linux-kernel - Re: [PATCH 1/2] mm/ksm: Reset KSM counters in mm

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Message-ID: <53a7bf3d-843d-4e19-b0a3-cc6852fe72c1@redhat.com>
Date: Tue, 26 Aug 2025 16:09:15 +0200
From: David Hildenbrand <david@...hat.com>
To: Giorgi Tchankvetadze <giorgitchankvetadze1997@...il.com>,
 donettom@...ux.ibm.com
Cc: aboorvad@...ux.ibm.com, akpm@...ux-foundation.org,
 chengming.zhou@...ux.dev, linux-kernel@...r.kernel.org, linux-mm@...ck.org,
 richard.weiyang@...il.com, ritesh.list@...il.com, xu.xin16@....com.cn
Subject: Re: [PATCH 1/2] mm/ksm: Reset KSM counters in mm_struct during fork

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