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@xxxxxxxxxx>
>
> ---
>
> 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
