Re: [PATCH v2 for v5.9] mm/page_alloc: handle a missing case for memalloc_nocma_{save/restore} APIs

[Michal Hocko]

On Mon 28-09-20 17:50:46, Joonsoo Kim wrote:
> From: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> 
> memalloc_nocma_{save/restore} APIs can be used to skip page allocation
> on CMA area, but, there is a missing case and the page on CMA area could
> be allocated even if APIs are used. This patch handles this case to fix
> the potential issue.
> 
> Missing case is an allocation from the pcplist. MIGRATE_MOVABLE pcplist
> could have the pages on CMA area so we need to skip it if ALLOC_CMA isn't
> specified.
> 
> Fixes: 8510e69c8efe (mm/page_alloc: fix memalloc_nocma_{save/restore} APIs)
> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> ---
>  mm/page_alloc.c | 13 ++++++++++---
>  1 file changed, 10 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index fab5e97..104d2e1 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -3367,9 +3367,16 @@ struct page *rmqueue(struct zone *preferred_zone,
>  	struct page *page;
>  
>  	if (likely(order == 0)) {
> -		page = rmqueue_pcplist(preferred_zone, zone, gfp_flags,
> +		/*
> +		 * MIGRATE_MOVABLE pcplist could have the pages on CMA area and
> +		 * we need to skip it when CMA area isn't allowed.
> +		 */
> +		if (!IS_ENABLED(CONFIG_CMA) || alloc_flags & ALLOC_CMA ||
> +				migratetype != MIGRATE_MOVABLE) {
> +			page = rmqueue_pcplist(preferred_zone, zone, gfp_flags,
>  					migratetype, alloc_flags);
> -		goto out;
> +			goto out;
> +		}
>  	}

This approach looks definitely better than the previous version.

>  
>  	/*
> @@ -3381,7 +3388,7 @@ struct page *rmqueue(struct zone *preferred_zone,
>  
>  	do {
>  		page = NULL;
> -		if (alloc_flags & ALLOC_HARDER) {
> +		if (order > 0 && alloc_flags & ALLOC_HARDER) {
>  			page = __rmqueue_smallest(zone, order, MIGRATE_HIGHATOMIC);
>  			if (page)
>  				trace_mm_page_alloc_zone_locked(page, order, migratetype);

But this condition is not clear to me. __rmqueue_smallest doesn't access
pcp lists. Maybe I have missed the point in the original discussion but
this deserves a comment at least.

> -- 
> 2.7.4

-- 
Michal Hocko
SUSE Labs