Re: [RFC PATCH 1/3][RESEND] fs: add infrastructure for opportunistic high-res ctime/mtime updates

Jeff Layton
2023-04-11T14:54:38Z

On Tue, 2023-04-11 at 07:42 -0700, Darrick J. Wong wrote:
> On Tue, Apr 11, 2023 at 10:37:00AM -0400, 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 metadata updates.
> >
> > 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 problem of timestamp granularity. Other
> > applications have similar issues (e.g backup applications).
> >
> > Switching to always using high resolution timestamps would improve the
> > situation for NFS, but that becomes rather expensive, as we'd have to
> > log a lot more metadata updates.
> >
> > This patch grabs a new i_state bit to use as a flag that filesystems can
> > set in their getattr routine to indicate that the mtime or ctime was
> > queried since it was last updated.
> >
> > It then adds a new current_cmtime function that acts like the
> > current_time helper, but will conditionally grab high-res timestamps
> > when the i_state flag is set in the inode.
> >
> > This allows NFS and other applications to reap the benefits of high-res
> > ctime and mtime timestamps, but at a substantially lower cost than
> > fetching them every time.
> >
> > Cc: Dave Chinner <david@fromorbit.com>
> > Signed-off-by: Jeff Layton <jlayton@kernel.org>
> > ---
> > fs/inode.c | 40 ++++++++++++++++++++++++++++++++++++++--
> > fs/stat.c | 10 ++++++++++
> > include/linux/fs.h | 5 ++++-
> > 3 files changed, 52 insertions(+), 3 deletions(-)
> >
> > diff --git a/fs/inode.c b/fs/inode.c
> > index 4558dc2f1355..3630f67fd042 100644
> > --- a/fs/inode.c
> > +++ b/fs/inode.c
> > @@ -2062,6 +2062,42 @@ static int __file_update_time(struct file *file, struct timespec64 *now,
> > return ret;
> > }
> >
> > +/**
> > + * current_cmtime - Return FS time (possibly high-res)
> > + * @inode: inode.
> > + *
> > + * Return the current time truncated to the time granularity supported by
> > + * the fs, as suitable for a ctime or mtime change. If something recently
> > + * fetched the ctime or mtime out of the inode via getattr, then get a
> > + * high-resolution timestamp.
> > + *
> > + * Note that inode and inode->sb cannot be NULL.
> > + * Otherwise, the function warns and returns coarse time without truncation.
> > + */
> > +struct timespec64 current_cmtime(struct inode *inode)
> > +{
> > + struct timespec64 now;
> > +
> > + if (unlikely(!inode->i_sb)) {
> > + WARN(1, "%s() called with uninitialized super_block in the inode", __func__);
> > + ktime_get_coarse_real_ts64(&now);
> > + return now;
> > + }
> > +
> > + /* Do a lockless check for the flag before taking the spinlock */
> > + if (READ_ONCE(inode->i_state) & I_CMTIME_QUERIED) {
> > + ktime_get_real_ts64(&now);
> > + spin_lock(&inode->i_lock);
> > + inode->i_state &= ~I_CMTIME_QUERIED;
> > + spin_unlock(&inode->i_lock);
> > + } else {
> > + ktime_get_coarse_real_ts64(&now);
> > + }
> > +
> > + return timestamp_truncate(now, inode);
>
> I wonder, under which conditions (arch+fs) would it be worth the effort
> to check s_time_gran as part of deciding whether or not to sample a high
> res timestamp?
>
> I suppose that would only help us for the situation where "ktime
> sampling is not fast" and "fs timestamp granularity is awful"?
>
> (Mechanically, the function body looks ok to me...)
>

Thanks for looking! Yeah, that is a good point. No reason to fetch a
high res value if we can't store it anyway. That shouldn't be hard to
optimize away. I'll plan to have a look at that.

> > +}
> > +EXPORT_SYMBOL(current_cmtime);
> > +
> > /**
> > * file_update_time - update mtime and ctime time
> > * @file: file accessed
> > @@ -2080,7 +2116,7 @@ int file_update_time(struct file *file)
> > {
> > int ret;
> > struct inode *inode = file_inode(file);
> > - struct timespec64 now = current_time(inode);
> > + struct timespec64 now = current_cmtime(inode);
> >
> > ret = inode_needs_update_time(inode, &now);
> > if (ret <= 0)
> > @@ -2109,7 +2145,7 @@ static int file_modified_flags(struct file *file, int flags)
> > {
> > int ret;
> > struct inode *inode = file_inode(file);
> > - struct timespec64 now = current_time(inode);
> > + struct timespec64 now = current_cmtime(inode);
> >
> > /*
> > * Clear the security bits if the process is not being run by root.
> > diff --git a/fs/stat.c b/fs/stat.c
> > index 7c238da22ef0..d8b80a2e36b7 100644
> > --- a/fs/stat.c
> > +++ b/fs/stat.c
> > @@ -64,6 +64,16 @@ void generic_fillattr(struct mnt_idmap *idmap, struct inode *inode,
> > }
> > EXPORT_SYMBOL(generic_fillattr);
> >
> > +void fill_cmtime_and_mark(struct inode *inode, struct kstat *stat)
> > +{
> > + spin_lock(&inode->i_lock);
> > + inode->i_state |= I_CMTIME_QUERIED;
> > + stat->ctime = inode->i_ctime;
> > + stat->mtime = inode->i_mtime;
> > + spin_unlock(&inode->i_lock);
> > +}
> > +EXPORT_SYMBOL(fill_cmtime_and_mark);
> > +
> > /**
> > * generic_fill_statx_attr - Fill in the statx attributes from the inode flags
> > * @inode: Inode to use as the source
> > diff --git a/include/linux/fs.h b/include/linux/fs.h
> > index c85916e9f7db..7dece4390979 100644
> > --- a/include/linux/fs.h
> > +++ b/include/linux/fs.h
> > @@ -1457,7 +1457,8 @@ static inline bool fsuidgid_has_mapping(struct super_block *sb,
> > kgid_has_mapping(fs_userns, kgid);
> > }
> >
> > -extern struct timespec64 current_time(struct inode *inode);
> > +struct timespec64 current_time(struct inode *inode);
> > +struct timespec64 current_cmtime(struct inode *inode);
> >
> > /*
> > * Snapshotting support.
> > @@ -2116,6 +2117,7 @@ static inline void kiocb_clone(struct kiocb *kiocb, struct kiocb *kiocb_src,
> > #define I_DONTCACHE (1 << 16)
> > #define I_SYNC_QUEUED (1 << 17)
> > #define I_PINNING_FSCACHE_WB (1 << 18)
> > +#define I_CMTIME_QUERIED (1 << 19)
> >
> > #define I_DIRTY_INODE (I_DIRTY_SYNC | I_DIRTY_DATASYNC)
> > #define I_DIRTY (I_DIRTY_INODE | I_DIRTY_PAGES)
> > @@ -2839,6 +2841,7 @@ extern int page_symlink(struct inode *inode, const char *symname, int len);
> > extern const struct inode_operations page_symlink_inode_operations;
> > extern void kfree_link(void *);
> > void generic_fillattr(struct mnt_idmap *, struct inode *, struct kstat *);
> > +void fill_cmtime_and_mark(struct inode *inode, struct kstat *stat);
> > void generic_fill_statx_attr(struct inode *inode, struct kstat *stat);
> > extern int vfs_getattr_nosec(const struct path *, struct kstat *, u32, unsigned int);
> > extern int vfs_getattr(const struct path *, struct kstat *, u32, unsigned int);
> > --
> > 2.39.2
> >

--
Jeff Layton <jlayton@kernel.org>