From mboxrd@z Thu Jan 1 00:00:00 1970 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 48F39C77B73 for ; Tue, 2 May 2023 14:40:14 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id CE26F6B0075; Tue, 2 May 2023 10:40:13 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id C92DD6B0078; Tue, 2 May 2023 10:40:13 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id B822E6B007D; Tue, 2 May 2023 10:40:13 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from desiato.infradead.org (desiato.infradead.org [90.155.92.199]) by kanga.kvack.org (Postfix) with ESMTP id 7D7416B0075 for ; Tue, 2 May 2023 10:40:13 -0400 (EDT) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=desiato.20200630; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=qSL9OkS9lUf+/Y0rKTYhB5gamqdgUqvemLYpwvoIblc=; b=nyzYxVwkDYSGs+p8GZKdRUmbVj Guk04quwpkt4KIxKlgvRq3hDoGXk8vII7ZwOwysVQJCDxD43IcwITOv8Ukdr45u9zmDX6QMz4+ZLF LqkGMCkKLaN//YYSjQyG5rR4N3Zm3NnDNwEoajETpYIv0qstEhNJpklrqzN+7T7GAfsy1KwEUWIp3 2ZzcofszrGMZajnGYhtwC+KoakXv3sPRbf2mmipIl0Okn41UYHFcjQqBRK0YkzxsIQRNfunJH19jn gmExS18rx3m3bAnr1WnkjVxjqHvFlbaQ4e4wrs7y3THRwDT87WlmVeGXRICl7RWDw8PvbZ2LrvrCF 9u27uBeQ==; Received: from j130084.upc-j.chello.nl ([24.132.130.84] helo=noisy.programming.kicks-ass.net) by desiato.infradead.org with esmtpsa (Exim 4.96 #2 (Red Hat Linux)) id 1ptnwv-00GI5F-13; Tue, 02 May 2023 11:13:37 +0000 Received: from hirez.programming.kicks-ass.net (hirez.programming.kicks-ass.net [192.168.1.225]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (4096 bits)) (Client did not present a certificate) by noisy.programming.kicks-ass.net (Postfix) with ESMTPS id BEDB2300348; Tue, 2 May 2023 13:13:34 +0200 (CEST) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id 79C6A23C5C34E; Tue, 2 May 2023 13:13:34 +0200 (CEST) Date: Tue, 2 May 2023 13:13:34 +0200 From: Peter Zijlstra To: Lorenzo Stoakes Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Andrew Morton , Jason Gunthorpe , Jens Axboe , Matthew Wilcox , Dennis Dalessandro , Leon Romanovsky , Christian Benvenuti , Nelson Escobar , Bernard Metzler , Ingo Molnar , Arnaldo Carvalho de Melo , Mark Rutland , Alexander Shishkin , Jiri Olsa , Namhyung Kim , Ian Rogers , Adrian Hunter , Bjorn Topel , Magnus Karlsson , Maciej Fijalkowski , Jonathan Lemon , "David S . Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Christian Brauner , Richard Cochran , Alexei Starovoitov , Daniel Borkmann , Jesper Dangaard Brouer , John Fastabend , linux-fsdevel@vger.kernel.org, linux-perf-users@vger.kernel.org, netdev@vger.kernel.org, bpf@vger.kernel.org, Oleg Nesterov , Jason Gunthorpe , John Hubbard , Jan Kara , "Kirill A . Shutemov" , Pavel Begunkov , Mika Penttila , David Hildenbrand , Dave Chinner , Theodore Ts'o , Peter Xu Subject: Re: [PATCH v6 3/3] mm/gup: disallow FOLL_LONGTERM GUP-fast writing to file-backed mappings Message-ID: <20230502111334.GP1597476@hirez.programming.kicks-ass.net> References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: 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, May 02, 2023 at 12:11:49AM +0100, Lorenzo Stoakes wrote: > @@ -95,6 +96,77 @@ static inline struct folio *try_get_folio(struct page *page, int refs) > return folio; > } > > +#ifdef CONFIG_MMU_GATHER_RCU_TABLE_FREE > +static bool stabilise_mapping_rcu(struct folio *folio) > +{ > + struct address_space *mapping = READ_ONCE(folio->mapping); > + > + rcu_read_lock(); > + > + return mapping == READ_ONCE(folio->mapping); This doesn't make sense; why bother reading the same thing twice? Who cares if the thing changes from before; what you care about is that the value you see has stable storage, this doesn't help with that. > +} > + > +static void unlock_rcu(void) > +{ > + rcu_read_unlock(); > +} > +#else > +static bool stabilise_mapping_rcu(struct folio *) > +{ > + return true; > +} > + > +static void unlock_rcu(void) > +{ > +} > +#endif Anyway, this all can go away. RCU can't progress while you have interrupts disabled anyway. > +/* > + * Used in the GUP-fast path to determine whether a FOLL_PIN | FOLL_LONGTERM | > + * FOLL_WRITE pin is permitted for a specific folio. > + * > + * This assumes the folio is stable and pinned. > + * > + * Writing to pinned file-backed dirty tracked folios is inherently problematic > + * (see comment describing the writeable_file_mapping_allowed() function). We > + * therefore try to avoid the most egregious case of a long-term mapping doing > + * so. > + * > + * This function cannot be as thorough as that one as the VMA is not available > + * in the fast path, so instead we whitelist known good cases. > + * > + * The folio is stable, but the mapping might not be. When truncating for > + * instance, a zap is performed which triggers TLB shootdown. IRQs are disabled > + * so we are safe from an IPI, but some architectures use an RCU lock for this > + * operation, so we acquire an RCU lock to ensure the mapping is stable. > + */ > +static bool folio_longterm_write_pin_allowed(struct folio *folio) > +{ > + bool ret; > + > + /* hugetlb mappings do not require dirty tracking. */ > + if (folio_test_hugetlb(folio)) > + return true; > + This: > + if (stabilise_mapping_rcu(folio)) { > + struct address_space *mapping = folio_mapping(folio); And this is 3rd read of folio->mapping, just for giggles? > + > + /* > + * Neither anonymous nor shmem-backed folios require > + * dirty tracking. > + */ > + ret = folio_test_anon(folio) || > + (mapping && shmem_mapping(mapping)); > + } else { > + /* If the mapping is unstable, fallback to the slow path. */ > + ret = false; > + } > + > + unlock_rcu(); > + > + return ret; then becomes: if (folio_test_anon(folio)) return true; /* * Having IRQs disabled (as per GUP-fast) also inhibits RCU * grace periods from making progress, IOW. they imply * rcu_read_lock(). */ lockdep_assert_irqs_disabled(); /* * Inodes and thus address_space are RCU freed and thus safe to * access at this point. */ mapping = folio_mapping(folio); if (mapping && shmem_mapping(mapping)) return true; return false; > +}