linux-kernel - Re: [PATCH v10 06/10] mm/memory.c: Allow different return codes for copy_nonpresent

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Date:   Tue, 8 Jun 2021 11:19:41 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Alistair Popple <apopple@...dia.com>
Cc:     linux-mm@...ck.org, akpm@...ux-foundation.org,
        rcampbell@...dia.com, linux-doc@...r.kernel.org,
        nouveau@...ts.freedesktop.org, hughd@...gle.com,
        linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        hch@...radead.org, bskeggs@...hat.com, jgg@...dia.com,
        shakeelb@...gle.com, jhubbard@...dia.com, willy@...radead.org
Subject: Re: [PATCH v10 06/10] mm/memory.c: Allow different return codes for
 copy_nonpresent_pte()

On Mon, Jun 07, 2021 at 05:58:51PM +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@...dia.com>
> 
> ---
> 
> v10:
> 
> Use a unique error code and only check return codes for handling.
> 
> v9:
> 
> New for v9 to allow device exclusive handling to occur in
> copy_nonpresent_pte().
> ---
>  mm/memory.c | 26 ++++++++++++++++----------
>  1 file changed, 16 insertions(+), 10 deletions(-)
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index 2fb455c365c2..0982cab37ecb 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 -EIO;
>  
>  		/* 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 == -EIO) {
> +				entry = pte_to_swp_entry(*src_pte);
>  				break;
> +			}
>  			progress += 8;
>  			continue;
>  		}
> @@ -1011,20 +1013,24 @@ copy_pte_range(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma,
>  	pte_unmap_unlock(orig_dst_pte, dst_ptl);
>  	cond_resched();
>  
> -	if (entry.val) {
> +	if (ret == -EIO) {
> +		VM_WARN_ON_ONCE(!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);
> +	} else if (ret ==  -EAGAIN) {
                          ^
                          |----------------------------- one more space here

>  		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;
> +	} else if (ret) {
> +		VM_WARN_ON_ONCE(1);
>  	}
> +
> +	/* We've captured and resolved the error. Reset, try again. */

Maybe better as:

      /*
       * We've resolved all error even if there is, reset error code and try
       * again if necessary.
       */

as it also covers the no-error path.  But I guess not a big deal..

Reviewed-by: Peter Xu <peterx@...hat.com>

Thanks,

> +	ret = 0;
> +
>  	if (addr != end)
>  		goto again;
>  out:
> -- 
> 2.20.1
> 

-- 
Peter Xu