Re: [PATCH v9 06/10] mm/memory.c: Allow different return codes for copy_nonpresent_pte()

[Peter Xu <peterx@redhat.com>]

On Mon, May 24, 2021 at 11:27:21PM +1000, Alistair Popple wrote:
> Currently if copy_nonpresent_pte() returns a non-zero value it is
> assumed to be a swap entry which requires further processing outside the
> loop in copy_pte_range() after dropping locks. This prevents other
> values being returned to signal conditions such as failure which a
> subsequent change requires.
> 
> Instead make copy_nonpresent_pte() return an error code if further
> processing is required and read the value for the swap entry in the main
> loop under the ptl.
> 
> Signed-off-by: Alistair Popple <apopple@nvidia.com>
> 
> ---
> 
> v9:
> 
> New for v9 to allow device exclusive handling to occur in
> copy_nonpresent_pte().
> ---
>  mm/memory.c | 12 +++++++-----
>  1 file changed, 7 insertions(+), 5 deletions(-)
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index 2fb455c365c2..e061cfa18c11 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -718,7 +718,7 @@ copy_nonpresent_pte(struct mm_struct *dst_mm, struct mm_struct *src_mm,
>  
>  	if (likely(!non_swap_entry(entry))) {
>  		if (swap_duplicate(entry) < 0)
> -			return entry.val;
> +			return -EAGAIN;
>  
>  		/* make sure dst_mm is on swapoff's mmlist. */
>  		if (unlikely(list_empty(&dst_mm->mmlist))) {
> @@ -974,11 +974,13 @@ copy_pte_range(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma,
>  			continue;
>  		}
>  		if (unlikely(!pte_present(*src_pte))) {
> -			entry.val = copy_nonpresent_pte(dst_mm, src_mm,
> -							dst_pte, src_pte,
> -							src_vma, addr, rss);
> -			if (entry.val)
> +			ret = copy_nonpresent_pte(dst_mm, src_mm,
> +						dst_pte, src_pte,
> +						src_vma, addr, rss);
> +			if (ret == -EAGAIN) {
> +				entry = pte_to_swp_entry(*src_pte);
>  				break;
> +			}
>  			progress += 8;
>  			continue;
>  		}

Note that -EAGAIN was previously used by copy_present_page() for early cow
use.  Here later although we check entry.val first:

	if (entry.val) {
		if (add_swap_count_continuation(entry, GFP_KERNEL) < 0) {
			ret = -ENOMEM;
			goto out;
		}
		entry.val = 0;
	} else if (ret) {
		WARN_ON_ONCE(ret != -EAGAIN);
		prealloc = page_copy_prealloc(src_mm, src_vma, addr);
		if (!prealloc)
			return -ENOMEM;
		/* We've captured and resolved the error. Reset, try again. */
		ret = 0;
	}

We didn't reset "ret" in entry.val case (maybe we should?). Then in the next
round of "goto again" if "ret" is unluckily untouched, it could reach the 2nd
if check, and I think it could cause an unexpected page_copy_prealloc().

-- 
Peter Xu