Re: [PATCH 3/4] mm/page_alloc: Further fix __alloc_pages_bulk() return value

[Chuck Lever III]

> On Jul 13, 2021, at 11:20 AM, Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx> wrote:
> 
> From: Chuck Lever <chuck.lever@xxxxxxxxxx>
> 
> The author of commit b3b64ebd3822 ("mm/page_alloc: do bulk array
> bounds check after checking populated elements") was possibly
> confused by the mixture of return values throughout the function.
> 
> The API contract is clear that the function "Returns the number of
> pages on the list or array." It does not list zero as a unique
> return value with a special meaning. Therefore zero is a plausible
> return value only if @nr_pages is zero or less.
> 
> Clean up the return logic to make it clear that the returned value
> is always the total number of pages in the array/list, not the
> number of pages that were allocated during this call.
> 
> The only change in behavior with this patch is the value returned
> if prepare_alloc_pages() fails. To match the API contract, the
> number of pages currently in the array/list is returned in this
> case.
> 
> The call site in __page_pool_alloc_pages_slow() also seems to be
> confused on this matter. It should be attended to by someone who
> is familiar with that code.
> 
> [mel@xxxxxxxxxxxxxxxxxxx: Return nr_populated if 0 pages are requested]
> Signed-off-by: Chuck Lever <chuck.lever@xxxxxxxxxx>
> Acked-by: Jesper Dangaard Brouer <brouer@xxxxxxxxxx>
> Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
> ---
> mm/page_alloc.c | 14 ++++++++------
> 1 file changed, 8 insertions(+), 6 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 803414ce9264..c66f1e6204c2 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -5221,9 +5221,6 @@ unsigned long __alloc_pages_bulk(gfp_t gfp, int preferred_nid,
> 	unsigned int alloc_flags = ALLOC_WMARK_LOW;
> 	int nr_populated = 0, nr_account = 0;
> 
> -	if (unlikely(nr_pages <= 0))
> -		return 0;
> -
> 	/*
> 	 * Skip populated array elements to determine if any pages need
> 	 * to be allocated before disabling IRQs.
> @@ -5231,9 +5228,13 @@ unsigned long __alloc_pages_bulk(gfp_t gfp, int preferred_nid,
> 	while (page_array && nr_populated < nr_pages && page_array[nr_populated])
> 		nr_populated++;
> 
> +	/* No pages requested? */
> +	if (unlikely(nr_pages <= 0))
> +		goto out;
> +
> 	/* Already populated array? */
> 	if (unlikely(page_array && nr_pages - nr_populated == 0))
> -		return nr_populated;
> +		goto out;
> 
> 	/* Use the single page allocator for one page. */
> 	if (nr_pages - nr_populated == 1)
> @@ -5255,7 +5256,7 @@ unsigned long __alloc_pages_bulk(gfp_t gfp, int preferred_nid,
> 	gfp &= gfp_allowed_mask;
> 	alloc_gfp = gfp;
> 	if (!prepare_alloc_pages(gfp, 0, preferred_nid, nodemask, &ac, &alloc_gfp, &alloc_flags))
> -		return nr_populated;
> +		goto out;

:thumbsup:  Thanks!


> 	gfp = alloc_gfp;
> 
> 	/* Find an allowed local zone that meets the low watermark. */
> @@ -5323,6 +5324,7 @@ unsigned long __alloc_pages_bulk(gfp_t gfp, int preferred_nid,
> 	__count_zid_vm_events(PGALLOC, zone_idx(zone), nr_account);
> 	zone_statistics(ac.preferred_zoneref->zone, zone, nr_account);
> 
> +out:
> 	return nr_populated;
> 
> failed_irq:
> @@ -5338,7 +5340,7 @@ unsigned long __alloc_pages_bulk(gfp_t gfp, int preferred_nid,
> 		nr_populated++;
> 	}
> 
> -	return nr_populated;
> +	goto out;
> }
> EXPORT_SYMBOL_GPL(__alloc_pages_bulk);
> 
> -- 
> 2.26.2
> 

--
Chuck Lever