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[98.15.154.102]) by smtp.gmail.com with ESMTPSA id g8sm1383337qkm.25.2021.08.19.04.16.34 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 19 Aug 2021 04:16:34 -0700 (PDT) Date: Thu, 19 Aug 2021 07:18:13 -0400 From: Johannes Weiner To: Muchun Song Cc: Andrew Morton , Roman Gushchin , Michal Hocko , Shakeel Butt , Vladimir Davydov , Matthew Wilcox , LKML , Linux Memory Management List Subject: Re: [PATCH] mm: introduce PAGEFLAGS_MASK to replace ((1UL << NR_PAGEFLAGS) - 1) Message-ID: References: <20210817033032.76089-1-songmuchun@bytedance.com> <20210818003919.5bd008fec6cb0436af2443c4@linux-foundation.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Aug 19, 2021 at 02:33:37PM +0800, Muchun Song wrote: > On Wed, Aug 18, 2021 at 3:39 PM Andrew Morton wrote: > > > > On Tue, 17 Aug 2021 21:44:36 -0700 Roman Gushchin wrote: > > > > > On Wed, Aug 18, 2021 at 12:35:08PM +0800, Muchun Song wrote: > > > > On Wed, Aug 18, 2021 at 10:16 AM Roman Gushchin wrote: > > > > > > > > > > On Tue, Aug 17, 2021 at 11:30:32AM +0800, Muchun Song wrote: > > > > > > Instead of hard-coding ((1UL << NR_PAGEFLAGS) - 1) everywhere, introducing > > > > > > PAGEFLAGS_MASK to make the code clear to get the page flags. > > > > > > > > > > > > Signed-off-by: Muchun Song > > > > > > --- > > > > > > include/linux/page-flags.h | 4 +++- > > > > > > include/trace/events/page_ref.h | 4 ++-- > > > > > > lib/test_printf.c | 2 +- > > > > > > lib/vsprintf.c | 2 +- > > > > > > 4 files changed, 7 insertions(+), 5 deletions(-) > > > > > > > > > > > > diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h > > > > > > index 54c4af35c628..1f951ac24a5e 100644 > > > > > > --- a/include/linux/page-flags.h > > > > > > +++ b/include/linux/page-flags.h > > > > > > @@ -180,6 +180,8 @@ enum pageflags { > > > > > > PG_reported = PG_uptodate, > > > > > > }; > > > > > > > > > > > > +#define PAGEFLAGS_MASK (~((1UL << NR_PAGEFLAGS) - 1)) > > > > > > > > > > Hm, isn't it better to invert it? Like > > > > > #define PAGEFLAGS_MASK ((1UL << NR_PAGEFLAGS) - 1) > > > > > > > > > > It feels more usual and will simplify the rest of the patch. > > > > > > > > Actually, I learned from PAGE_MASK. So I thought the macro > > > > like xxx_MASK should be the format of 0xff...ff00...00. I don't > > > > know if it is an unwritten rule. I can invert PAGEFLAGS_MASK > > > > if it's not a rule. > > > > > > There are many examples of both approached in the kernel tree, > > > however I'd say the more common is without "~" (out of my head). > > > > > > It's definitely OK to define it like > > > #define PAGEFLAGS_MASK ((1UL << NR_PAGEFLAGS) - 1) > > > > > > > PAGE_MASK has always seemed weird to me but I figured that emulating it > > would be the approach of least surprise. Might be wrong about that... > > IIUC, you seem to agree with the current approach. Right? We do use both schemes. PAGE_MASK, as you pointed out. PMD_MASK, PUD_MASK etc. also *hide* rather than extract the named bits. However, we already have a series of masks for page->flags in mm.h that follow the more common scheme of *extracting* the named bits: #define ZONES_MASK ((1UL << ZONES_WIDTH) - 1) #define NODES_MASK ((1UL << NODES_WIDTH) - 1) #define SECTIONS_MASK ((1UL << SECTIONS_WIDTH) - 1) #define LAST_CPUPID_MASK ((1UL << LAST_CPUPID_SHIFT) - 1) #define KASAN_TAG_MASK ((1UL << KASAN_TAG_WIDTH) - 1) #define ZONEID_MASK ((1UL << ZONEID_SHIFT) - 1) Because of that, I would agree with Roman here and say it probably makes sense to put the PAGEFLAGS_MASK definition next to those and follow the "mask everything but" semantics.