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 D4F29C433F5 for ; Wed, 13 Apr 2022 14:55:48 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 51D006B0073; Wed, 13 Apr 2022 10:55:48 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 4A53E6B0074; Wed, 13 Apr 2022 10:55:48 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 2F8486B0075; Wed, 13 Apr 2022 10:55:48 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (relay.hostedemail.com [64.99.140.28]) by kanga.kvack.org (Postfix) with ESMTP id 1A7826B0073 for ; Wed, 13 Apr 2022 10:55:48 -0400 (EDT) Received: from smtpin13.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay11.hostedemail.com (Postfix) with ESMTP id D46CE82558 for ; Wed, 13 Apr 2022 14:55:47 +0000 (UTC) X-FDA: 79352155134.13.CDA548D Received: from smtp-out2.suse.de (smtp-out2.suse.de [195.135.220.29]) by imf30.hostedemail.com (Postfix) with ESMTP id 273DC8000A for ; Wed, 13 Apr 2022 14:55:47 +0000 (UTC) Received: from imap2.suse-dmz.suse.de (imap2.suse-dmz.suse.de [192.168.254.74]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (P-521) server-digest SHA512) (No client certificate requested) by smtp-out2.suse.de (Postfix) with ESMTPS id 8B0F31F869; Wed, 13 Apr 2022 14:55:45 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.cz; s=susede2_rsa; t=1649861745; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=BcWSoqX6MF2jNoXvZrSaKVBHkNeLeKaasZJ9GM7J/mw=; b=kRuThKFn7qs28R959Nsasud/3cMNFp1t+MNSDsmA6H9gRax1E7daj2Z19n4AW5dc9KX73p pVFMArBKUV/n16ycOsVrppwWNaIJfzv5e04Nt+mFPfVaPUMpDlvzZuCFDyIGiaN8HSjkdJ l79GzukUL8K/wwXfaMzci7m3PPUIxFs= DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=suse.cz; s=susede2_ed25519; t=1649861745; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=BcWSoqX6MF2jNoXvZrSaKVBHkNeLeKaasZJ9GM7J/mw=; b=B9nD+lFBRxtBKKHMzUxYi9cEjbZ3h2bXt41cWrObQwxkpZxxDAT08w173fAcYn89XbWEa5 T5jXysjHroNAULBg== Received: from imap2.suse-dmz.suse.de (imap2.suse-dmz.suse.de [192.168.254.74]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (P-521) server-digest SHA512) (No client certificate requested) by imap2.suse-dmz.suse.de (Postfix) with ESMTPS id 31C6813A91; Wed, 13 Apr 2022 14:55:45 +0000 (UTC) Received: from dovecot-director2.suse.de ([192.168.254.65]) by imap2.suse-dmz.suse.de with ESMTPSA id dteZC3HkVmIDJQAAMHmgww (envelope-from ); Wed, 13 Apr 2022 14:55:45 +0000 Message-ID: Date: Wed, 13 Apr 2022 16:55:44 +0200 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.7.0 Subject: Re: [PATCH v3 11/16] mm/page-flags: reuse PG_mappedtodisk as PG_anon_exclusive for PageAnon() pages Content-Language: en-US To: David Hildenbrand , linux-kernel@vger.kernel.org Cc: Andrew Morton , Hugh Dickins , Linus Torvalds , David Rientjes , Shakeel Butt , John Hubbard , Jason Gunthorpe , Mike Kravetz , Mike Rapoport , Yang Shi , "Kirill A . Shutemov" , Matthew Wilcox , Jann Horn , Michal Hocko , Nadav Amit , Rik van Riel , Roman Gushchin , Andrea Arcangeli , Peter Xu , Donald Dutile , Christoph Hellwig , Oleg Nesterov , Jan Kara , Liang Zhang , Pedro Gomes , Oded Gabbay , linux-mm@kvack.org References: <20220329160440.193848-1-david@redhat.com> <20220329160440.193848-12-david@redhat.com> <84c0bcbb-5c8f-d3b2-2a8c-d68462d0bc04@suse.cz> From: Vlastimil Babka In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Stat-Signature: mynt6birjbhybya6mzgfgbjtqjt4imck Authentication-Results: imf30.hostedemail.com; dkim=pass header.d=suse.cz header.s=susede2_rsa header.b=kRuThKFn; dkim=pass header.d=suse.cz header.s=susede2_ed25519 header.b=B9nD+lFB; dmarc=none; spf=pass (imf30.hostedemail.com: domain of vbabka@suse.cz designates 195.135.220.29 as permitted sender) smtp.mailfrom=vbabka@suse.cz X-Rspam-User: X-Rspamd-Server: rspam11 X-Rspamd-Queue-Id: 273DC8000A X-HE-Tag: 1649861747-424623 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 4/13/22 12:28, David Hildenbrand wrote: > On 13.04.22 10:25, Vlastimil Babka wrote: >> On 3/29/22 18:04, David Hildenbrand wrote: >>> the pin will be fully reliable and stay consistent with the pages >>> mapped into the page table, as the bit cannot get cleared (e.g., by >>> fork(), KSM) while the page is pinned. For anonymous pages that >>> are mapped R/W, PG_anon_exclusive can be assumed to always be set >>> because such pages cannot possibly be shared. >>> >>> The page table lock protecting the page table entry is the primary >>> synchronization mechanism for PG_anon_exclusive; GUP-fast that does >>> not take the PT lock needs special care when trying to clear the >>> flag. >>> >>> Page table entry types and PG_anon_exclusive: >>> * Present: PG_anon_exclusive applies. >>> * Swap: the information is lost. PG_anon_exclusive was cleared. >>> * Migration: the entry holds this information instead. >>> PG_anon_exclusive was cleared. >>> * Device private: PG_anon_exclusive applies. >>> * Device exclusive: PG_anon_exclusive applies. >>> * HW Poison: PG_anon_exclusive is stale and not changed. >>> >>> If the page may be pinned (FOLL_PIN), clearing PG_anon_exclusive is >>> not allowed and the flag will stick around until the page is freed >>> and folio->mapping is cleared. >> >> Or also if it's unpinned? > > I'm afraid I didn't get your question. Once the page is no longer > pinned, we can succeed in clearing PG_anon_exclusive (just like pinning > never happened). Does that answer your question? Yeah it looked like a scenario that's oddly missing in that description, yet probably obvious. Now I feel it's indeed obvious, so nevermind :) >>> We won't be clearing PG_anon_exclusive on destructive unmapping (i.e., >>> zapping) of page table entries, page freeing code will handle that when >>> also invalidate page->mapping to not indicate PageAnon() anymore. >>> Letting information about exclusivity stick around will be an important >>> property when adding sanity checks to unpinning code. >>> >>> Note that we properly clear the flag in free_pages_prepare() via >>> PAGE_FLAGS_CHECK_AT_PREP for each individual subpage of a compound page, >>> so there is no need to manually clear the flag. >>> >>> Signed-off-by: David Hildenbrand >> >> Acked-by: Vlastimil Babka > > Thanks! > >> >>> --- a/mm/memory.c >>> +++ b/mm/memory.c >>> @@ -3663,6 +3663,17 @@ vm_fault_t do_swap_page(struct vm_fault *vmf) >>> goto out_nomap; >>> } >>> >>> + /* >>> + * PG_anon_exclusive reuses PG_mappedtodisk for anon pages. A swap pte >>> + * must never point at an anonymous page in the swapcache that is >>> + * PG_anon_exclusive. Sanity check that this holds and especially, that >>> + * no filesystem set PG_mappedtodisk on a page in the swapcache. Sanity >>> + * check after taking the PT lock and making sure that nobody >>> + * concurrently faulted in this page and set PG_anon_exclusive. >>> + */ >>> + BUG_ON(!PageAnon(page) && PageMappedToDisk(page)); >>> + BUG_ON(PageAnon(page) && PageAnonExclusive(page)); >>> + >> >> Hmm, dunno why not VM_BUG_ON? > > Getting PageAnonExclusive accidentally set by a file system would result > in an extremely unpleasant security issue. I most surely want to catch > something like that in any case, especially in the foreseeable future. OK then.