Re: [PATCH] mm/rmap: remove unnecessary page_table_lock

Qi Zheng
2024-04-22T11:25:04Z

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@linux.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))