linux-kernel - Re: [PATCH 2/4] mm/ksm: fix ksm_zero

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Message-ID: <7cac6762-4486-4c42-885d-dd5715eb6ba4@redhat.com>
Date: Wed, 8 May 2024 14:36:26 +0200
From: David Hildenbrand <david@...hat.com>
To: Chengming Zhou <chengming.zhou@...ux.dev>,
 Andrew Morton <akpm@...ux-foundation.org>, Stefan Roesch <shr@...kernel.io>,
 xu xin <xu.xin16@....com.cn>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org,
 zhouchengming@...edance.com
Subject: Re: [PATCH 2/4] mm/ksm: fix ksm_zero_pages accounting

On 08.05.24 11:55, Chengming Zhou wrote:
> We normally ksm_zero_pages++ in ksmd when page is merged with zero page,
> but ksm_zero_pages-- is done from page tables side, which can't protected
> by the ksmd mutex.
> 
> So we can read very exceptional value of ksm_zero_pages in rare cases,
> such as -1, which is very confusing to users.
> 
> Fix it by changing to use atomic_long_t, and the same case with the
> mm->ksm_zero_pages.
> 
> Signed-off-by: Chengming Zhou <chengming.zhou@...ux.dev>
> ---
>   fs/proc/base.c           |  2 +-
>   include/linux/ksm.h      | 22 +++++++++++++++++++---
>   include/linux/mm_types.h |  2 +-
>   mm/ksm.c                 | 11 +++++------
>   4 files changed, 26 insertions(+), 11 deletions(-)
> 
> diff --git a/fs/proc/base.c b/fs/proc/base.c
> index 18550c071d71..72a1acd03675 100644
> --- a/fs/proc/base.c
> +++ b/fs/proc/base.c
> @@ -3214,7 +3214,7 @@ static int proc_pid_ksm_stat(struct seq_file *m, struct pid_namespace *ns,
>   	mm = get_task_mm(task);
>   	if (mm) {
>   		seq_printf(m, "ksm_rmap_items %lu\n", mm->ksm_rmap_items);
> -		seq_printf(m, "ksm_zero_pages %lu\n", mm->ksm_zero_pages);
> +		seq_printf(m, "ksm_zero_pages %ld\n", mm_ksm_zero_pages(mm));
>   		seq_printf(m, "ksm_merging_pages %lu\n", mm->ksm_merging_pages);
>   		seq_printf(m, "ksm_process_profit %ld\n", ksm_process_profit(mm));
>   		mmput(mm);
> diff --git a/include/linux/ksm.h b/include/linux/ksm.h
> index 52c63a9c5a9c..bfc2cf756b0d 100644
> --- a/include/linux/ksm.h
> +++ b/include/linux/ksm.h
> @@ -33,16 +33,32 @@ void __ksm_exit(struct mm_struct *mm);
>    */
>   #define is_ksm_zero_pte(pte)	(is_zero_pfn(pte_pfn(pte)) && pte_dirty(pte))
>   
> -extern unsigned long ksm_zero_pages;
> +extern atomic_long_t ksm_zero_pages;
> +
> +static inline void ksm_map_zero_page(struct mm_struct *mm)
> +{
> +	atomic_long_inc(&ksm_zero_pages);
> +	atomic_long_inc(&mm->ksm_zero_pages);
> +}
>   
>   static inline void ksm_might_unmap_zero_page(struct mm_struct *mm, pte_t pte)
>   {
>   	if (is_ksm_zero_pte(pte)) {
> -		ksm_zero_pages--;
> -		mm->ksm_zero_pages--;
> +		atomic_long_dec(&ksm_zero_pages);
> +		atomic_long_dec(&mm->ksm_zero_pages);
>   	}
>   }
>   
> +static inline long get_ksm_zero_pages(void)
> +{
> +	return atomic_long_read(&ksm_zero_pages);
> +}

I suggest inlining that one. The naming of the function also is a bit 
inconsistent staring at the others.

> +
> +static inline long mm_ksm_zero_pages(struct mm_struct *mm)
> +{
> +	return atomic_long_read(&mm->ksm_zero_pages);
> +}
> +

Apart from that LGTM

Acked-by: David Hildenbrand <david@...hat.com>

-- 
Cheers,

David / dhildenb