Re: [PATCH] mm: clarify why we want kmalloc before falling backto vmallock

[Vlastimil Babka]

On 05/17/2017 10:09 AM, Michal Hocko wrote:
> From: Michal Hocko <mhocko@xxxxxxxx>
> 
> While converting drm_[cm]alloc* helpers to kvmalloc* variants Chris
> Wilson has wondered why we want to try kmalloc before vmalloc fallback
> even for larger allocations requests. Let's clarify that one larger
> physically contiguous block is less likely to fragment memory than many
> scattered pages which can prevent more large blocks from being created.
> 
> Suggested-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
> Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>

Acked-by: Vlastimil Babka <vbabka@xxxxxxx>

> ---
>  mm/util.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/util.c b/mm/util.c
> index 464df3489903..87499f8119f2 100644
> --- a/mm/util.c
> +++ b/mm/util.c
> @@ -357,7 +357,10 @@ void *kvmalloc_node(size_t size, gfp_t flags, int node)
>  	WARN_ON_ONCE((flags & GFP_KERNEL) != GFP_KERNEL);
>  
>  	/*
> -	 * Make sure that larger requests are not too disruptive - no OOM
> +	 * We want to attempt a large physically contiguous block first because
> +	 * it is less likely to fragment multiple larger blocks and therefore
> +	 * contribute to a long term fragmentation less than vmalloc fallback.
> +	 * However make sure that larger requests are not too disruptive - no OOM
>  	 * killer and no allocation failure warnings as we have a fallback
>  	 */
>  	if (size > PAGE_SIZE) {
>