Re: [PATCH v6 3/6] ksm: count all zero pages placed by KSM

[Claudio Imbrenda]

On Fri, 10 Feb 2023 09:18:52 +0800 (CST)
<yang.yang29@xxxxxxxxxx> wrote:

> From: xu xin <xu.xin16@xxxxxxxxxx>
> 
> As pages_sharing and pages_shared don't include the number of zero pages
> merged by KSM, we cannot know how many pages are zero pages placed by KSM
> when enabling use_zero_pages, which leads to KSM not being transparent with
> all actual merged pages by KSM. In the early days of use_zero_pages,
> zero-pages was unable to get unshared by the ways like MADV_UNMERGEABLE so
> it's hard to count how many times one of those zeropages was then unmerged.
> 
> But now, unsharing KSM-placed zero page accurately has been achieved, so we
> can easily count both how many times a page full of zeroes was merged with
> zero-page and how many times one of those pages was then unmerged. and so,
> it helps to estimate memory demands when each and every shared page could
> get unshared.
> 
> So we add zero_pages_sharing under /sys/kernel/mm/ksm/ to show the number
> of all zero pages placed by KSM.
> 
> Signed-off-by: xu xin <xu.xin16@xxxxxxxxxx>
> Cc: Claudio Imbrenda <imbrenda@xxxxxxxxxxxxx>
> Cc: David Hildenbrand <david@xxxxxxxxxx>
> Cc: Xuexin Jiang <jiang.xuexin@xxxxxxxxxx>
> Reviewed-by: Xiaokai Ran <ran.xiaokai@xxxxxxxxxx>
> Reviewed-by: Yang Yang <yang.yang29@xxxxxxxxxx>

Reviewed-by: Claudio Imbrenda <imbrenda@xxxxxxxxxxxxx>

> 
>  v4->v5:
>  fix warning  mm/ksm.c:3238:9: warning: no previous prototype for
>  'zero_pages_sharing_show' [-Wmissing-prototypes].
> ---
>  mm/ksm.c | 19 +++++++++++++++++--
>  1 file changed, 17 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/ksm.c b/mm/ksm.c
> index ab04b44679c8..1fa668e1fe82 100644
> --- a/mm/ksm.c
> +++ b/mm/ksm.c
> @@ -276,6 +276,9 @@ static unsigned int zero_checksum __read_mostly;
>  /* Whether to merge empty (zeroed) pages with actual zero pages */
>  static bool ksm_use_zero_pages __read_mostly;
> 
> +/* The number of zero pages placed by KSM use_zero_pages */
> +static unsigned long ksm_zero_pages_sharing;
> +
>  #ifdef CONFIG_NUMA
>  /* Zeroed when merging across nodes is not allowed */
>  static unsigned int ksm_merge_across_nodes = 1;
> @@ -789,8 +792,10 @@ static struct page *get_ksm_page(struct ksm_stable_node *stable_node,
>   */
>  static inline void clean_rmap_item_zero_flag(struct ksm_rmap_item *rmap_item)
>  {
> -	if (rmap_item->address & ZERO_PAGE_FLAG)
> +	if (rmap_item->address & ZERO_PAGE_FLAG) {
> +		ksm_zero_pages_sharing--;
>  		rmap_item->address &= PAGE_MASK;
> +	}
>  }
> 
>  /* Only called when rmap_item is going to be freed */
> @@ -2109,8 +2114,10 @@ static int try_to_merge_with_kernel_zero_page(struct ksm_rmap_item *rmap_item,
>  		if (vma) {
>  			err = try_to_merge_one_page(vma, page,
>  						ZERO_PAGE(rmap_item->address));
> -			if (!err)
> +			if (!err) {
>  				rmap_item->address |= ZERO_PAGE_FLAG;
> +				ksm_zero_pages_sharing++;
> +			}
>  		} else {
>  			/* If the vma is out of date, we do not need to continue. */
>  			err = 0;
> @@ -3230,6 +3237,13 @@ static ssize_t pages_volatile_show(struct kobject *kobj,
>  }
>  KSM_ATTR_RO(pages_volatile);
> 
> +static ssize_t zero_pages_sharing_show(struct kobject *kobj,
> +				struct kobj_attribute *attr, char *buf)
> +{
> +	return sysfs_emit(buf, "%ld\n", ksm_zero_pages_sharing);
> +}
> +KSM_ATTR_RO(zero_pages_sharing);
> +
>  static ssize_t stable_node_dups_show(struct kobject *kobj,
>  				     struct kobj_attribute *attr, char *buf)
>  {
> @@ -3285,6 +3299,7 @@ static struct attribute *ksm_attrs[] = {
>  	&pages_sharing_attr.attr,
>  	&pages_unshared_attr.attr,
>  	&pages_volatile_attr.attr,
> +	&zero_pages_sharing_attr.attr,
>  	&full_scans_attr.attr,
>  #ifdef CONFIG_NUMA
>  	&merge_across_nodes_attr.attr,