Re: [PATCH v3 09/14] mm: mremap: move_ptes() use pte_offset_map_rw_nolock()

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

On 2024/9/4 16:40, Qi Zheng wrote:
> In move_ptes(), we may modify the new_pte after acquiring the new_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 do a pmd_same() check after holding the PTL.

retract_page_tables() and move_ptes() are synchronized with
i_mmap_lock, right?

Muchun,
Thanks.

>
> Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
> ---
>   mm/mremap.c | 20 ++++++++++++++++++--
>   1 file changed, 18 insertions(+), 2 deletions(-)
>
> diff --git a/mm/mremap.c b/mm/mremap.c
> index 24712f8dbb6b5..16e54151395ad 100644
> --- a/mm/mremap.c
> +++ b/mm/mremap.c
> @@ -143,6 +143,7 @@ static int move_ptes(struct vm_area_struct *vma, pmd_t *old_pmd,
>   	spinlock_t *old_ptl, *new_ptl;
>   	bool force_flush = false;
>   	unsigned long len = old_end - old_addr;
> +	pmd_t pmdval;
>   	int err = 0;
>   
>   	/*
> @@ -175,14 +176,29 @@ static int move_ptes(struct vm_area_struct *vma, pmd_t *old_pmd,
>   		err = -EAGAIN;
>   		goto out;
>   	}
> -	new_pte = pte_offset_map_nolock(mm, new_pmd, new_addr, &new_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.
> +	 */
> +	new_pte = pte_offset_map_rw_nolock(mm, new_pmd, new_addr, &pmdval,
> +					   &new_ptl);
>   	if (!new_pte) {
>   		pte_unmap_unlock(old_pte, old_ptl);
>   		err = -EAGAIN;
>   		goto out;
>   	}
> -	if (new_ptl != old_ptl)
> +	if (new_ptl != old_ptl) {
>   		spin_lock_nested(new_ptl, SINGLE_DEPTH_NESTING);
> +
> +		if (unlikely(!pmd_same(pmdval, pmdp_get_lockless(new_pmd)))) {
> +			pte_unmap_unlock(new_pte, new_ptl);
> +			pte_unmap_unlock(old_pte, old_ptl);
> +			err = -EAGAIN;
> +			goto out;
> +		}
> +	}
> +
>   	flush_tlb_batched_pending(vma->vm_mm);
>   	arch_enter_lazy_mmu_mode();
>