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Wed, 20 Sep 2023 10:35:26 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 04647C433C7; Wed, 20 Sep 2023 10:35:18 +0000 (UTC) Message-ID: <317d84b1b909b6c6519a2406fcb302ce22dafa41.camel@kernel.org> From: Jeff Layton To: Jan Kara , Christian Brauner 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> User-Agent: Evolution 3.48.4 (3.48.4-1.fc38) MIME-Version: 1.0 X-Mimecast-Impersonation-Protect: Policy=CLT - Impersonation Protection Definition; Similar Internal Domain=false; Similar Monitored External Domain=false; Custom External Domain=false; Mimecast External Domain=false; Newly Observed Domain=false; Internal User Name=false; Custom Display Name List=false; Reply-to Address Mismatch=false; Targeted Threat Dictionary=false; Mimecast Threat Dictionary=false; Custom Threat Dictionary=false X-Scanned-By: MIMEDefang 3.1 on 10.11.54.2 Subject: Re: [Cluster-devel] [PATCH v7 12/13] ext4: switch to multigrain timestamps X-BeenThere: cluster-devel@redhat.com X-Mailman-Version: 2.1.29 Precedence: list List-Id: "\[Cluster devel\]" List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Latchesar Ionkov , Martin Brandenburg , Konstantin Komarov , linux-xfs@vger.kernel.org, "Darrick J. Wong" , Dominique Martinet , Christian Schoenebeck , linux-unionfs@vger.kernel.org, David Howells , Chris Mason , Andreas Dilger , Hans de Goede , Marc Dionne , codalist@coda.cs.cmu.edu, linux-afs@lists.infradead.org, linux-mtd@lists.infradead.org, Mike Marshall , Paulo Alcantara , Amir Goldstein , Eric Van Hensbergen , bug-gnulib@gnu.org, Miklos Szeredi , Richard Weinberger , Mark Fasheh , Hugh Dickins , Tyler Hicks , cluster-devel@redhat.com, coda@cs.cmu.edu, linux-mm@kvack.org, Gao Xiang , Iurii Zaikin , Namjae Jeon , Trond Myklebust , Xi Ruoyao , Shyam Prasad N , ecryptfs@vger.kernel.org, Kees Cook , ocfs2-devel@lists.linux.dev, linux-cifs@vger.kernel.org, Chao Yu , linux-erofs@lists.ozlabs.org, Josef Bacik , Tom Talpey , Tejun Heo , Yue Hu , Alexander Viro , Ronnie Sahlberg , David Sterba , Jaegeuk Kim , ceph-devel@vger.kernel.org, Xiubo Li , Ilya Dryomov , OGAWA Hirofumi , Jan Harkes , linux-nfs@vger.kernel.org, linux-ext4@vger.kernel.org, Theodore Ts'o , Joseph Qi , Greg Kroah-Hartman , v9fs@lists.linux.dev, ntfs3@lists.linux.dev, samba-technical@lists.samba.org, linux-kernel@vger.kernel.org, linux-f2fs-devel@lists.sourceforge.net, Steve French , Sergey Senozhatsky , Luis Chamberlain , Jeffle Xu , devel@lists.orangefs.org, Anna Schumaker , Jan Kara , linux-fsdevel@vger.kernel.org, Andrew Morton , Sungjong Seo , Bruno Haible , linux-btrfs@vger.kernel.org, Joel Becker Errors-To: cluster-devel-bounces@redhat.com Sender: "Cluster-devel" X-Scanned-By: MIMEDefang 3.1 on 10.11.54.8 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: kernel.org Content-Type: text/plain; charset="ISO-8859-15" Content-Transfer-Encoding: quoted-printable 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 { > > =09{ "mand",=09SB_MANDLOCK }, > > =09{ "ro",=09=09SB_RDONLY }, > > =09{ "sync",=09SB_SYNCHRONOUS }, > > +=09{ "mgtime",=09SB_MGTIME }, > > =09{ }, > > }; > > =20 > > @@ -52,18 +53,32 @@ static const struct constant_table common_clear_sb_= flag[] =3D { > > =09{ "nolazytime",=09SB_LAZYTIME }, > > =09{ "nomand",=09SB_MANDLOCK }, > > =09{ "rw",=09=09SB_RDONLY }, > > +=09{ "nomgtime",=09SB_MGTIME }, > > =09{ }, > > }; > > =20 > > +static inline int check_mgtime(unsigned int token, const struct fs_con= text *fc) > > +{ > > +=09if (token !=3D SB_MGTIME) > > +=09=09return 0; > > +=09if (!(fc->fs_type->fs_flags & FS_MGTIME)) > > +=09=09return invalf(fc, "Filesystem doesn't support multi-grain timest= amps"); > > +=09return 0; > > +} > > + > > /* > > * Check for a common mount option that manipulates s_flags. > > */ > > static int vfs_parse_sb_flag(struct fs_context *fc, const char *key) > > { > > =09unsigned int token; > > +=09int ret; > > =20 > > =09token =3D lookup_constant(common_set_sb_flag, key, 0); > > =09if (token) { > > +=09=09ret =3D check_mgtime(token, fc); > > +=09=09if (ret) > > +=09=09=09return ret; > > =09=09fc->sb_flags |=3D token; > > =09=09fc->sb_flags_mask |=3D token; > > =09=09return 0; > > @@ -71,6 +86,9 @@ static int vfs_parse_sb_flag(struct fs_context *fc, c= onst char *key) > > =20 > > =09token =3D lookup_constant(common_clear_sb_flag, key, 0); > > =09if (token) { > > +=09=09ret =3D check_mgtime(token, fc); > > +=09=09if (ret) > > +=09=09=09return ret; > > =09=09fc->sb_flags &=3D ~token; > > =09=09fc->sb_flags_mask |=3D token; > > =09=09return 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) > > { > > -=09if (is_mgtime(inode)) > > +=09if (IS_MGTIME(inode)) > > =09=09return current_mgtime(inode); > > =09return current_time(inode); > > } > > @@ -2588,7 +2588,7 @@ struct timespec64 inode_set_ctime_current(struct = inode *inode) > > =09=09now =3D current_time(inode); > > =20 > > =09=09/* Just copy it into place if it's not multigrain */ > > -=09=09if (!is_mgtime(inode)) { > > +=09=09if (!IS_MGTIME(inode)) { > > =09=09=09inode_set_ctime_to_ts(inode, now); > > =09=09=09return now; > > =09=09} > > 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) > > =09=09{ SB_DIRSYNC, ",dirsync" }, > > =09=09{ SB_MANDLOCK, ",mand" }, > > =09=09{ SB_LAZYTIME, ",lazytime" }, > > +=09=09{ SB_MGTIME, ",mgtime" }, > > =09=09{ 0, NULL } > > =09}; > > =09const 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, > > =09stat->size =3D i_size_read(inode); > > =09stat->atime =3D inode->i_atime; > > =20 > > -=09if (is_mgtime(inode)) { > > +=09if (IS_MGTIME(inode)) { > > =09=09fill_mg_cmtime(stat, request_mask, inode); > > =09} else { > > =09=09stat->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)=09/* Disallow access to device special = files */ > > #define SB_NOEXEC BIT(3)=09/* Disallow program execution */ > > #define SB_SYNCHRONOUS BIT(4)=09/* Writes are synced at once */ > > +#define SB_MGTIME=09BIT(5)=09/* Use multi-grain timestamps */ > > #define SB_MANDLOCK BIT(6)=09/* Allow mandatory locks on an FS */ > > #define SB_DIRSYNC BIT(7)=09/* Directory modifications are synchr= onous */ > > #define SB_NOATIME BIT(10)=09/* Do not update access times. */ > > @@ -2105,6 +2106,7 @@ static inline bool sb_rdonly(const struct super_b= lock *sb) { return sb->s_flags > > =09=09=09=09=09((inode)->i_flags & (S_SYNC|S_DIRSYNC))) > > #define IS_MANDLOCK(inode)=09__IS_FLG(inode, SB_MANDLOCK) > > #define IS_NOATIME(inode)=09__IS_FLG(inode, SB_RDONLY|SB_NOATIME) > > +#define IS_MGTIME(inode)=09__IS_FLG(inode, SB_MGTIME) > > #define IS_I_VERSION(inode)=09__IS_FLG(inode, SB_I_VERSION) > > =20 > > #define IS_NOQUOTA(inode)=09((inode)->i_flags & S_NOQUOTA) > > @@ -2366,7 +2368,7 @@ struct file_system_type { > > */ > > static inline bool is_mgtime(const struct inode *inode) > > { > > -=09return inode->i_sb->s_type->fs_flags & FS_MGTIME; > > +=09return 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