linux-kernel - Re: [PATCH v2] mm: ksm: Consider the number of ksm_mm_slot in the general

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Date: Thu, 20 Jun 2024 21:38:21 +0200
From: David Hildenbrand <david@...hat.com>
To: Sung-hun Kim <sfoon.kim@...sung.com>, akpm@...ux-foundation.org,
 linux-mm@...ck.org, linux-kernel@...r.kernel.org
Cc: sungguk.na@...sung.com, sw0312.kim@...sung.com, sebuns@...il.com
Subject: Re: [PATCH v2] mm: ksm: Consider the number of ksm_mm_slot in the
 general_profit calculation

On 20.06.24 06:39, Sung-hun Kim wrote:
> The current version of KSM does not take into account the number of
> used ksm_mm_slot. Therefore, when users want to obtain profits of
> KSM, KSM omits the memory used for allocating ksm_mm_slots.
> 
> This patch introduces a new variable to keep track of the number of
> allocated ksm_mm_slots. By doing so, KSM will be able to provide a
> more accurate number of the gains made.

If you take a look at the calculation explained in 
Documentation/admin-guide/mm/ksm.rst, we only care about rmap_items, 
which can grow rather substantially in size.

We also don't consider other metadata, such as the size of the stable 
nodes etc. So why should the ksm_mm_slots matter that much that we 
should track them and account them?

Any real life examples where this is relevant / a problem.

> 
> Signed-off-by: Sung-hun Kim <sfoon.kim@...sung.com>
> ---
> Changelog in V2:
> - Add an MMF_VM_MERGEABLE flag check in ksm_process_profit for
> untracked processes
> ---
>   mm/ksm.c | 13 +++++++++++--
>   1 file changed, 11 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/ksm.c b/mm/ksm.c
> index 34c4820e0d3d..c8ced991ccda 100644
> --- a/mm/ksm.c
> +++ b/mm/ksm.c
> @@ -267,6 +267,9 @@ static unsigned long ksm_pages_unshared;
>   /* The number of rmap_items in use: to calculate pages_volatile */
>   static unsigned long ksm_rmap_items;
>   
> +/* The number of ksm_mm_slot in use */
> +static atomic_long_t ksm_mm_slots = ATOMIC_LONG_INIT(0);
> +
>   /* The number of stable_node chains */
>   static unsigned long ksm_stable_node_chains;
>   
> @@ -1245,6 +1248,7 @@ static int unmerge_and_remove_all_rmap_items(void)
>   			spin_unlock(&ksm_mmlist_lock);
>   
>   			mm_slot_free(mm_slot_cache, mm_slot);
> +			atomic_long_dec(&ksm_mm_slots);
>   			clear_bit(MMF_VM_MERGEABLE, &mm->flags);
>   			clear_bit(MMF_VM_MERGE_ANY, &mm->flags);
>   			mmdrop(mm);
> @@ -2717,6 +2721,7 @@ static struct ksm_rmap_item *scan_get_next_rmap_item(struct page **page)
>   		spin_unlock(&ksm_mmlist_lock);
>   
>   		mm_slot_free(mm_slot_cache, mm_slot);
> +		atomic_long_dec(&ksm_mm_slots);
>   		clear_bit(MMF_VM_MERGEABLE, &mm->flags);
>   		clear_bit(MMF_VM_MERGE_ANY, &mm->flags);
>   		mmap_read_unlock(mm);
> @@ -3000,6 +3005,7 @@ int __ksm_enter(struct mm_struct *mm)
>   		list_add_tail(&slot->mm_node, &ksm_scan.mm_slot->slot.mm_node);
>   	spin_unlock(&ksm_mmlist_lock);
>   
> +	atomic_long_inc(&ksm_mm_slots);
>   	set_bit(MMF_VM_MERGEABLE, &mm->flags);
>   	mmgrab(mm);
>   
> @@ -3042,6 +3048,7 @@ void __ksm_exit(struct mm_struct *mm)
>   
>   	if (easy_to_free) {
>   		mm_slot_free(mm_slot_cache, mm_slot);
> +		atomic_long_dec(&ksm_mm_slots);
>   		clear_bit(MMF_VM_MERGE_ANY, &mm->flags);
>   		clear_bit(MMF_VM_MERGEABLE, &mm->flags);
>   		mmdrop(mm);
> @@ -3374,7 +3381,8 @@ static void wait_while_offlining(void)
>   long ksm_process_profit(struct mm_struct *mm)
>   {
>   	return (long)(mm->ksm_merging_pages + mm_ksm_zero_pages(mm)) * PAGE_SIZE -
> -		mm->ksm_rmap_items * sizeof(struct ksm_rmap_item);
> +		mm->ksm_rmap_items * sizeof(struct ksm_rmap_item) -
> +		(test_bit(MMF_VM_MERGEABLE, &mm->flags) ? sizeof(struct ksm_mm_slot) : 0);
>   }
>   #endif /* CONFIG_PROC_FS */
>   
> @@ -3672,7 +3680,8 @@ static ssize_t general_profit_show(struct kobject *kobj,
>   	long general_profit;
>   
>   	general_profit = (ksm_pages_sharing + atomic_long_read(&ksm_zero_pages)) * PAGE_SIZE -
> -				ksm_rmap_items * sizeof(struct ksm_rmap_item);
> +				ksm_rmap_items * sizeof(struct ksm_rmap_item) -
> +				atomic_long_read(&ksm_mm_slots) * sizeof(struct ksm_mm_slot);
>   
>   	return sysfs_emit(buf, "%ld\n", general_profit);
>   }

-- 
Cheers,

David / dhildenb