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 vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 3614DC7EE29 for ; Tue, 23 May 2023 10:17:29 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S236478AbjEWKR2 (ORCPT ); Tue, 23 May 2023 06:17:28 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:49630 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S235984AbjEWKR0 (ORCPT ); Tue, 23 May 2023 06:17:26 -0400 Received: from smtp-out2.suse.de (smtp-out2.suse.de [195.135.220.29]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 2C4D894; Tue, 23 May 2023 03:17:25 -0700 (PDT) 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 DBA97204B2; Tue, 23 May 2023 10:17:23 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.cz; s=susede2_rsa; t=1684837043; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=pJearGp7XvoKQH8r+JXuFUaSaDT86GZEmHPZFyporzw=; b=yJZJT1OgORi5RtyIixB/rOLuR+h4YOf/eNgFYXfGwJwrUNG0IuaqlOt1xJwUh2rdfYfa3z TRq3yl9IzwjdiXabcxYBb7c/VkGt/gWrkXIOV6zWQqw/TlF8q9NzxPsA6vuoHaKhRfDVsx 7dDUe7LfbfgHwItCuI47Lu50P+j1AYg= DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=suse.cz; s=susede2_ed25519; t=1684837043; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=pJearGp7XvoKQH8r+JXuFUaSaDT86GZEmHPZFyporzw=; b=smBRmOb08zACD6WGFlZMUpHyGypof+tbliDQZkhMAqleKrtZ7ouMKaKN/V4ftcEBe+q1Hl HJ7kYRewaUgDqADg== 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 C61E113588; Tue, 23 May 2023 10:17:23 +0000 (UTC) Received: from dovecot-director2.suse.de ([192.168.254.65]) by imap2.suse-dmz.suse.de with ESMTPSA id EVhVMLOSbGQlbAAAMHmgww (envelope-from ); Tue, 23 May 2023 10:17:23 +0000 Received: by quack3.suse.cz (Postfix, from userid 1000) id 4F79CA075D; Tue, 23 May 2023 12:17:23 +0200 (CEST) Date: Tue, 23 May 2023 12:17:23 +0200 From: Jan Kara To: Jeff Layton Cc: Alexander Viro , Christian Brauner , "Darrick J. Wong" , Hugh Dickins , Andrew Morton , Dave Chinner , Chuck Lever , Jan Kara , Amir Goldstein , David Howells , Neil Brown , Matthew Wilcox , Andreas Dilger , Theodore T'so , Chris Mason , Josef Bacik , David Sterba , Namjae Jeon , Steve French , Sergey Senozhatsky , Tom Talpey , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-xfs@vger.kernel.org, linux-btrfs@vger.kernel.org, linux-ext4@vger.kernel.org, linux-mm@kvack.org, linux-nfs@vger.kernel.org, linux-cifs@vger.kernel.org Subject: Re: [PATCH v4 2/9] fs: add infrastructure for multigrain inode i_m/ctime Message-ID: <20230523101723.xmy7mylbczhki6aa@quack3> References: <20230518114742.128950-1-jlayton@kernel.org> <20230518114742.128950-3-jlayton@kernel.org> <20230523100240.mgeu4y46friv7hau@quack3> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20230523100240.mgeu4y46friv7hau@quack3> Precedence: bulk List-ID: X-Mailing-List: linux-ext4@vger.kernel.org On Tue 23-05-23 12:02:40, Jan Kara wrote: > On Thu 18-05-23 07:47:35, Jeff Layton wrote: > > The VFS always uses coarse-grained timestamp updates for filling out the > > ctime and mtime after a change. This has the benefit of allowing > > filesystems to optimize away a lot metadata updates, down to around 1 > > per jiffy, even when a file is under heavy writes. > > > > Unfortunately, this has always been an issue when we're exporting via > > NFSv3, which relies on timestamps to validate caches. Even with NFSv4, a > > lot of exported filesystems don't properly support a change attribute > > and are subject to the same problems with timestamp granularity. Other > > applications have similar issues (e.g backup applications). > > > > Switching to always using fine-grained timestamps would improve the > > situation, but that becomes rather expensive, as the underlying > > filesystem will have to log a lot more metadata updates. > > > > What we need is a way to only use fine-grained timestamps when they are > > being actively queried. > > > > The kernel always stores normalized ctime values, so only the first 30 > > bits of the tv_nsec field are ever used. Whenever the mtime changes, the > > ctime must also change. > > > > Use the 31st bit of the ctime tv_nsec field to indicate that something > > has queried the inode for the i_mtime or i_ctime. When this flag is set, > > on the next timestamp update, the kernel can fetch a fine-grained > > timestamp instead of the usual coarse-grained one. > > > > This patch adds the infrastructure this scheme. Filesytems can opt > > into it by setting the FS_MULTIGRAIN_TS flag in the fstype. > > > > Later patches will convert individual filesystems over to use it. > > > > Signed-off-by: Jeff Layton > > So there are two things I dislike about this series because I think they > are fragile: > > 1) If we have a filesystem supporting multigrain ts and someone > accidentally directly uses the value of inode->i_ctime, he can get bogus > value (with QUERIED flag). This mistake is very easy to do. So I think we > should rename i_ctime to something like __i_ctime and always use accessor > function for it. > > 2) As I already commented in a previous version of the series, the scheme > with just one flag for both ctime and mtime and flag getting cleared in > current_time() relies on the fact that filesystems always do an equivalent > of: > > inode->i_mtime = inode->i_ctime = current_time(); > > Otherwise we can do coarse grained update where we should have done a fine > grained one. Filesystems often update timestamps like this but not > universally. Grepping shows some instances where only inode->i_mtime is set > from current_time() e.g. in autofs or bfs. Again a mistake that is rather > easy to make and results in subtle issues. I think this would be also > nicely solved by renaming i_ctime to __i_ctime and using a function to set > ctime. Mtime could then be updated with inode->i_mtime = ctime_peek(). > > I understand this is quite some churn but a very mechanical one that could > be just done with Coccinelle and a few manual fixups. So IMHO it is worth > the more robust result. Also as I'm thinking about it your current scheme is slightly racy. Suppose the filesystem does: CPU1 CPU2 statx() inode->i_ctime = current_time() current_mg_time() nsec = atomic_long_fetch_andnot(QUERIED, &inode->i_ctime.tv_nsec) nsec = atomic_long_fetch_or(QUERIED, &inode->i_ctime.tv_nsec) if (nsec & QUERIED) - not set ktime_get_coarse_real_ts64(&now) return timestamp_truncate(now, inode); - QUERIED flag in the inode->i_ctime gets overwritten by the assignment => we need not update ctime due to granularity although it was queried One more reason to use explicit function to update inode->i_ctime ;) Honza -- Jan Kara SUSE Labs, CR