Re: [PATCH 1/2] mm/ksm: Reset KSM counters in mm_struct during fork

[David Hildenbrand]

On 26.08.25 15:47, Giorgi Tchankvetadze wrote:
> What if we only allocate KSM stats when a process actually uses KSM?
>
> struct ksm_mm_stats {
> 	atomic_long_t merging_pages;
> 	atomic_long_t rmap_items;
> 	atomic_long_t zero_pages;
> };
> struct ksm_mm_stats *mm->ksm_stats; // NULL until first enter
>
> static inline struct ksm_mm_stats *mm_get_ksm_stats(struct mm_struct *mm)
> {
> 	if (likely(mm->ksm_stats))
> 		return mm->ksm_stats;
> 	return ksm_alloc_stats_if_needed(mm); // Slow path
> }

The fork'ed child uses KSM. It just doesn't have any stable rmap entries.

We have to copy the zero_pages counter such that 
ksm_might_unmap_zero_page() will do the right thing.

But you're comment made me realize that there is likely another bug:

When copying zero_pages during fork(), we have to increment 
&ksm_zero_pages as well. Otherwise we will get an underflow later.

@Donet, can you look into that as well?

--
Cheers

David / dhildenb