Re: [PATCH] mm/mm_slot: make sure slot is the first element of its wrapper structure

[Lance Yang <lance.yang@linux.dev>]

On 2025/9/14 15:29, Dev Jain wrote:
> 
> On 14/09/25 12:33 pm, Lance Yang wrote:
>>
>>
>> On 2025/9/14 14:21, Dev Jain wrote:
>>>
>>> On 14/09/25 5:30 am, Wei Yang wrote:
>>>> When using mm_slot in ksm/khugepaged, there is code snip like:
>>>>
>>>>       slot = mm_slot_lookup(mm_slots_hash, mm);
>>>>       mm_slot = mm_slot_entry(slot, struct ksm_mm_slot, slot);
>>>>       if (mm_slot && ..) {
>>>>       }
>>>>
>>>> This is only valid when mm_slot is the first element of its wrapper
>>>> structure, otherwise a NULL slot would converted to a mm_slot with
>>>> negative value. And current code thinks it is valid and continue.
>>>
>>> Shouldn't you fix the code for the case when you can't find the slot
>>> in the hashtable, i.e slot == NULL? Like, if (!slot) return.
>>
>> Right. For khugepaged specifically, the slot == NULL case in
>> __khugepaged_exit() (only user of mm_slot_lookup) should probably
>> be treated as a kernel BUG for new.
>>
>> But I'm not sure if the same logic applies to KSM ;)
> 
> I haven't seen the KSM analogue, but restricting the position of an element
> 
> in a struct to make the code work should imply that the code is wrong in
> 
> the first place :)

Yep, you're spot on :)

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