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[70.30.145.63]) by smtp.gmail.com with ESMTPSA id t197-20020a3746ce000000b007417e60f621sm11748357qka.126.2023.03.08.07.17.26 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 08 Mar 2023 07:17:27 -0800 (PST) Date: Wed, 8 Mar 2023 10:17:25 -0500 From: Peter Xu To: Axel Rasmussen Cc: Alexander Viro , Andrew Morton , Hugh Dickins , Jan Kara , "Liam R. Howlett" , Matthew Wilcox , Mike Kravetz , Mike Rapoport , Muchun Song , Nadav Amit , Shuah Khan , James Houghton , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-kselftest@vger.kernel.org Subject: Re: [PATCH v3 3/5] mm: userfaultfd: combine 'mode' and 'wp_copy' arguments Message-ID: References: <20230306225024.264858-1-axelrasmussen@google.com> <20230306225024.264858-4-axelrasmussen@google.com> MIME-Version: 1.0 In-Reply-To: X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Disposition: inline X-Rspamd-Queue-Id: 463BD140015 X-Rspamd-Server: rspam09 X-Rspam-User: X-Stat-Signature: h564hafhndy58nejdf9bdzj6d4enhjnk X-HE-Tag: 1678288691-225471 X-HE-Meta: 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 5DMcinZw q5jkfEgbTg86j3LrD49OR6G73QG8MGUNKPFmil/zJifmGhT89F+PorZviXuOdqPUjqNXoVG+a/c1jeZa9618VCqtnfGDO6GcDvNsWVDDtltJ0UmOp0RqsViO3ympf9g3lkmbcLtUN2bwIKUF5ETxhEQIDWXJaT/mj/+khbu0vqeqqr40rdrt+qwn396cOoLPMv7Jz/KtBIXS0udb6ezokeJ67Hs9Y1O+Rs9O6l/XTK1te7i1Of+Hy7uOL6TvLmHEl5XQH+5SoaJHWnWOMhMnzpuoyD5U/d1Pc3h4sMfUnMV2e9nCeXfO5B/1WGc+J4Ev8l4EGzyX8zEr/egqoH8ZN5XZQv0ele8EUxEXh2NZsbJ7Anj+grP/GdBPZeKZ96UQeijoogsTVJ/w1qSg= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Tue, Mar 07, 2023 at 03:27:17PM -0800, Axel Rasmussen wrote: > > > > > > > > +#define MFILL_ATOMIC_MODE_BITS (const_ilog2(NR_MFILL_ATOMIC_MODES - 1) + 1) > > > > Here IIUC it should be "const_ilog2(NR_MFILL_ATOMIC_MODES) + 1", but > > maybe.. we don't bother and define every bit explicitly? > > If my reading of const_ilog2's definition is correct, then: > > const_ilog2(4) = 2 > const_ilog2(3) = 1 > const_ilog2(2) = 1 > > For either 3 or 4 modes, we need 2 bits to represent them (0, 1, 2, > 3), i.e. we want MFILL_ATOMIC_MODE_BITS = 2. I think this is correct > as is, because const_ilog2(4 - 1) + 1 = 2, and const_ilog2(3 - 1) + 1 > = 2. > > In other words, I think const_ilog2 is defined as floor(log2()), > whereas what we want is ceil(log2()). You're right. > > The benefit of doing this vs. just doing defines with fixed values is, > if we ever added a new mode, we wouldn't have to do bit twiddling and > update the mask, flag bits, etc. - it would happen "automatically". I > prefer it this way, but I agree it is a matter of opinion / taste. :) > If you or others feel strongly this is overcomplicated, I can take the > other approach. I don't know what this will look like at last. The thing is if you plan to define MFILL_ATOMIC_* with __bitwise I think it'll stop working with any calculations upon it. I don't worry on growing modes, as I don't expect it to happen a lot. No strong opinion here, as long as sparse won't complain. Thanks, -- Peter Xu