel.org>
To: Baoquan He <bhe@...hat.com>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org, x86@...nel.org,
	linuxppc-dev@...ts.ozlabs.org, akpm@...ux-foundation.org
Subject: Re: [PATCH v2 4/6] mm/mm_init.c: remove meaningless calculation of
 zone->managed_pages in free_area_init_core()

On Mon, Mar 25, 2024 at 10:56:44PM +0800, Baoquan He wrote:
> Currently, in free_area_init_core(), when initialize zone's field, a
> rough value is set to zone->managed_pages. That value is calculated by
> (zone->present_pages - memmap_pages).
> 
> In the meantime, add the value to nr_all_pages and nr_kernel_pages which
> represent all free pages of system (only low memory or including HIGHMEM
> memory separately). Both of them are gonna be used in
> alloc_large_system_hash().
> 
> However, the rough calculation and setting of zone->managed_pages is
> meaningless because
>   a) memmap pages are allocated on units of node in sparse_init() or
>      alloc_node_mem_map(pgdat); The simple (zone->present_pages -
>      memmap_pages) is too rough to make sense for zone;
>   b) the set zone->managed_pages will be zeroed out and reset with
>      acutal value in mem_init() via memblock_free_all(). Before the
>      resetting, no buddy allocation request is issued.
> 
> Here, remove the meaningless and complicated calculation of
> (zone->present_pages - memmap_pages), initialize zone->managed_pages as 0
> which reflect its actual value because no any page is added into buddy
> system right now. It will be reset in mem_init().
> 
> And also remove the assignment of nr_all_pages and nr_kernel_pages in
> free_area_init_core(). Instead, call the newly added calc_nr_kernel_pages()
> to count up all free but not reserved memory in memblock and assign to
> nr_all_pages and nr_kernel_pages. The counting excludes memmap_pages,
> and other kernel used data, which is more accurate than old way and
> simpler, and can also cover the ppc required arch_reserved_kernel_pages()
> case.
> 
> And also clean up the outdated code comment above free_area_init_core().
> And free_area_init_core() is easy to understand now, no need to add
> words to explain.
> 
> Signed-off-by: Baoquan He <bhe@...hat.com>

Reviewed-by: Mike Rapoport (IBM) <rppt@...nel.org>

> ---
>  mm/mm_init.c | 46 +++++-----------------------------------------
>  1 file changed, 5 insertions(+), 41 deletions(-)