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

[Johannes Weiner <hannes@cmpxchg.org>]

On Tue, Dec 10, 2024 at 10:34:37AM +0100, 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@suse.cz>
> Closes: https://lkml.kernel.org/r/e1a898ba-a717-4d20-9144-29df1a6c8813@suse.cz
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: Zi Yan <ziy@nvidia.com>
> Cc: Yu Zhao <yuzhao@google.com>
> Cc: <stable@vger.kernel.org>
> Signed-off-by: David Hildenbrand <david@redhat.com>

Acked-by: Johannes Weiner <hannes@cmpxchg.org>