Re: [PATCH v3 08/14] mm: copy_pte_range() use pte_offset_map_rw_nolock()

[Muchun Song <muchun.song@linux.dev>]

On 2024/9/4 16:40, Qi Zheng wrote:
> In copy_pte_range(), we may modify the src_pte entry after holding the
> src_ptl, so convert it to using pte_offset_map_rw_nolock(). Since we may
> free the PTE page in retract_page_tables() without holding the read lock
> of mmap_lock, so we still need to get pmdval and do pmd_same() check after
> the ptl is held.

See commit 3db82b9374ca92, copy_pte_range and retract_page_tables
are using vma->anon_vma to be exclusive.

retract_page_tables()                    copy_page_range()
     vma_interval_tree_foreach()              if (!vma_needs_copy())
         if (READ_ONCE(vma->anon_vma))            return 0;
             continue;                        copy_pte_range()

So I think mmap write lock here is also used for keeping ->anon_vma stable.
And we do not need pmd_same().

Muchun,
Thanks.
>
> Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
> ---
> Hi Muchun, since the code has changed, I dropped your Reviewed-by tag here.
>
>   mm/memory.c | 18 +++++++++++++++++-
>   1 file changed, 17 insertions(+), 1 deletion(-)
>
> diff --git a/mm/memory.c b/mm/memory.c
> index 06674f94b7a4e..47974cc4bd7f2 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -1082,6 +1082,7 @@ copy_pte_range(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma,
>   	struct mm_struct *src_mm = src_vma->vm_mm;
>   	pte_t *orig_src_pte, *orig_dst_pte;
>   	pte_t *src_pte, *dst_pte;
> +	pmd_t pmdval;
>   	pte_t ptent;
>   	spinlock_t *src_ptl, *dst_ptl;
>   	int progress, max_nr, ret = 0;
> @@ -1107,13 +1108,28 @@ copy_pte_range(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma,
>   		ret = -ENOMEM;
>   		goto out;
>   	}
> -	src_pte = pte_offset_map_nolock(src_mm, src_pmd, addr, &src_ptl);
> +
> +	/*
> +	 * Since we may free the PTE page in retract_page_tables() without
> +	 * holding the read lock of mmap_lock, so we still need to do a
> +	 * pmd_same() check after holding the PTL.
> +	 */
> +	src_pte = pte_offset_map_rw_nolock(src_mm, src_pmd, addr, &pmdval,
> +					   &src_ptl);
>   	if (!src_pte) {
>   		pte_unmap_unlock(dst_pte, dst_ptl);
>   		/* ret == 0 */
>   		goto out;
>   	}
>   	spin_lock_nested(src_ptl, SINGLE_DEPTH_NESTING);
> +
> +	if (unlikely(!pmd_same(pmdval, pmdp_get_lockless(src_pmd)))) {
> +		pte_unmap_unlock(src_pte, src_ptl);
> +		pte_unmap_unlock(dst_pte, dst_ptl);
> +		/* ret == 0 */
> +		goto out;
> +	}
> +
>   	orig_src_pte = src_pte;
>   	orig_dst_pte = dst_pte;
>   	arch_enter_lazy_mmu_mode();