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 <brau...@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.
>
> Signed-off-by: Christian Brauner <brau...@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,
The mount option looks reasonable. Thanks for throwing together the
patch. Maybe in the future we can come up with a way to mitigate the
problems and do this unconditionally?
Reviewed-by: Jeff Layton <jlay...@kernel.org>
