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

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Message-ID: <b451b318-133b-45f6-87b3-3dc3fa1f75a8@linux.ibm.com>
Date: Wed, 27 Aug 2025 23:39:13 +0530
From: Donet Tom <donettom@...ux.ibm.com>
To: David Hildenbrand <david@...hat.com>,
        Giorgi Tchankvetadze <giorgitchankvetadze1997@...il.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 8/26/25 7:39 PM, David Hildenbrand wrote:
> 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.


Yes, David, you are right. I added a test to check this scenario, and I 
am seeing ksm_zero_pages go negative.

# cat /sys/kernel/mm/ksm/ksm_zero_pages
-128
#


>
> @Donet, can you look into that as well?


Sure, I will add a fix for this issue in the next version.