linux-kernel - Re: [PATCH] mm/rmap: remove unnecessary page_table

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Message-ID: <abe2a739-d50c-42b4-8701-ae89435da23e@bytedance.com>
Date: Mon, 22 Apr 2024 19:24:58 +0800
From: Qi Zheng <zhengqi.arch@...edance.com>
To: Yajun Deng <yajun.deng@...ux.dev>
Cc: akpm@...ux-foundation.org, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/rmap: remove unnecessary page_table_lock



On 2024/4/22 18:52, Yajun Deng wrote:
> page_table_lock is a lock that for page table, we won't change page
> table in __anon_vma_prepare(). As we can see, it works well in
> anon_vma_clone(). They do the same operation.
> 
> Remove unnecessary page_table_lock in __anon_vma_prepare().

IIUC, the page_table_lock here is not to protect page table, but as
mentioned in the comments, to prevent concurrent modification by
multiple threads.

> 
> Signed-off-by: Yajun Deng <yajun.deng@...ux.dev>
> ---
>   mm/rmap.c | 4 ----
>   1 file changed, 4 deletions(-)
> 
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 2608c40dffad..e894640a9327 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -187,7 +187,6 @@ static void anon_vma_chain_link(struct vm_area_struct *vma,
>    */
>   int __anon_vma_prepare(struct vm_area_struct *vma)
>   {
> -	struct mm_struct *mm = vma->vm_mm;
>   	struct anon_vma *anon_vma, *allocated;
>   	struct anon_vma_chain *avc;
>   
> @@ -208,8 +207,6 @@ int __anon_vma_prepare(struct vm_area_struct *vma)
>   	}
>   
>   	anon_vma_lock_write(anon_vma);
> -	/* page_table_lock to protect against threads */
> -	spin_lock(&mm->page_table_lock);
>   	if (likely(!vma->anon_vma)) {
>   		vma->anon_vma = anon_vma;
>   		anon_vma_chain_link(vma, avc, anon_vma);
> @@ -217,7 +214,6 @@ int __anon_vma_prepare(struct vm_area_struct *vma)
>   		allocated = NULL;
>   		avc = NULL;
>   	}
> -	spin_unlock(&mm->page_table_lock);
>   	anon_vma_unlock_write(anon_vma);
>   
>   	if (unlikely(allocated))