Re: [PATCH 3/4] mm: slub: Do not take expensive steps for SLUBs speculative high-order allocations

[David Rientjes <rientjes@google.com>]

On Fri, 13 May 2011, Mel Gorman wrote:

> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 9f8a97b..057f1e2 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1972,6 +1972,7 @@ gfp_to_alloc_flags(gfp_t gfp_mask)
>  {
>  	int alloc_flags = ALLOC_WMARK_MIN | ALLOC_CPUSET;
>  	const gfp_t wait = gfp_mask & __GFP_WAIT;
> +	const gfp_t can_wake_kswapd = !(gfp_mask & __GFP_NO_KSWAPD);
>  
>  	/* __GFP_HIGH is assumed to be the same as ALLOC_HIGH to save a branch. */
>  	BUILD_BUG_ON(__GFP_HIGH != (__force gfp_t) ALLOC_HIGH);
> @@ -1984,7 +1985,7 @@ gfp_to_alloc_flags(gfp_t gfp_mask)
>  	 */
>  	alloc_flags |= (__force int) (gfp_mask & __GFP_HIGH);
>  
> -	if (!wait) {
> +	if (!wait && can_wake_kswapd) {
>  		/*
>  		 * Not worth trying to allocate harder for
>  		 * __GFP_NOMEMALLOC even if it can't schedule.
> diff --git a/mm/slub.c b/mm/slub.c
> index 98c358d..c5797ab 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -1170,7 +1170,8 @@ static struct page *allocate_slab(struct kmem_cache *s, gfp_t flags, int node)
>  	 * Let the initial higher-order allocation fail under memory pressure
>  	 * so we fall-back to the minimum order allocation.
>  	 */
> -	alloc_gfp = (flags | __GFP_NOWARN | __GFP_NORETRY | __GFP_NO_KSWAPD) & ~__GFP_NOFAIL;
> +	alloc_gfp = (flags | __GFP_NOWARN | __GFP_NO_KSWAPD) &
> +			~(__GFP_NOFAIL | __GFP_WAIT | __GFP_REPEAT);
>  
>  	page = alloc_slab_page(alloc_gfp, node, oo);
>  	if (unlikely(!page)) {

It's unnecessary to clear __GFP_REPEAT, these !__GFP_NOFAIL allocations 
will immediately fail.

alloc_gfp would probably benefit from having a comment about why 
__GFP_WAIT should be masked off here: that we don't want to do compaction 
or direct reclaim or retry the allocation more than once (so both 
__GFP_NORETRY and __GFP_REPEAT are no-ops).

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