From patchwork Wed Nov 9 20:30:48 2022 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Linus Torvalds X-Patchwork-Id: 13038031 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by smtp.lore.kernel.org (Postfix) with ESMTP id A3BC7C433FE for ; Wed, 9 Nov 2022 20:30:57 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 15E4D6B0072; Wed, 9 Nov 2022 15:30:57 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id 10EF86B0073; Wed, 9 Nov 2022 15:30:57 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id F18F06B0074; Wed, 9 Nov 2022 15:30:56 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0012.hostedemail.com [216.40.44.12]) by kanga.kvack.org (Postfix) with ESMTP id E26016B0072 for ; Wed, 9 Nov 2022 15:30:56 -0500 (EST) Received: from smtpin23.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay05.hostedemail.com (Postfix) with ESMTP id B0FF64046A for ; Wed, 9 Nov 2022 20:30:56 +0000 (UTC) X-FDA: 80115047712.23.7B553EB Received: from ams.source.kernel.org (ams.source.kernel.org [145.40.68.75]) by imf02.hostedemail.com (Postfix) with ESMTP id 0AFF480013 for ; Wed, 9 Nov 2022 20:30:55 +0000 (UTC) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by ams.source.kernel.org (Postfix) with ESMTPS id C6223B81F9D; Wed, 9 Nov 2022 20:30:53 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 6098AC433C1; Wed, 9 Nov 2022 20:30:52 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linux-foundation.org; s=korg; t=1668025852; bh=penp5eGjlAzwEMHIlYfO2UthsDqmi4JE3eNVxwebGdU=; h=From:To:Cc:Subject:Date:From; b=QDtR/oMNFrNnuSC8rkqWQuyDgSXMdz7cE6sGNbUSY63i7rPywtVF2bs/QeWo1hR6D E+ZSTT3gQ1V/p4cp0tg271dSgkqpaQYCwj4Hw+IDg1mhCSpAbyrGxAwL6Aja826eaJ t6IFNeMUW7uI0RdE4bX2KpSOHjrhnD9sK5lLsPGY= From: Linus Torvalds To: Hugh Dickins , Johannes Weiner , Andrew Morton Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, Alexander Gordeev Subject: [PATCH 1/4] mm: introduce 'encoded' page pointers with embedded extra bits Date: Wed, 9 Nov 2022 12:30:48 -0800 Message-Id: <20221109203051.1835763-1-torvalds@linux-foundation.org> X-Mailer: git-send-email 2.38.1.284.gfd9468d787 MIME-Version: 1.0 ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1668025856; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-transfer-encoding:content-transfer-encoding: in-reply-to:references:dkim-signature; bh=UAFQiOpzZMBKv2AQjP7YaqcakaLTBmk9LN48Ag/gOS8=; b=yP/N87LpAfqwO5QVwSHOMgMG23QpmB2kxkbZ7GdCqcudnkekB83O+o/D8ih77Yg9t4fld/ PFP3ePWb/BmExSYyvJCmOY15bbLeXw8MxcK1VG2Iy+mb2hHSzuG7eaNykJfTCsPGm/OH51 qmOWUZ9qU8u7eRbVDwJJbUEjfQGYXdc= ARC-Authentication-Results: i=1; imf02.hostedemail.com; dkim=pass header.d=linux-foundation.org header.s=korg header.b="QDtR/oMN"; spf=pass (imf02.hostedemail.com: domain of torvalds@linux-foundation.org designates 145.40.68.75 as permitted sender) smtp.mailfrom=torvalds@linux-foundation.org; dmarc=none ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1668025856; a=rsa-sha256; cv=none; b=4szcMsSUiskCdjEay8fpNJg0ugngfo21g/ekYZhGppV14dAAeanFELjVktULNCemQIzAH6 ShX2KGO+3jKnmyPHnYnmSTG0xahOTbKIP23BskWhVTSmkg4tp//uuR8X6lKGI7c+lqBZ8X j5DNLWw8xvzM3/x4o9pNfQMxdZJzMg4= X-Rspam-User: X-Stat-Signature: 9y7pycnex8p5jz5wkaxratgxpsqb45df X-Rspamd-Queue-Id: 0AFF480013 Authentication-Results: imf02.hostedemail.com; dkim=pass header.d=linux-foundation.org header.s=korg header.b="QDtR/oMN"; spf=pass (imf02.hostedemail.com: domain of torvalds@linux-foundation.org designates 145.40.68.75 as permitted sender) smtp.mailfrom=torvalds@linux-foundation.org; dmarc=none X-Rspamd-Server: rspam02 X-HE-Tag: 1668025855-948553 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: 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. 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. Note that this makes the alignment of 'struct page' explicit even for the case where CONFIG_HAVE_ALIGNED_STRUCT_PAGE is not set. That is entirely redundant in almost all cases, since the page structure already contains several word-sized entries. However, on m68k, the alignment of even 32-bit data is just 16 bits, and as such in theory the alignment of 'struct page' could be too. So let's just make it very very explicit that the alignment needs to be at least 32 bits, giving us a guarantee of two unused low bits in the pointer. Now, in practice, our page struct array is aligned much more than that anyway, even on m68k, and our existing code in mm/mlock.c obviously already depended on that. But since the whole point of this change is to be careful about the type system when hiding extra bits in the pointer, let's also be explicit about the assumptions we make. NOTE! This is being very careful in another way too: it has a build-time assertion that the 'flags' added to the page pointer actually fit in the two bits. That means that this helper must be inlined, and can only be used in contexts where the compiler can statically determine that the value fits in the available bits. Link: https://lore.kernel.org/all/Y2tKixpO4RO6DgW5@tuxmaker.boeblingen.de.ibm.com/ Cc: Alexander Gordeev Acked-by: Johannes Weiner Acked-by: Hugh Dickins Signed-off-by: Linus Torvalds Reviewed-by: David Hildenbrand --- include/linux/mm_types.h | 34 +++++++++++++++++++++++++++++++++- 1 file changed, 33 insertions(+), 1 deletion(-) diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h index 500e536796ca..0a38fcb08d85 100644 --- a/include/linux/mm_types.h +++ b/include/linux/mm_types.h @@ -67,7 +67,7 @@ struct mem_cgroup; #ifdef CONFIG_HAVE_ALIGNED_STRUCT_PAGE #define _struct_page_alignment __aligned(2 * sizeof(unsigned long)) #else -#define _struct_page_alignment +#define _struct_page_alignment __aligned(sizeof(unsigned long)) #endif struct page { @@ -241,6 +241,38 @@ struct page { #endif } _struct_page_alignment; +/** + * struct encoded_page - a nonexistent type marking this pointer + * + * An 'encoded_page' pointer is a pointer to a regular 'struct page', but + * with the low bits of the pointer indicating extra context-dependent + * information. Not super-common, but happens in mmu_gather and mlock + * handling, and this acts as a type system check on that use. + * + * We only really have two guaranteed bits in general, although you could + * play with 'struct page' alignment (see CONFIG_HAVE_ALIGNED_STRUCT_PAGE) + * for more. + * + * Use the supplied helper functions to endcode/decode the pointer and bits. + */ +struct encoded_page; +#define ENCODE_PAGE_BITS 3ul +static __always_inline struct encoded_page *encode_page(struct page *page, unsigned long flags) +{ + BUILD_BUG_ON(flags > ENCODE_PAGE_BITS); + return (struct encoded_page *)(flags | (unsigned long)page); +} + +static inline unsigned long encoded_page_flags(struct encoded_page *page) +{ + return ENCODE_PAGE_BITS & (unsigned long)page; +} + +static inline struct page *encoded_page_ptr(struct encoded_page *page) +{ + return (struct page *)(~ENCODE_PAGE_BITS & (unsigned long)page); +} + /** * struct folio - Represents a contiguous set of bytes. * @flags: Identical to the page flags.