linux-kernel - Re: [PATCH 2/2] mm: page_alloc: avoid slowpath for more than MAX

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Message-ID: <20130722163836.GD715@cmpxchg.org>
Date:	Mon, 22 Jul 2013 12:38:36 -0400
From:	Johannes Weiner <hannes@...xchg.org>
To:	Pintu Kumar <pintu.k@...sung.com>
Cc:	akpm@...ux-foundation.org, mgorman@...e.de, jiang.liu@...wei.com,
	minchan@...nel.org, cody@...ux.vnet.ibm.com, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, cpgs@...sung.com,
	pintu_agarwal@...oo.com
Subject: Re: [PATCH 2/2] mm: page_alloc: avoid slowpath for more than
 MAX_ORDER allocation.

Hi Pintu,

On Mon, Jul 22, 2013 at 05:02:42PM +0530, Pintu Kumar wrote:
> It was observed that if order is passed as more than MAX_ORDER
> allocation in __alloc_pages_nodemask, it will unnecessarily go to
> slowpath and then return failure.
> Since we know that more than MAX_ORDER will anyways fail, we can
> avoid slowpath by returning failure in nodemask itself.
> 
> Signed-off-by: Pintu Kumar <pintu.k@...sung.com>
> ---
>  mm/page_alloc.c |    4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 202ab58..6d38e75 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1564,6 +1564,10 @@ __setup("fail_page_alloc=", setup_fail_page_alloc);
>  
>  static bool should_fail_alloc_page(gfp_t gfp_mask, unsigned int order)
>  {
> +	if (order >= MAX_ORDER) {
> +		WARN_ON(!(gfp_mask & __GFP_NOWARN));
> +		return false;
> +	}

I don't see how this solves what you describe (should return true?)

It would also not be a good place to put performance optimization,
because this function is only called as part of a debugging mechanism
that is usually disabled.

Lastly, order >= MAX_ORDER is not supported by the page allocator, and
we do not want to punish 99.999% of all legitimate page allocations in
the fast path in order to catch an unlikely situation like this.
Having the check only in the slowpath is a good thing.
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