From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ed1-f69.google.com (mail-ed1-f69.google.com [209.85.208.69]) by kanga.kvack.org (Postfix) with ESMTP id 2CD888E0001 for ; Mon, 10 Dec 2018 02:46:56 -0500 (EST) Received: by mail-ed1-f69.google.com with SMTP id f17so3781460edm.20 for ; Sun, 09 Dec 2018 23:46:56 -0800 (PST) Received: from mx1.suse.de (mx2.suse.de. [195.135.220.15]) by mx.google.com with ESMTPS id d12si610585edv.158.2018.12.09.23.46.54 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Sun, 09 Dec 2018 23:46:54 -0800 (PST) Subject: Re: [PATCHv2] mm/pageblock: throw compiling time error if pageblock_bits can not hold MIGRATE_TYPES References: <1544158388-20832-1-git-send-email-kernelfans@gmail.com> <7aa8d326-cffc-f2b6-2c03-01d9bd4c54b4@suse.cz> From: Vlastimil Babka Message-ID: Date: Mon, 10 Dec 2018 08:46:52 +0100 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Pingfan Liu Cc: linux-mm@kvack.org, Andrew Morton , Michal Hocko , Pavel Tatashin , Oscar Salvador , Mike Rapoport , Joonsoo Kim , Alexander Duyck On 12/10/18 4:15 AM, Pingfan Liu wrote: > On Fri, Dec 7, 2018 at 3:36 PM Vlastimil Babka wrote: >> >> On 12/7/18 5:53 AM, Pingfan Liu wrote: >>> Currently, NR_PAGEBLOCK_BITS and MIGRATE_TYPES are not associated by code. >>> If someone adds extra migrate type, then he may forget to enlarge the >>> NR_PAGEBLOCK_BITS. Hence it requires some way to fix. >>> NR_PAGEBLOCK_BITS depends on MIGRATE_TYPES, while these macro >>> spread on two different .h file with reverse dependency, it is a little >>> hard to refer to MIGRATE_TYPES in pageblock-flag.h. This patch tries to >>> remind such relation in compiling-time. >>> >>> Signed-off-by: Pingfan Liu >>> Cc: Andrew Morton >>> Cc: Michal Hocko >>> Cc: Pavel Tatashin >>> Cc: Vlastimil Babka >>> Cc: Oscar Salvador >>> Cc: Mike Rapoport >>> Cc: Joonsoo Kim >>> Cc: Alexander Duyck >>> --- >>> include/linux/pageblock-flags.h | 5 +++-- >>> mm/page_alloc.c | 3 ++- >>> 2 files changed, 5 insertions(+), 3 deletions(-) >>> >>> diff --git a/include/linux/pageblock-flags.h b/include/linux/pageblock-flags.h >>> index 9132c5c..fe0aec4 100644 >>> --- a/include/linux/pageblock-flags.h >>> +++ b/include/linux/pageblock-flags.h >>> @@ -25,11 +25,12 @@ >>> >>> #include >>> >>> +#define PB_migratetype_bits 3 >>> /* Bit indices that affect a whole block of pages */ >>> enum pageblock_bits { >>> PB_migrate, >>> - PB_migrate_end = PB_migrate + 3 - 1, >>> - /* 3 bits required for migrate types */ >>> + PB_migrate_end = PB_migrate + PB_migratetype_bits - 1, >>> + /* n bits required for migrate types */ >>> PB_migrate_skip,/* If set the block is skipped by compaction */ >>> >>> /* >>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c >>> index 2ec9cc4..1a22d8d 100644 >>> --- a/mm/page_alloc.c >>> +++ b/mm/page_alloc.c >>> @@ -425,7 +425,8 @@ void set_pfnblock_flags_mask(struct page *page, unsigned long flags, >>> unsigned long bitidx, word_bitidx; >>> unsigned long old_word, word; >>> >>> - BUILD_BUG_ON(NR_PAGEBLOCK_BITS != 4); >> >> Why delete this one? It's for something a bit different and also still >> valid. >> > > I think it is dependent on PB_migratetype_bits (plus 1 dedicated bit > for skip), hence the later one implies it. No, NR_PAGEBLOCK_BITS simply to be 4 (or other number that can divide the number of bits in a word with no remainder), or set_pfnblock_flags_mask() will not function correctly. >>> + BUILD_BUG_ON(order_base_2(MIGRATE_TYPES) >>> + != (PB_migratetype_bits - 1)); >> >> I think this should use the '>' operator. It's fine if there are less > > If ">" is chosen, then it allows wasted bits. I had thought it is not > the purpose of the design, hence disard the ">". We do allow wasted bits, because we need NR_PAGEBLOCK_BITS to be 4 in any case, as explained above. > Otherwise, what about > using warning on ">"? I think it would be needed. >> types than what can fit into 3 bits. AFAICS for !CONFIG_DMA and >> !CONFIG_MEMORY_ISOLATION there are just 4 types that fit into 2 bits... >> > Oh, yes, you are right. How about this: > #if defined(CONFIG_DMA) || defined(CONFIG_MEMORY_ISOLATION) > #define PB_migratetype_bits 3 > #else > #define PB_migratetype_bits 2 > #endif I think it's not necessary. Really, we need to have 4 NR_PAGEBLOCK_BITS with the current code, and this leaves us with 3 bits for migratetype. The only thing to check is whether migratetypes fit into these 3 bits, IMHO. > Thanks for your kindly review. > > Regards, > Pingfan >>> >>> bitmap = get_pageblock_bitmap(page, pfn); >>> bitidx = pfn_to_bitidx(page, pfn); >>> >>