Re: [PATCH v2 4/6] mm/mm_init.c: remove meaningless calculation of zone->managed_pages in free_area_init_core()

[Baoquan He]

On 03/25/24 at 10:56pm, 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 And also clean up the outdated code comment above free_area_init_core().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@xxxxxxxxxx>
> ---
>  mm/mm_init.c | 46 +++++-----------------------------------------
>  1 file changed, 5 insertions(+), 41 deletions(-)
> 
> diff --git a/mm/mm_init.c b/mm/mm_init.c
> index c57a7fc97a16..7f71e56e83f3 100644
> --- a/mm/mm_init.c
> +++ b/mm/mm_init.c
> @@ -1565,15 +1565,6 @@ void __ref free_area_init_core_hotplug(struct pglist_data *pgdat)
>  }
>  #endif
>  
> -/*
> - * Set up the zone data structures:
> - *   - mark all pages reserved
> - *   - mark all memory queues empty
> - *   - clear the memory bitmaps
> - *
> - * NOTE: pgdat should get zeroed by caller.
> - * NOTE: this function is only called during early init.
> - */
>  static void __init free_area_init_core(struct pglist_data *pgdat)
>  {
>  	enum zone_type j;
> @@ -1584,41 +1575,13 @@ 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.
> +		 * Initialize zone->managed_pages as 0 , it will be reset
> +		 * when memblock allocator frees pages into buddy system.
>  		 */
> -		zone_init_internals(zone, j, nid, freesize);
> +		zone_init_internals(zone, j, nid, 0);

Here, we should initialize zone->managed_pages as zone->present_pages
because later page_group_by_mobility_disabled need be set according to
zone->managed_pages. Otherwise page_group_by_mobility_disabled will be
set to 1 always. I will sent out v3.