Re: [PATCH] mm, page_alloc: check pfn is valid before moving to freelist

[Andrew Morton <akpm@linux-foundation.org>]

On Tue, 12 Apr 2022 13:16:23 -0700 Sudarshan Rajagopalan <quic_sudaraja@quicinc.com> wrote:

> Check if pfn is valid before or not before moving it to freelist.
> 
> There are possible scenario where a pageblock can have partial physical
> hole and partial part of System RAM. This happens when base address in RAM
> partition table is not aligned to pageblock size.
> 
> ...
>
> 
> Signed-off-by: Sudarshan Rajagopalan <quic_sudaraja@quicinc.com>
> Fixes: 4c7b9896621be ("mm: remove pfn_valid_within() and CONFIG_HOLES_IN_ZONE")

I made that 859a85ddf90e714092dea71a0e54c7b9896621be and added
cc:stable.  I'll await reviewer input before proceeding further.

> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -2521,6 +2521,11 @@ static int move_freepages(struct zone *zone,
>  	int pages_moved = 0;
>  
>  	for (pfn = start_pfn; pfn <= end_pfn;) {
> +		if (!pfn_valid(pfn)) {

Readers will wonder how we can encounter an invalid pfn here.  A small
comment might help clue them in.

> +			pfn++;
> +			continue;
> +		}
> +
>  		page = pfn_to_page(pfn);
>  		if (!PageBuddy(page)) {
>  			/*