Subject: Re: [PATCH 4/6] mm/mm_init.c: remove meaningless calculation of zone->managed_pages in free_area_init_core()

From: Mike Rapoport <[email-masked]>
Date: Tue, 19 Mar 2024 18:17:07 +0200

On Mon, Mar 18, 2024 at 10:21:36PM +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), directly set zone->present_pages to
> zone->managed_pages. It will be adjusted in mem_init().

Do you mean "set zone->managed_pages to zone->present_pages"?

I think we can just set zone->managed_pages to 0 in free_area_init_core().
Anyway it will be reset before the first use.
 
> 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.
> 
> Signed-off-by: Baoquan He <[email-masked]>
> ---
>  mm/mm_init.c | 38 ++++++--------------------------------
>  1 file changed, 6 insertions(+), 32 deletions(-)
> 
> diff --git a/mm/mm_init.c b/mm/mm_init.c
> index c57a7fc97a16..55a2b886b7a6 100644
> --- a/mm/mm_init.c
> +++ b/mm/mm_init.c
> @@ -1584,41 +1584,14 @@ static void __init free_area_init_core(struct pglist_data *pgdat)
>  
>  	for (j = 0; j < MAX_NR_ZONES; j++) {
>  		struct zone *zone = pgdat->node_zones + j;
> -		unsigned long size, freesize, memmap_pages;
> -
> -		size = zone->spanned_pages;
> -		freesize = zone->present_pages;
> -
> -		/*
> -		 * Adjust freesize so that it accounts for how much memory
> -		 * is used by this zone for memmap. This affects the watermark
> -		 * and per-cpu initialisations
> -		 */
> -		memmap_pages = calc_memmap_size(size, freesize);
> -		if (!is_highmem_idx(j)) {
> -			if (freesize >= memmap_pages) {
> -				freesize -= memmap_pages;
> -				if (memmap_pages)
> -					pr_debug("  %s zone: %lu pages used for memmap\n",
> -						 zone_names[j], memmap_pages);
> -			} else
> -				pr_warn("  %s zone: %lu memmap pages exceeds freesize %lu\n",
> -					zone_names[j], memmap_pages, freesize);
> -		}
> -
> -		if (!is_highmem_idx(j))
> -			nr_kernel_pages += freesize;
> -		/* Charge for highmem memmap if there are enough kernel pages */
> -		else if (nr_kernel_pages > memmap_pages * 2)
> -			nr_kernel_pages -= memmap_pages;
> -		nr_all_pages += freesize;
> +		unsigned long size = zone->spanned_pages;
>  
>  		/*
> -		 * Set an approximate value for lowmem here, it will be adjusted
> -		 * when the bootmem allocator frees pages into the buddy system.
> -		 * And all highmem pages will be managed by the buddy system.
> +		 * Set the zone->managed_pages as zone->present_pages roughly, it
> +		 * be zeroed out and reset when memblock allocator frees pages into
> +		 * buddy system.
>  		 */
> -		zone_init_internals(zone, j, nid, freesize);
> +		zone_init_internals(zone, j, nid, zone->present_pages);
>  
>  		if (!size)
>  			continue;
> @@ -1915,6 +1888,7 @@ void __init free_area_init(unsigned long *max_zone_pfn)
>  		check_for_memory(pgdat);
>  	}
>  
> +	calc_nr_kernel_pages();
>  	memmap_init();
>  
>  	/* disable hash distribution for systems with a single node */
> -- 
> 2.41.0
> 

-- 
Sincerely yours,
Mike.