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

[David Hildenbrand <david@redhat.com>]

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@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>
> 
> Reviewed-by: Vlastimil Babka <vbabka@suse.cz>

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