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 C6243CE79CB for ; Wed, 20 Sep 2023 10:35:32 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S234365AbjITKff (ORCPT ); Wed, 20 Sep 2023 06:35:35 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:51648 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233376AbjITKfc (ORCPT ); Wed, 20 Sep 2023 06:35:32 -0400 Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id EFBE3AD; Wed, 20 Sep 2023 03:35:25 -0700 (PDT) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 04647C433C7; Wed, 20 Sep 2023 10:35:18 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1695206125; bh=K2lTJnCgosym5o0oHZC21PlpMcbDPAGRzGpQMsH+RNE=; h=Subject:From:To:Cc:Date:In-Reply-To:References:From; b=DQraYbgnbA8u42xIprJlGq919tAZP4oSOdbkdBTrPg4E020AuJU3vm+IxDgIAtub4 0ualIwtp20Irlnr5UkTgSXB/6dLY+vOyf0EwAPBcmnJdBqIsbV/q8wd+oKJBDukCmC /GGkmmgnqoNA7fAJABwddnMG0NDO1gl1EM1jlFcVcGauNmnScyO0k32UToSskpKj7Z IDirVhj/0pNpBeARBmUePperJeyjvV4b3EGol9t/AUaUdbdstRRauxjUUZvhy5PZAK 37rI5uERCd6DxALjpO+Wq3SS9dt0mP5kZEJ0WiE3vmZJqNiMvhEhUe+Cvf5i6voNam ksh/5ZQ5h3mdg== Message-ID: <317d84b1b909b6c6519a2406fcb302ce22dafa41.camel@kernel.org> Subject: Re: [PATCH v7 12/13] ext4: switch to multigrain timestamps From: Jeff Layton To: Jan Kara , Christian Brauner Cc: Bruno Haible , Xi Ruoyao , bug-gnulib@gnu.org, Alexander Viro , Eric Van Hensbergen , Latchesar Ionkov , Dominique Martinet , Christian Schoenebeck , David Howells , Marc Dionne , Chris Mason , Josef Bacik , David Sterba , Xiubo Li , Ilya Dryomov , Jan Harkes , coda@cs.cmu.edu, Tyler Hicks , Gao Xiang , Chao Yu , Yue Hu , Jeffle Xu , Namjae Jeon , Sungjong Seo , Jan Kara , Theodore Ts'o , Andreas Dilger , Jaegeuk Kim , OGAWA Hirofumi , Miklos Szeredi , Bo b Peterson , Andreas Gruenbacher , Greg Kroah-Hartman , Tejun Heo , Trond Myklebust , Anna Schumaker , Konstantin Komarov , Mark Fasheh , Joel Becker , Joseph Qi , Mike Marshall , Martin Brandenburg , Luis Chamberlain , Kees Cook , Iurii Zaikin , Steve French , Paulo Alcantara , Ronnie Sahlberg , Shyam Prasad N , Tom Talpey , Sergey Senozhatsky , Richard Weinberger , Hans de Goede , Hugh Dickins , Andrew Morton , Amir Goldstein , "Darrick J. Wong" , Benjamin Coddington , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, v9fs@lists.linux.dev, linux-afs@lists.infradead.org, linux-btrfs@vger.kernel.org, ceph-devel@vger.kernel.org, codalist@coda.cs.cmu.edu, ecryptfs@vger.kernel.org, linux-erofs@lists.ozlabs.org, linux-ext4@vger.kernel.org, linux-f2fs-devel@lists.sourceforge.net, cluster-devel@redhat.com, linux-nfs@vger.kernel.org, ntfs3@lists.linux.dev, ocfs2-devel@lists.linux.dev, devel@lists.orangefs.org, linux-cifs@vger.kernel.org, samba-technical@lists.samba.org, linux-mtd@lists.infradead.org, linux-mm@kvack.org, linux-unionfs@vger.kernel.org, linux-xfs@vger.kernel.org Date: Wed, 20 Sep 2023 06:35:18 -0400 In-Reply-To: <20230920101731.ym6pahcvkl57guto@quack3> References: <20230807-mgctime-v7-0-d1dec143a704@kernel.org> <20230919110457.7fnmzo4nqsi43yqq@quack3> <1f29102c09c60661758c5376018eac43f774c462.camel@kernel.org> <4511209.uG2h0Jr0uP@nimes> <08b5c6fd3b08b87fa564bb562d89381dd4e05b6a.camel@kernel.org> <20230920-leerung-krokodil-52ec6cb44707@brauner> <20230920101731.ym6pahcvkl57guto@quack3> Content-Type: text/plain; charset="ISO-8859-15" Content-Transfer-Encoding: quoted-printable User-Agent: Evolution 3.48.4 (3.48.4-1.fc38) MIME-Version: 1.0 Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On Wed, 2023-09-20 at 12:17 +0200, Jan Kara wrote: > On Wed 20-09-23 10:41:30, Christian Brauner wrote: > > > > f1 was last written to *after* f2 was last written to. If the times= tamp of f1 > > > > is then lower than the timestamp of f2, timestamps are fundamentall= y broken. > > > >=20 > > > > Many things in user-space depend on timestamps, such as build syste= m > > > > centered around 'make', but also 'find ... -newer ...'. > > > >=20 > > >=20 > > >=20 > > > What does breakage with make look like in this situation? The "fuzz" > > > here is going to be on the order of a jiffy. The typical case for mak= e > > > timestamp comparisons is comparing source files vs. a build target. I= f > > > those are being written nearly simultaneously, then that could be an > > > issue, but is that a typical behavior? It seems like it would be hard= to > > > rely on that anyway, esp. given filesystems like NFS that can do lazy > > > writeback. > > >=20 > > > One of the operating principles with this series is that timestamps c= an > > > be of varying granularity between different files. Note that Linux > > > already violates this assumption when you're working across filesyste= ms > > > of different types. > > >=20 > > > As to potential fixes if this is a real problem: > > >=20 > > > I don't really want to put this behind a mount or mkfs option (a'la > > > relatime, etc.), but that is one possibility. > > >=20 > > > I wonder if it would be feasible to just advance the coarse-grained > > > current_time whenever we end up updating a ctime with a fine-grained > > > timestamp? It might produce some inode write amplification. Files tha= t > >=20 > > Less than ideal imho. > >=20 > > If this risks breaking existing workloads by enabling it unconditionall= y > > and there isn't a clear way to detect and handle these situations > > without risk of regression then we should move this behind a mount > > option. > >=20 > > So how about the following: > >=20 > > From cb14add421967f6e374eb77c36cc4a0526b10d17 Mon Sep 17 00:00:00 2001 > > From: Christian Brauner > > Date: Wed, 20 Sep 2023 10:00:08 +0200 > > Subject: [PATCH] vfs: move multi-grain timestamps behind a mount option > >=20 > > While we initially thought we can do this unconditionally it turns out > > that this might break existing workloads that rely on timestamps in ver= y > > specific ways and we always knew this was a possibility. Move > > multi-grain timestamps behind a vfs mount option. > >=20 > > Signed-off-by: Christian Brauner >=20 > Surely this is a safe choice as it moves the responsibility to the sysadm= in > and the cases where finegrained timestamps are required. But I kind of > wonder how is the sysadmin going to decide whether mgtime is safe for his > system or not? Because the possible breakage needn't be obvious at the > first sight... >=20 That's the main reason I really didn't want to go with a mount option. Documenting that may be difficult. While there is some pessimism around it, I may still take a stab at just advancing the coarse clock whenever we fetch a fine-grained timestamp. It'd be nice to remove this option in the future if that turns out to be feasible. > If I were a sysadmin, I'd rather opt for something like > finegrained timestamps + lazytime (if I needed the finegrained timestamps > functionality). That should avoid the IO overhead of finegrained timestam= ps > as well and I'd know I can have problems with timestamps only after a > system crash. > I've just got another idea how we could solve the problem: Couldn't we > always just report coarsegrained timestamp to userspace and provide acces= s > to finegrained value only to NFS which should know what it's doing? >=20 I think that'd be hard. First of all, where would we store the second timestamp? We can't just truncate the fine-grained ones to come up with a coarse-grained one. It might also be confusing having nfsd and local filesystems present different attributes. > > --- > > fs/fs_context.c | 18 ++++++++++++++++++ > > fs/inode.c | 4 ++-- > > fs/proc_namespace.c | 1 + > > fs/stat.c | 2 +- > > include/linux/fs.h | 4 +++- > > 5 files changed, 25 insertions(+), 4 deletions(-) > >=20 > > diff --git a/fs/fs_context.c b/fs/fs_context.c > > index a0ad7a0c4680..dd4dade0bb9e 100644 > > --- a/fs/fs_context.c > > +++ b/fs/fs_context.c > > @@ -44,6 +44,7 @@ static const struct constant_table common_set_sb_flag= [] =3D { > > { "mand", SB_MANDLOCK }, > > { "ro", SB_RDONLY }, > > { "sync", SB_SYNCHRONOUS }, > > + { "mgtime", SB_MGTIME }, > > { }, > > }; > > =20 > > @@ -52,18 +53,32 @@ static const struct constant_table common_clear_sb_= flag[] =3D { > > { "nolazytime", SB_LAZYTIME }, > > { "nomand", SB_MANDLOCK }, > > { "rw", SB_RDONLY }, > > + { "nomgtime", SB_MGTIME }, > > { }, > > }; > > =20 > > +static inline int check_mgtime(unsigned int token, const struct fs_con= text *fc) > > +{ > > + if (token !=3D SB_MGTIME) > > + return 0; > > + if (!(fc->fs_type->fs_flags & FS_MGTIME)) > > + return invalf(fc, "Filesystem doesn't support multi-grain timestamps= "); > > + return 0; > > +} > > + > > /* > > * Check for a common mount option that manipulates s_flags. > > */ > > static int vfs_parse_sb_flag(struct fs_context *fc, const char *key) > > { > > unsigned int token; > > + int ret; > > =20 > > token =3D lookup_constant(common_set_sb_flag, key, 0); > > if (token) { > > + ret =3D check_mgtime(token, fc); > > + if (ret) > > + return ret; > > fc->sb_flags |=3D token; > > fc->sb_flags_mask |=3D token; > > return 0; > > @@ -71,6 +86,9 @@ static int vfs_parse_sb_flag(struct fs_context *fc, c= onst char *key) > > =20 > > token =3D lookup_constant(common_clear_sb_flag, key, 0); > > if (token) { > > + ret =3D check_mgtime(token, fc); > > + if (ret) > > + return ret; > > fc->sb_flags &=3D ~token; > > fc->sb_flags_mask |=3D token; > > return 0; > > diff --git a/fs/inode.c b/fs/inode.c > > index 54237f4242ff..fd1a2390aaa3 100644 > > --- a/fs/inode.c > > +++ b/fs/inode.c > > @@ -2141,7 +2141,7 @@ EXPORT_SYMBOL(current_mgtime); > > =20 > > static struct timespec64 current_ctime(struct inode *inode) > > { > > - if (is_mgtime(inode)) > > + if (IS_MGTIME(inode)) > > return current_mgtime(inode); > > return current_time(inode); > > } > > @@ -2588,7 +2588,7 @@ struct timespec64 inode_set_ctime_current(struct = inode *inode) > > now =3D current_time(inode); > > =20 > > /* Just copy it into place if it's not multigrain */ > > - if (!is_mgtime(inode)) { > > + if (!IS_MGTIME(inode)) { > > inode_set_ctime_to_ts(inode, now); > > return now; > > } > > diff --git a/fs/proc_namespace.c b/fs/proc_namespace.c > > index 250eb5bf7b52..08f5bf4d2c6c 100644 > > --- a/fs/proc_namespace.c > > +++ b/fs/proc_namespace.c > > @@ -49,6 +49,7 @@ static int show_sb_opts(struct seq_file *m, struct su= per_block *sb) > > { SB_DIRSYNC, ",dirsync" }, > > { SB_MANDLOCK, ",mand" }, > > { SB_LAZYTIME, ",lazytime" }, > > + { SB_MGTIME, ",mgtime" }, > > { 0, NULL } > > }; > > const struct proc_fs_opts *fs_infop; > > diff --git a/fs/stat.c b/fs/stat.c > > index 6e60389d6a15..2f18dd5de18b 100644 > > --- a/fs/stat.c > > +++ b/fs/stat.c > > @@ -90,7 +90,7 @@ void generic_fillattr(struct mnt_idmap *idmap, u32 re= quest_mask, > > stat->size =3D i_size_read(inode); > > stat->atime =3D inode->i_atime; > > =20 > > - if (is_mgtime(inode)) { > > + if (IS_MGTIME(inode)) { > > fill_mg_cmtime(stat, request_mask, inode); > > } else { > > stat->mtime =3D inode->i_mtime; > > diff --git a/include/linux/fs.h b/include/linux/fs.h > > index 4aeb3fa11927..03e415fb3a7c 100644 > > --- a/include/linux/fs.h > > +++ b/include/linux/fs.h > > @@ -1114,6 +1114,7 @@ extern int send_sigurg(struct fown_struct *fown); > > #define SB_NODEV BIT(2) /* Disallow access to device special fi= les */ > > #define SB_NOEXEC BIT(3) /* Disallow program execution */ > > #define SB_SYNCHRONOUS BIT(4) /* Writes are synced at once */ > > +#define SB_MGTIME BIT(5) /* Use multi-grain timestamps */ > > #define SB_MANDLOCK BIT(6) /* Allow mandatory locks on an FS */ > > #define SB_DIRSYNC BIT(7) /* Directory modifications are synchron= ous */ > > #define SB_NOATIME BIT(10) /* Do not update access times. */ > > @@ -2105,6 +2106,7 @@ static inline bool sb_rdonly(const struct super_b= lock *sb) { return sb->s_flags > > ((inode)->i_flags & (S_SYNC|S_DIRSYNC))) > > #define IS_MANDLOCK(inode) __IS_FLG(inode, SB_MANDLOCK) > > #define IS_NOATIME(inode) __IS_FLG(inode, SB_RDONLY|SB_NOATIME) > > +#define IS_MGTIME(inode) __IS_FLG(inode, SB_MGTIME) > > #define IS_I_VERSION(inode) __IS_FLG(inode, SB_I_VERSION) > > =20 > > #define IS_NOQUOTA(inode) ((inode)->i_flags & S_NOQUOTA) > > @@ -2366,7 +2368,7 @@ struct file_system_type { > > */ > > static inline bool is_mgtime(const struct inode *inode) > > { > > - return inode->i_sb->s_type->fs_flags & FS_MGTIME; > > + return inode->i_sb->s_flags & SB_MGTIME; > > } > > =20 > > extern struct dentry *mount_bdev(struct file_system_type *fs_type, > > --=20 > > 2.34.1 > >=20 --=20 Jeff Layton