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

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On 10.12.24 10:48, Vlastimil Babka wrote:
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>

BTW, staring at nr_isolate_pageblock accounting, I just stumbled over prep_move_freepages_block(), and I am not sure about the zone_spans_pfn() checks in there.

With overlapping zones, these are not reliably, which makes me believe that maybe these checks are not required at all. Or that there is a bug. Or that there is some implication that these checks are only required on systems without overlapping zones :)

--
Cheers,

David / dhildenb





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