Re: [PATCH] mm/mm_init: Don't iterate pages below ARCH_PFN_OFFSET

[Mike Rapoport <rppt@kernel.org>]

On Sat, Apr 19, 2025 at 12:27:27AM +0800, Ruihan Li wrote:
> Currently, memmap_init initializes pfn_hole with 0 instead of
> ARCH_PFN_OFFSET. Then init_unavailable_range will start iterating each
> page from the page at address zero to the first available page, but it
> won't do anything for pages below ARCH_PFN_OFFSET because pfn_valid
> won't pass.
> 
> If ARCH_PFN_OFFSET is very large (e.g., something like 2^64-2GiB if the
> kernel is used as a library and loaded at a very high address), the
> pointless iteration for pages below ARCH_PFN_OFFSET will take a very
> long time, and the kernel will look stuck at boot time.
> 
> This commit sets the initial value of pfn_hole to ARCH_PFN_OFFSET, which
> avoids the problematic and useless iteration mentioned above.
> 
> Fixes: 907ec5fca3dc ("mm: zero remaining unavailable struct pages")

I don't think it should be backported, this does not fix a bug.

> Signed-off-by: Ruihan Li <lrh2000@pku.edu.cn>
> ---
>  mm/mm_init.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/mm_init.c b/mm/mm_init.c
> index 84f14fa12..b3ae9f797 100644
> --- a/mm/mm_init.c
> +++ b/mm/mm_init.c
> @@ -966,7 +966,7 @@ static void __init memmap_init_zone_range(struct zone *zone,
>  static void __init memmap_init(void)
>  {
>  	unsigned long start_pfn, end_pfn;
> -	unsigned long hole_pfn = 0;
> +	unsigned long hole_pfn = ARCH_PFN_OFFSET;

ARCH_PFN_OFFSET is defined only for FLATMEM, this won't even build for
SPARSEMEM.

>  	int i, j, zone_id = 0, nid;
>  
>  	for_each_mem_pfn_range(i, MAX_NUMNODES, &start_pfn, &end_pfn, &nid) {
> -- 
> 2.49.0
> 
> 

-- 
Sincerely yours,
Mike.