Re: [PATCH 2/2] mm/slub: don't use reserved highatomic pageblock for optimistic try

[Vlastimil Babka]

On 08/28/2017 03:11 AM, js1304@xxxxxxxxx wrote:
> From: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> 
> High-order atomic allocation is difficult to succeed since we cannot
> reclaim anything in this context. So, we reserves the pageblock for
> this kind of request.
> 
> In slub, we try to allocate higher-order page more than it actually
> needs in order to get the best performance. If this optimistic try is
> used with GFP_ATOMIC, alloc_flags will be set as ALLOC_HARDER and
> the pageblock reserved for high-order atomic allocation would be used.
> Moreover, this request would reserve the MIGRATE_HIGHATOMIC pageblock
> ,if succeed, to prepare further request. It would not be good to use
> MIGRATE_HIGHATOMIC pageblock in terms of fragmentation management
> since it unconditionally set a migratetype to request's migratetype
> when unreserving the pageblock without considering the migratetype of
> used pages in the pageblock.
> 
> This is not what we don't intend so fix it by unconditionally setting
> __GFP_NOMEMALLOC in order to not set ALLOC_HARDER.

I wonder if it would be more robust to strip GFP_ATOMIC from alloc_gfp.
E.g. __GFP_NOMEMALLOC does seem to prevent ALLOC_HARDER, but not
ALLOC_HIGH. Or maybe we should adjust __GFP_NOMEMALLOC implementation
and document it more thoroughly? CC Michal Hocko

Also, were these 2 patches done via code inspection or you noticed
suboptimal behavior which got fixed? Thanks.

> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> ---
>  mm/slub.c | 6 ++----
>  1 file changed, 2 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index e1e442c..fd8dd89 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -1579,10 +1579,8 @@ static struct page *allocate_slab(struct kmem_cache *s, gfp_t flags, int node)
>  	 */
>  	alloc_gfp = (flags | __GFP_NOWARN | __GFP_NORETRY) & ~__GFP_NOFAIL;
>  	if (oo_order(oo) > oo_order(s->min)) {
> -		if (alloc_gfp & __GFP_DIRECT_RECLAIM) {
> -			alloc_gfp |= __GFP_NOMEMALLOC;
> -			alloc_gfp &= ~__GFP_DIRECT_RECLAIM;
> -		}
> +		alloc_gfp |= __GFP_NOMEMALLOC;
> +		alloc_gfp &= ~__GFP_DIRECT_RECLAIM;
>  	}
>  
>  	page = alloc_slab_page(s, alloc_gfp, node, oo);
>