Re: [RFC] mm/huge_memory: prevent potential NULL pointer dereference

[Antonio Quartulli <antonio@mandelbit.com>]

On 16/07/2025 21:10, Lorenzo Stoakes wrote:
> On Wed, Jul 16, 2025 at 09:05:14PM +0200, Antonio Quartulli wrote:
>> Hi Lorenzo,
>>
>> On 16/07/2025 17:07, Lorenzo Stoakes wrote:
>>>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>>>> index 389620c65a5f..d649026db95a 100644
>>>> --- a/mm/huge_memory.c
>>>> +++ b/mm/huge_memory.c
>>>> @@ -3802,7 +3802,7 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
>>>>
>>>>    			/* Some pages can be beyond EOF: drop them from cache */
>>>>    			if (new_folio->index >= end) {
>>>
>>> It's kind of not _hugely_ clear but, if anon - which implies no mapping - we set
>>> end == -1.
>>>
>>> At which point this condition cannot evaluate true (index is at page granularity
>>> so even MAX_UINT64 would be page shifted and still not equal -1).
>>
>> I may be missing something, but why can't "index >= -1" be true?
> 
> These are unsigned long's, -1 in two's complement this means -1 translates
> to the maximum possible unsigned long. Index cannot == the maximum number
> as this is a _page_ index so a folio would need to reference a page larger
> than could be represented in a 64-bit system for that to be so.

Got it!
Thanks a lot for the clarification.

Cheers,


-- 
Antonio Quartulli

CEO and Co-Founder
Mandelbit Srl
https://www.mandelbit.com