Re: [PATCH v1] mm/page_alloc: don't call pfn_to_page() on possibly non-existent PFN in split_large_buddy()

[Vlastimil Babka]

On 12/10/24 10:34, David Hildenbrand wrote:
> In split_large_buddy(), we might call pfn_to_page() on a PFN that might
> not exist. In corner cases, such as when freeing the highest pageblock in
> the last memory section, this could result with CONFIG_SPARSEMEM &&
> !CONFIG_SPARSEMEM_EXTREME in __pfn_to_section() returning NULL and
> and __section_mem_map_addr() dereferencing that NULL pointer.
> 
> Let's fix it, and avoid doing a pfn_to_page() call for the first
> iteration, where we already have the page.
> 
> So far this was found by code inspection, but let's just CC stable as
> the fix is easy.
> 
> Fixes: fd919a85cd55 ("mm: page_isolation: prepare for hygienic freelists")
> Reported-by: Vlastimil Babka <vbabka@xxxxxxx>
> Closes: https://lkml.kernel.org/r/e1a898ba-a717-4d20-9144-29df1a6c8813@xxxxxxx
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> Cc: Zi Yan <ziy@xxxxxxxxxx>
> Cc: Yu Zhao <yuzhao@xxxxxxxxxx>
> Cc: <stable@xxxxxxxxxxxxxxx>
> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>

Reviewed-by: Vlastimil Babka <vbabka@xxxxxxx>

Thanks!

> ---
>  mm/page_alloc.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 48a291c485df4..a52c6022c65cb 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1238,13 +1238,15 @@ static void split_large_buddy(struct zone *zone, struct page *page,
>  	if (order > pageblock_order)
>  		order = pageblock_order;
>  
> -	while (pfn != end) {
> +	do {
>  		int mt = get_pfnblock_migratetype(page, pfn);
>  
>  		__free_one_page(page, pfn, zone, order, mt, fpi);
>  		pfn += 1 << order;
> +		if (pfn == end)
> +			break;
>  		page = pfn_to_page(pfn);
> -	}
> +	} while (1);
>  }
>  
>  static void free_one_page(struct zone *zone, struct page *page,