Re: [PATCH v2 07/19] mm/fork: Accept huge pfnmap entries

[Yan Zhao <yan.y.zhao@intel.com>]

On Mon, Aug 26, 2024 at 04:43:41PM -0400, Peter Xu wrote:
> Teach the fork code to properly copy pfnmaps for pmd/pud levels.  Pud is
> much easier, the write bit needs to be persisted though for writable and
> shared pud mappings like PFNMAP ones, otherwise a follow up write in either
> parent or child process will trigger a write fault.
> 
> Do the same for pmd level.
> 
> Signed-off-by: Peter Xu <peterx@redhat.com>
> ---
>  mm/huge_memory.c | 29 ++++++++++++++++++++++++++---
>  1 file changed, 26 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index e2c314f631f3..15418ffdd377 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -1559,6 +1559,24 @@ int copy_huge_pmd(struct mm_struct *dst_mm, struct mm_struct *src_mm,
>  	pgtable_t pgtable = NULL;
>  	int ret = -ENOMEM;
>  
> +	pmd = pmdp_get_lockless(src_pmd);
> +	if (unlikely(pmd_special(pmd))) {
> +		dst_ptl = pmd_lock(dst_mm, dst_pmd);
> +		src_ptl = pmd_lockptr(src_mm, src_pmd);
> +		spin_lock_nested(src_ptl, SINGLE_DEPTH_NESTING);
> +		/*
> +		 * No need to recheck the pmd, it can't change with write
> +		 * mmap lock held here.
> +		 *
> +		 * Meanwhile, making sure it's not a CoW VMA with writable
> +		 * mapping, otherwise it means either the anon page wrongly
> +		 * applied special bit, or we made the PRIVATE mapping be
> +		 * able to wrongly write to the backend MMIO.
> +		 */
> +		VM_WARN_ON_ONCE(is_cow_mapping(src_vma->vm_flags) && pmd_write(pmd));
> +		goto set_pmd;
> +	}
> +
>  	/* Skip if can be re-fill on fault */
>  	if (!vma_is_anonymous(dst_vma))
>  		return 0;
> @@ -1640,7 +1658,9 @@ int copy_huge_pmd(struct mm_struct *dst_mm, struct mm_struct *src_mm,
>  	pmdp_set_wrprotect(src_mm, addr, src_pmd);
>  	if (!userfaultfd_wp(dst_vma))
>  		pmd = pmd_clear_uffd_wp(pmd);
> -	pmd = pmd_mkold(pmd_wrprotect(pmd));
> +	pmd = pmd_wrprotect(pmd);
> +set_pmd:
> +	pmd = pmd_mkold(pmd);
>  	set_pmd_at(dst_mm, addr, dst_pmd, pmd);
>  
>  	ret = 0;
> @@ -1686,8 +1706,11 @@ int copy_huge_pud(struct mm_struct *dst_mm, struct mm_struct *src_mm,
>  	 * TODO: once we support anonymous pages, use
>  	 * folio_try_dup_anon_rmap_*() and split if duplicating fails.
>  	 */
> -	pudp_set_wrprotect(src_mm, addr, src_pud);
> -	pud = pud_mkold(pud_wrprotect(pud));
> +	if (is_cow_mapping(vma->vm_flags) && pud_write(pud)) {
> +		pudp_set_wrprotect(src_mm, addr, src_pud);
> +		pud = pud_wrprotect(pud);
> +	}
Do we need the logic to clear dirty bit in the child as that in
__copy_present_ptes()?  (and also for the pmd's case).

e.g.
if (vma->vm_flags & VM_SHARED)
	pud = pud_mkclean(pud);

> +	pud = pud_mkold(pud);
>  	set_pud_at(dst_mm, addr, dst_pud, pud);
>  
>  	ret = 0;
> -- 
> 2.45.0
>