From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <owner-linux-mm@kvack.org>
Received: from mail-wr0-f198.google.com (mail-wr0-f198.google.com [209.85.128.198])
	by kanga.kvack.org (Postfix) with ESMTP id 8D3B06B03BD
	for <linux-mm@kvack.org>; Wed,  8 Mar 2017 02:37:59 -0500 (EST)
Received: by mail-wr0-f198.google.com with SMTP id v66so8379378wrc.4
        for <linux-mm@kvack.org>; Tue, 07 Mar 2017 23:37:59 -0800 (PST)
Received: from mx2.suse.de (mx2.suse.de. [195.135.220.15])
        by mx.google.com with ESMTPS id o74si3732784wmg.57.2017.03.07.23.37.58
        for <linux-mm@kvack.org>
        (version=TLS1 cipher=AES128-SHA bits=128/128);
        Tue, 07 Mar 2017 23:37:58 -0800 (PST)
Subject: Re: [PATCH] mm, vmalloc: use __GFP_HIGHMEM implicitly
References: <20170307141020.29107-1-mhocko@kernel.org>
 <20170307182841.GS16328@bombadil.infradead.org>
 <20170307185748.GU16328@bombadil.infradead.org>
From: Vlastimil Babka <vbabka@suse.cz>
Message-ID: <1d8e9dd2-a542-250b-846f-5d1d7941a6b5@suse.cz>
Date: Wed, 8 Mar 2017 08:37:57 +0100
MIME-Version: 1.0
In-Reply-To: <20170307185748.GU16328@bombadil.infradead.org>
Content-Type: text/plain; charset=windows-1252
Content-Transfer-Encoding: 7bit
Sender: owner-linux-mm@kvack.org
List-ID: <linux-mm.kvack.org>
To: Matthew Wilcox <willy@infradead.org>, Michal Hocko <mhocko@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>, Al Viro <viro@zeniv.linux.org.uk>, David Rientjes <rientjes@google.com>, Cristopher Lameter <cl@linux.com>, linux-mm@kvack.org, LKML <linux-kernel@vger.kernel.org>, Michal Hocko <mhocko@suse.com>

On 03/07/2017 07:57 PM, Matthew Wilcox wrote:
> On Tue, Mar 07, 2017 at 10:28:41AM -0800, Matthew Wilcox wrote:
>> On Tue, Mar 07, 2017 at 03:10:20PM +0100, Michal Hocko wrote:
>>> This patch simply uses __GFP_HIGHMEM implicitly when allocating pages to
>>> be mapped to the vmalloc space. Current users which add __GFP_HIGHMEM
>>> are simplified and drop the flag.
> 
> btw, I had another idea for GFP_HIGHMEM -- remove it when CONFIG_HIGHMEM
> isn't enabled.  Saves 26 bytes of .text and 64 bytes of .data on my
> laptop's kernel build.  What do you think?
> 
> Also, I suspect the layout of bits is suboptimal from an assembly
> language perspective.  I still mostly care about x86 which doesn't
> benefit, so I'm not inclined to do the work, but certainly ARM, PA-RISC,
> SPARC and Itanium would all benefit from having frequently-used bits
> (ie those used in GFP_KERNEL and GFP_ATOMIC) placed in the low 8 bits.
> 
> diff --git a/include/linux/gfp.h b/include/linux/gfp.h
> index 0fe0b6295ab5..d88cb532d7c8 100644
> --- a/include/linux/gfp.h
> +++ b/include/linux/gfp.h
> @@ -16,7 +16,11 @@ struct vm_area_struct;
>  
>  /* Plain integer GFP bitmasks. Do not use this directly. */
>  #define ___GFP_DMA		0x01u
> +#ifdef CONFIG_HIGHMEM
>  #define ___GFP_HIGHMEM		0x02u
> +#else
> +#define ___GFP_HIGHMEM		0x0u

Make sure you don't break the users of __def_gfpflag_names e.g.
format_flags(). IIRC zero is a terminator in the table.

But the savings don't seem to be worth the trouble.

> +#endif
>  #define ___GFP_DMA32		0x04u
>  #define ___GFP_MOVABLE		0x08u
>  #define ___GFP_RECLAIMABLE	0x10u
> 

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>