Re: [PATCH v7 12/13] ext4: switch to multigrain timestamps

Xi Ruoyao
2023-09-20T08:51:02Z

On Wed, 2023-09-20 at 10:41 +0200, Christian Brauner wrote:
> > > f1 was last written to *after* f2 was last written to. If the timestamp of f1
> > > is then lower than the timestamp of f2, timestamps are fundamentally broken.
> > >
> > > Many things in user-space depend on timestamps, such as build system
> > > centered around 'make', but also 'find ... -newer ...'.
> > >
> >
> >
> > 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 make
> > timestamp comparisons is comparing source files vs. a build target. If
> > 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.
> >
> > One of the operating principles with this series is that timestamps can
> > be of varying granularity between different files. Note that Linux
> > already violates this assumption when you're working across filesystems
> > of different types.
> >
> > As to potential fixes if this is a real problem:
> >
> > I don't really want to put this behind a mount or mkfs option (a'la
> > relatime, etc.), but that is one possibility.
> >
> > 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 that
>
> Less than ideal imho.
>
> If this risks breaking existing workloads by enabling it unconditionally
> 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.
>
> So how about the following:
>
> From cb14add421967f6e374eb77c36cc4a0526b10d17 Mon Sep 17 00:00:00 2001
> From: Christian Brauner <brauner@kernel.org>
> Date: Wed, 20 Sep 2023 10:00:08 +0200
> Subject: [PATCH] vfs: move multi-grain timestamps behind a mount option
>
> While we initially thought we can do this unconditionally it turns out
> that this might break existing workloads that rely on timestamps in very
> specific ways and we always knew this was a possibility. Move
> multi-grain timestamps behind a vfs mount option.

I agree with this solution.

You can add some metainfo:

Reported-by: Ken Moffat <ken@linuxfromscratch.org>
Bisected-by: Xi Ruoyao <xry111@linuxfromscratch.org>
Link: https://lists.linuxfromscratch.org/sympa/arc/lfs-dev/2023-09/msg00036.html

> Signed-off-by: Christian Brauner <brauner@kernel.org>
> ---
>  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(-)
>
> 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[] = {
>   { "mand", SB_MANDLOCK },
>   { "ro", SB_RDONLY },
>   { "sync", SB_SYNCHRONOUS },
> + { "mgtime", SB_MGTIME },
>   { },
>  };
>
> @@ -52,18 +53,32 @@ static const struct constant_table common_clear_sb_flag[] = {
>   { "nolazytime", SB_LAZYTIME },
>   { "nomand", SB_MANDLOCK },
>   { "rw", SB_RDONLY },
> + { "nomgtime", SB_MGTIME },
>   { },
>  };
>
> +static inline int check_mgtime(unsigned int token, const struct fs_context *fc)
> +{
> + if (token != 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;
>
>   token = lookup_constant(common_set_sb_flag, key, 0);
>   if (token) {
> + ret = check_mgtime(token, fc);
> + if (ret)
> + return ret;
>   fc->sb_flags |= token;
>   fc->sb_flags_mask |= token;
>   return 0;
> @@ -71,6 +86,9 @@ static int vfs_parse_sb_flag(struct fs_context *fc, const char *key)
>
>   token = lookup_constant(common_clear_sb_flag, key, 0);
>   if (token) {
> + ret = check_mgtime(token, fc);
> + if (ret)
> + return ret;
>   fc->sb_flags &= ~token;
>   fc->sb_flags_mask |= 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);
>
>  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 = current_time(inode);
>
>   /* 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 super_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 request_mask,
>   stat->size = i_size_read(inode);
>   stat->atime = inode->i_atime;
>
> - if (is_mgtime(inode)) {
> + if (IS_MGTIME(inode)) {
>   fill_mg_cmtime(stat, request_mask, inode);
>   } else {
>   stat->mtime = 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 files */
>  #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 synchronous */
>  #define SB_NOATIME      BIT(10) /* Do not update access times. */
> @@ -2105,6 +2106,7 @@ static inline bool sb_rdonly(const struct super_block *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)
>
>  #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;
>  }
>
>  extern struct dentry *mount_bdev(struct file_system_type *fs_type,