From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-it0-f70.google.com (mail-it0-f70.google.com [209.85.214.70]) by kanga.kvack.org (Postfix) with ESMTP id C799E6B0008 for ; Mon, 7 May 2018 20:25:50 -0400 (EDT) Received: by mail-it0-f70.google.com with SMTP id n83-v6so10636453itg.2 for ; Mon, 07 May 2018 17:25:50 -0700 (PDT) Received: from mail1.bemta12.messagelabs.com (mail1.bemta12.messagelabs.com. [216.82.251.7]) by mx.google.com with ESMTPS id e6-v6si8137464itb.35.2018.05.07.17.25.49 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 07 May 2018 17:25:49 -0700 (PDT) From: Huaisheng HS1 Ye Subject: RE: [External] Re: [PATCH 2/3] include/linux/gfp.h: use unsigned int in gfp_zone Date: Tue, 8 May 2018 00:25:31 +0000 Message-ID: References: <1525416729-108201-1-git-send-email-yehs1@lenovo.com> <1525416729-108201-3-git-send-email-yehs1@lenovo.com> <20180504133533.GR4535@dhcp22.suse.cz> <20180504154004.GB29829@bombadil.infradead.org> <20180506134814.GB7362@bombadil.infradead.org> <20180506185532.GA13604@bombadil.infradead.org> <20180507184410.GA12361@bombadil.infradead.org> In-Reply-To: <20180507184410.GA12361@bombadil.infradead.org> Content-Language: zh-CN Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: quoted-printable MIME-Version: 1.0 Sender: owner-linux-mm@kvack.org List-ID: To: Matthew Wilcox Cc: Michal Hocko , "akpm@linux-foundation.org" , "linux-mm@kvack.org" , "vbabka@suse.cz" , "mgorman@techsingularity.net" , "pasha.tatashin@oracle.com" , "alexander.levin@verizon.com" , "hannes@cmpxchg.org" , "penguin-kernel@I-love.SAKURA.ne.jp" , "colyli@suse.de" , NingTing Cheng , "linux-kernel@vger.kernel.org" > On Mon, May 07, 2018 at 05:16:50PM +0000, Huaisheng HS1 Ye wrote: > > I hope it couldn't cause problem, but based on my analyzation it has th= e > potential to go wrong if users still use the flags as usual, which are __= GFP_DMA, > __GFP_DMA32 and __GFP_HIGHMEM. > > Let me take an example with my testing platform, these logics are much > abstract, an example will be helpful. > > > > There is a two sockets X86_64 server, No HIGHMEM and it has 16 + 16GB > memories. > > Its zone types shall be like this below, > > > > ZONE_DMA 0 0b0000 > > ZONE_DMA32 1 0b0001 > > ZONE_NORMAL 2 0b0010 > > (OPT_ZONE_HIGHMEM) 2 0b0010 > > ZONE_MOVABLE 3 0b0011 > > ZONE_DEVICE 4 0b0100 (virtual zone) > > __MAX_NR_ZONES 5 > > > > __GFP_DMA =3D ZONE_DMA ^ ZONE_NORMAL=3D 0b0010 > > __GFP_DMA32 =3D ZONE_DMA32 ^ ZONE_NORMAL=3D 0b0011 > > __GFP_HIGHMEM =3D OPT_ZONE_HIGHMEM ^ ZONE_NORMAL =3D 0b0000 > > __GFP_MOVABLE =3D ZONE_MOVABLE ^ ZONE_NORMAL | > ___GFP_MOVABLE =3D 0b1001 > > > > Eg. > > If a driver uses flags like this below, > > Step 1: > > gfp_mask | __GFP_DMA32; > > (0b 0000 | 0b 0011 =3D 0b 0011) > > gfp_mask's low four bits shall equal to 0011, assuming no __GFP_MOVABLE > > > > Step 2: > > gfp_mask & ~__GFP_DMA; > > (0b 0011 & ~0b0010 =3D 0b0001) > > gfp_mask's low four bits shall equal to 0001 now, then when it enter > gfp_zone(), > > > > return ((__force int)flags & ___GFP_ZONE_MASK) ^ ZONE_NORMAL; > > (0b0001 ^ 0b0010 =3D 0b0011) > > You know 0011 means that ZONE_MOVABLE will be returned. > > In this case, error can be found, because gfp_mask needs to get > ZONE_DMA32 originally. > > But with existing GFP_ZONE_TABLE/BAD, it is correct. Because the bits a= re > way of 0x1, 0x2, 0x4, 0x8 >=20 > Yes, I understand your point here. My point was that this was already a = bug; > the caller shouldn't simply be clearing __GFP_DMA; they really mean to cl= ear > all of the GFP_ZONE bits so that they allocate from ZONE_NORMAL. And for > that, they should be using ~GFP_ZONEMASK That is great, if they can follow this principle, I don't worry it. Maybe I= am too cautious. >=20 > Unless they already know, of course. For example, this one in > arch/x86/mm/pgtable.c is fine: >=20 > if (strcmp(arg, "nohigh") =3D=3D 0) > __userpte_alloc_gfp &=3D ~__GFP_HIGHMEM; >=20 > because it knows that __userpte_alloc_gfp can only have __GFP_HIGHMEM set= . >=20 > But something like btrfs should almost certainly be using ~GFP_ZONEMASK. > > > +#define __GFP_HIGHMEM ((__force gfp_t)OPT_ZONE_HIGHMEM ^ > > > ZONE_NORMAL) > > > -#define __GFP_MOVABLE ((__force gfp_t)___GFP_MOVABLE) /* > > > ZONE_MOVABLE allowed */ > > > +#define __GFP_MOVABLE ((__force gfp_t)ZONE_MOVABLE ^ > > > ZONE_NORMAL | \ > > > + ___GFP_MOVABLE) > > > > > > Then I think you can just make it: > > > > > > static inline enum zone_type gfp_zone(gfp_t flags) > > > { > > > return ((__force int)flags & ___GFP_ZONE_MASK) ^ ZONE_NORMAL; > > > } > > Sorry, I think it has risk in this way, let me introduce a failure case= for > example. > > > > Now suppose that, there is a flag should represent DMA flag with movabl= e. > > It should be like this below, > > __GFP_DMA | __GFP_MOVABLE > > (0b 0010 | 0b 1001 =3D 0b 1011) > > Normally, gfp_zone shall return ZONE_DMA but with MOVABLE policy, right= ? >=20 > No, if you somehow end up with __GFP_MOVABLE | __GFP_DMA, it should give > you ZONE_DMA. Exactly, it should return ZONE_DMA, that's what I thought. >=20 > > But with your code, gfp_zone will return ZONE_DMA32 with MOVABLE > >policy. > > (0b 1011 ^ 0b 0010 =3D 1001) >=20 > ___GFP_ZONE_MASK is 0x7, so it excludes __GFP_MOVABLE. Sorry, I made a mistake here. I rewrite it as below. ((__GFP_DMA | __GFP_MOVABLE) & ___GFP_ZONE_MASK) ((0b 0010 | 0b 1001 =3D 0b 1011) & 0b 0111) =3D 0b 0011 0b 0011 ^ 0b 0010 =3D 0b 0001 So ZONE_DMA32 will be returned, but what user needs is ZONE_DMA. Thanks, Huaisheng