linux-kernel - Re: [PATCH net-next 1/6] mm/page_alloc: modify page_frag_alloc

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Date: Fri, 05 Jan 2024 07:28:00 -0800
From: Alexander H Duyck <alexander.duyck@...il.com>
To: Yunsheng Lin <linyunsheng@...wei.com>, davem@...emloft.net,
 kuba@...nel.org,  pabeni@...hat.com
Cc: netdev@...r.kernel.org, linux-kernel@...r.kernel.org, Andrew Morton
	 <akpm@...ux-foundation.org>, Eric Dumazet <edumazet@...gle.com>, 
	linux-mm@...ck.org
Subject: Re: [PATCH net-next 1/6] mm/page_alloc: modify
 page_frag_alloc_align() to accept align as an argument

On Wed, 2024-01-03 at 17:56 +0800, Yunsheng Lin wrote:
> napi_alloc_frag_align() and netdev_alloc_frag_align() accept
> align as an argument, and they are thin wrappers around the
> __napi_alloc_frag_align() and __netdev_alloc_frag_align() APIs
> doing the align and align_mask conversion, in order to call
> page_frag_alloc_align() directly.
> 
> As __napi_alloc_frag_align() and __netdev_alloc_frag_align()
> APIs are only used by the above thin wrappers, it seems that
> it makes more sense to remove align and align_mask conversion
> and call page_frag_alloc_align() directly. By doing that, we
> can also avoid the confusion between napi_alloc_frag_align()
> accepting align as an argument and page_frag_alloc_align()
> accepting align_mask as an argument when they both have the
> 'align' suffix.
> 
> Signed-off-by: Yunsheng Lin <linyunsheng@...wei.com>
> CC: Alexander Duyck <alexander.duyck@...il.com>

This patch overlooks much of the main reason for having the wrappers.
By having the in-line wrapper and passing the argument as a mask we can
avoid having to verify the alignment value during execution time since
it can usually be handled during compile time.

By moving it into the function itself we are adding additional CPU
overhead per page as we will have to go through and validate the
alignment value for every single page instead of when the driver using
the function is compiled.

The overhead may not seem like much, but when you are having to deal
with it per page and you are processing pages at millions per second it
can quickly start to add up.

This is essentially a code cleanup at the cost of some amount of
performance.