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

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Message-Id: <20240620134752.9a63c12403282c0c7e728764@linux-foundation.org>
Date: Thu, 20 Jun 2024 13:47:52 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: Sung-hun Kim <sfoon.kim@...sung.com>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org,
 sungguk.na@...sung.com, sw0312.kim@...sung.com, sebuns@...il.com, Stefan
 Roesch <shr@...kernel.io>
Subject: Re: [PATCH v2] mm: ksm: Consider the number of ksm_mm_slot in the
 general_profit calculation

On Thu, 20 Jun 2024 13:39:14 +0900 Sung-hun Kim <sfoon.kim@...sung.com> 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.
> 

By how much does the improve the accuracy?  In other words, how much
difference does this make?

> @@ -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);

This assumes perfect slab packing, no?   Should it use ksize()?