Re: [PATCH 2/2] mm: support poison recovery from copy_present_page()

[Miaohe Lin <linmiaohe@huawei.com>]

On 2024/9/6 10:42, Kefeng Wang wrote:
> Similar to other poison recovery, use copy_mc_user_highpage() to
> avoid potentially kernel panic during copy page in copy_present_page()
> from fork, once copy failed due to hwpoison in source page, we need
> to break out of copy in copy_pte_range() and release prealloc folio,
> so copy_mc_user_highpage() is moved ahead before set *prealloc to NULL.
> 
> Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
> ---
>  mm/memory.c | 10 +++++++---
>  1 file changed, 7 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index d310c073a1b3..6e7b78e49d1a 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -926,8 +926,11 @@ copy_present_page(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma
>  	 * We have a prealloc page, all good!  Take it
>  	 * over and copy the page & arm it.
>  	 */
> +
> +	if (copy_mc_user_highpage(&new_folio->page, page, addr, src_vma))
> +		return -EHWPOISON;
> +
>  	*prealloc = NULL;
> -	copy_user_highpage(&new_folio->page, page, addr, src_vma);
>  	__folio_mark_uptodate(new_folio);
>  	folio_add_new_anon_rmap(new_folio, dst_vma, addr, RMAP_EXCLUSIVE);
>  	folio_add_lru_vma(new_folio, dst_vma);
> @@ -1166,8 +1169,9 @@ copy_pte_range(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma,
>  		/*
>  		 * If we need a pre-allocated page for this pte, drop the
>  		 * locks, allocate, and try again.
> +		 * If copy failed due to hwpoison in source page, break out.
>  		 */
> -		if (unlikely(ret == -EAGAIN))
> +		if (unlikely(ret == -EAGAIN || ret == -EHWPOISON))

Will it be better to put checking ret against -EHWPOISON in a new line? -EAGAIN case will enter the
loop again but -EHWPOISON case never does.

>  			break;
>  		if (unlikely(prealloc)) {
>  			/*
> @@ -1197,7 +1201,7 @@ copy_pte_range(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma,
>  			goto out;
>  		}
>  		entry.val = 0;
> -	} else if (ret == -EBUSY) {
> +	} else if (ret == -EBUSY || unlikely(ret == -EHWPOISON)) {

The caller of copy_pte_range() always set errno to -ENOMEM. So fork will failed with ENOMEM even if the real
cause is failed to copy due to hwpoison in source page. It's a pity.

Thanks.
.