Re: [PATCH] mm/page_alloc: fix alignment for alloc_contig_pages_noprof()

[Dev Jain <dev.jain@arm.com>]

On 23/09/25 5:49 am, Wei Yang wrote:
> Commit 5e27a2df03b8 ("mm/page_alloc: add alloc_contig_pages()")
> introduced generic method for alloc_contig_pages(). But the alignment
> calculation seems wrong.
>
> Since ALIGN() only accept power of two value, while nr_pages could be
> any positive one, the result is not defined.
>
> Use roundup() to calculate the correct alignment.
>
> Fixes: 5e27a2df03b8 ("mm/page_alloc: add alloc_contig_pages()")

The ALIGN() statement was even before this, but not sure if a potential
caller with non-power-of-2 nr_pages was present then.

> Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
> Cc: Anshuman Khandual <anshuman.khandual@arm.com>
> Cc: Oscar Salvador <osalvador@suse.de>
> Cc: Brendan Jackman <jackmanb@google.com>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: Michal Hocko <mhocko@suse.com>
> Cc: Suren Baghdasaryan <surenb@google.com>
> Cc: Vlastimil Babka <vbabka@suse.cz>
> Cc: Wei Yang <richard.weiyang@gmail.com>
> Cc: Zi Yan <ziy@nvidia.com>
> ---
>   mm/page_alloc.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index a1bcc1e003c7..a17a6014e3db 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -7095,7 +7095,7 @@ struct page *alloc_contig_pages_noprof(unsigned long nr_pages, gfp_t gfp_mask,
>   					gfp_zone(gfp_mask), nodemask) {
>   		spin_lock_irqsave(&zone->lock, flags);
>   
> -		pfn = ALIGN(zone->zone_start_pfn, nr_pages);
> +		pfn = roundup(zone->zone_start_pfn, nr_pages);
>   		while (zone_spans_last_pfn(zone, pfn, nr_pages)) {
>   			if (pfn_range_valid_contig(pfn, nr_pages)) {
>   				/*

The kernel-doc above alloc_contig_pages_noprof reads:

"If nr_pages is a power of two, then allocated range is always guaranteed
to be aligned to same nr_pages." This means that the function gives no
guarantee on alignment of non-power-of-2 requests - and why should it?
It doesn't make any sense to align pfn to 7 if nr_pages is 7.