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

[Wei Yang <richard.weiyang@gmail.com>]

On Mon, Sep 15, 2025 at 10:39:28AM +0100, Kiryl Shutsemau wrote:
>On Sun, Sep 14, 2025 at 12:00:26AM +0000, 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.
>> 
>> Current code works since mm_slot is the first element, but make sure it
>> won't be disturbed.
>
>Seems excessive for this kind of problem.
>
>Why not check slot in mm_slot_entry?
>
>diff --git a/mm/mm_slot.h b/mm/mm_slot.h
>index 83f18ed1c4bd..0de24c490b62 100644
>--- a/mm/mm_slot.h
>+++ b/mm/mm_slot.h
>@@ -19,7 +19,7 @@ struct mm_slot {
> };
>
> #define mm_slot_entry(ptr, type, member) \
>-	container_of(ptr, type, member)
>+	(ptr ? container_of(ptr, type, member) : NULL)
>

Hmm... I  have thought about this. But not choose this one without some strong
reason.

I am fine with this too...

> static inline void *mm_slot_alloc(struct kmem_cache *cache)
> {
>-- 
>  Kiryl Shutsemau / Kirill A. Shutemov

-- 
Wei Yang
Help you, Help me