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Tue, 08 Nov 2022 12:37:29 -0800 (PST) Content-Type: text/plain; charset=us-ascii Mime-Version: 1.0 (Mac OS X Mail 16.0 \(3696.120.41.1.1\)) Subject: Re: [PATCH 1/4] mm: introduce 'encoded' page pointers with embedded extra bits From: Nadav Amit In-Reply-To: <20221108194139.57604-1-torvalds@linux-foundation.org> Date: Tue, 8 Nov 2022 12:37:28 -0800 Cc: Hugh Dickins , Johannes Weiner , Andrew Morton , kernel list , linux-mm@kvack.org Content-Transfer-Encoding: quoted-printable Message-Id: <15A1A033-73DC-4ACA-87B0-3535E9236F7F@gmail.com> References: <20221108194139.57604-1-torvalds@linux-foundation.org> To: Linus Torvalds X-Mailer: Apple Mail (2.3696.120.41.1.1) ARC-Authentication-Results: i=1; imf24.hostedemail.com; dkim=pass header.d=gmail.com header.s=20210112 header.b="aO/Eszau"; spf=pass (imf24.hostedemail.com: domain of nadav.amit@gmail.com designates 209.85.215.180 as permitted sender) smtp.mailfrom=nadav.amit@gmail.com; dmarc=pass (policy=none) header.from=gmail.com ARC-Seal: i=1; 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spf=pass (imf24.hostedemail.com: domain of nadav.amit@gmail.com designates 209.85.215.180 as permitted sender) smtp.mailfrom=nadav.amit@gmail.com; dmarc=pass (policy=none) header.from=gmail.com X-Stat-Signature: b65rh9s8ubnsgt8t7bjpuhotnmrwid56 X-HE-Tag: 1667939908-106144 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 Nov 8, 2022, at 11:41 AM, Linus Torvalds = wrote: > We already have this notion in parts of the MM code (see the mlock = code > with the LRU_PAGE and NEW_PAGE) bits, but I'm going to introduce a new > case, and I refuse to do the same thing we've done before where we = just > put bits in the raw pointer and say it's still a normal pointer. >=20 > So this introduces a 'struct encoded_page' pointer that cannot be used > for anything else than to encode a real page pointer and a couple of > extra bits in the low bits. That way the compiler can trivially track > the state of the pointer and you just explicitly encode and decode the > extra bits. I tested again all of the patches with the PoC. They pass. >=20 > +struct encoded_page; > +#define ENCODE_PAGE_BITS 3ul > +static inline struct encoded_page *encode_page(struct page *page, = unsigned long flags) > +{ > + return (struct encoded_page *)(flags | (unsigned long)page); > +} > + > +static inline bool encoded_page_flags(struct encoded_page *page) > +{ > + return ENCODE_PAGE_BITS & (unsigned long)page; > +} I think this one wants to be some unsigned, as otherwise why have ENCODE_PAGE_BITS as 3ul ?