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

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Date:   Fri, 19 May 2017 17:46:58 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     Michal Hocko <mhocko@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>
CC:     Chris Wilson <chris@...is-wilson.co.uk>,
        Vlastimil Babka <vbabka@...e.cz>, <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Michal Hocko <mhocko@...e.com>
Subject: Re: [PATCH] mm: clarify why we want kmalloc before falling backto
 vmallock

On 05/17/2017 01:09 AM, Michal Hocko wrote:
> From: Michal Hocko <mhocko@...e.com>
> 
> 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@...is-wilson.co.uk>
> Signed-off-by: Michal Hocko <mhocko@...e.com>
> ---
>   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
>   	 */

Thanks for adding this, it's great to have. Here's a slightly polished version of your words, if you 
like:

	/*
	 * We want to attempt a large physically contiguous block first because
	 * it is less likely to fragment multiple larger blocks. This approach
	 * therefore contributes less to long term fragmentation than a vmalloc
	 * fallback would. However, make sure that larger requests are not too
	 * disruptive: no OOM killer and no allocation failure warnings, as we
	 * have a fallback.
	 */

thanks,
john h

>   	if (size > PAGE_SIZE) {
> -- 
> 2.11.0
> 
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