[PATCH v4 2/9] fs: add infrastructure for multigrain inode i_m/ctime

Jeff Layton
2023-05-18T11:48:21Z

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 a lot metadata updates, down to around 1
per jiffy, even when a file is under heavy writes.

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 problems with timestamp granularity. Other
applications have similar issues (e.g backup applications).

Switching to always using fine-grained timestamps would improve the
situation, but that becomes rather expensive, as the underlying
filesystem will have to log a lot more metadata updates.

What we need is a way to only use fine-grained timestamps when they are
being actively queried.

The kernel always stores normalized ctime values, so only the first 30
bits of the tv_nsec field are ever used. Whenever the mtime changes, the
ctime must also change.

Use the 31st bit of the ctime tv_nsec field to indicate that something
has queried the inode for the i_mtime or i_ctime. When this flag is set,
on the next timestamp update, the kernel can fetch a fine-grained
timestamp instead of the usual coarse-grained one.

This patch adds the infrastructure this scheme. Filesytems can opt
into it by setting the FS_MULTIGRAIN_TS flag in the fstype.

Later patches will convert individual filesystems over to use it.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
---
fs/inode.c | 48 ++++++++++++++++++++++++++++-----
fs/stat.c | 41 ++++++++++++++++++++++++++--
include/linux/fs.h | 66 +++++++++++++++++++++++++++++++++++++++++++++-
3 files changed, 145 insertions(+), 10 deletions(-)

diff --git a/fs/inode.c b/fs/inode.c
index 577799b7855f..24769e08fbaa 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -2029,6 +2029,7 @@ EXPORT_SYMBOL(file_remove_privs);
static int inode_needs_update_time(struct inode *inode, struct timespec64 *now)
{
int sync_it = 0;
+ struct timespec64 ctime;

/* First try to exhaust all avenues to not sync */
if (IS_NOCMTIME(inode))
@@ -2037,7 +2038,8 @@ static int inode_needs_update_time(struct inode *inode, struct timespec64 *now)
if (!timespec64_equal(&inode->i_mtime, now))
sync_it = S_MTIME;

- if (!timespec64_equal(&inode->i_ctime, now))
+ ctime = ctime_peek(inode);
+ if (!timespec64_equal(&ctime, now))
sync_it |= S_CTIME;

if (IS_I_VERSION(inode) && inode_iversion_need_inc(inode))
@@ -2431,6 +2433,40 @@ struct timespec64 timestamp_truncate(struct timespec64 t, struct inode *inode)
}
EXPORT_SYMBOL(timestamp_truncate);

+/**
+ * current_mg_time - Return FS time (possibly fine-grained)
+ * @inode: inode.
+ *
+ * Return the current time truncated to the time granularity supported by
+ * the fs, as suitable for a ctime/mtime change. If the ctime is flagged
+ * as having been QUERIED, get a fine-grained timestamp.
+ */
+static struct timespec64 current_mg_time(struct inode *inode)
+{
+ struct timespec64 now;
+ atomic_long_t *pnsec = (atomic_long_t *)&inode->i_ctime.tv_nsec;
+ long nsec = atomic_long_fetch_andnot(I_CTIME_QUERIED, pnsec);
+
+ if (nsec & I_CTIME_QUERIED) {
+ ktime_get_real_ts64(&now);
+ } else {
+ struct timespec64 ctime;
+
+ ktime_get_coarse_real_ts64(&now);
+
+ /*
+ * If we've recently fetched a fine-grained timestamp
+ * then the coarse-grained one may still be earlier than the
+ * existing one. Just keep the existing ctime if so.
+ */
+ ctime = ctime_peek(inode);
+ if (timespec64_compare(&ctime, &now) > 0)
+ now = ctime;
+ }
+
+ return now;
+}
+
/**
* current_time - Return FS time
* @inode: inode.
@@ -2445,12 +2481,10 @@ struct timespec64 current_time(struct inode *inode)
{
struct timespec64 now;

- ktime_get_coarse_real_ts64(&now);
-
- if (unlikely(!inode->i_sb)) {
- WARN(1, "current_time() called with uninitialized super_block in the inode");
- return now;
- }
+ if (is_multigrain_ts(inode))
+ now = current_mg_time(inode);
+ else
+ ktime_get_coarse_real_ts64(&now);

return timestamp_truncate(now, inode);
}
diff --git a/fs/stat.c b/fs/stat.c
index 9b513a142a56..74d8283cc5c6 100644
--- a/fs/stat.c
+++ b/fs/stat.c
@@ -26,6 +26,38 @@
#include "internal.h"
#include "mount.h"

+/**
+ * fill_multigrain_cmtime - Fill in the mtime and ctime and flag ctime as QUERIED
+ * @request_mask: STATX_* values requested
+ * @inode: inode from which to grab the c/mtime
+ * @stat: where to store the resulting values
+ *
+ * Given @inode, grab the ctime and mtime out if it and store the result
+ * in @stat. When fetching the value, flag it as queried so the next write
+ * will use a fine-grained timestamp.
+ */
+void fill_multigrain_cmtime(u32 request_mask, struct inode *inode,
+ struct kstat *stat)
+{
+ atomic_long_t *pnsec = (atomic_long_t *)&inode->i_ctime.tv_nsec;
+
+ /* If neither time was requested, then don't report them */
+ if (!(request_mask & (STATX_CTIME|STATX_MTIME))) {
+ stat->result_mask &= ~(STATX_CTIME|STATX_MTIME);
+ return;
+ }
+
+ stat->mtime = inode->i_mtime;
+ stat->ctime.tv_sec = inode->i_ctime.tv_sec;
+ /*
+ * Atomically set the QUERIED flag and fetch the new value with
+ * the flag masked off.
+ */
+ stat->ctime.tv_nsec = atomic_long_fetch_or(I_CTIME_QUERIED, pnsec) &
+ ~I_CTIME_QUERIED;
+}
+EXPORT_SYMBOL(fill_multigrain_cmtime);
+
/**
* generic_fillattr - Fill in the basic attributes from the inode struct
* @idmap: idmap of the mount the inode was found from
@@ -58,11 +90,16 @@ void generic_fillattr(struct mnt_idmap *idmap, u32 request_mask,
stat->rdev = inode->i_rdev;
stat->size = i_size_read(inode);
stat->atime = inode->i_atime;
- stat->mtime = inode->i_mtime;
- stat->ctime = inode->i_ctime;
stat->blksize = i_blocksize(inode);
stat->blocks = inode->i_blocks;

+ if (is_multigrain_ts(inode)) {
+ fill_multigrain_cmtime(request_mask, inode, stat);
+ } else {
+ stat->mtime = inode->i_mtime;
+ stat->ctime = inode->i_ctime;
+ }
+
if ((request_mask & STATX_CHANGE_COOKIE) && IS_I_VERSION(inode)) {
stat->result_mask |= STATX_CHANGE_COOKIE;
stat->change_cookie = inode_query_iversion(inode);
diff --git a/include/linux/fs.h b/include/linux/fs.h
index d5896f90093a..1f670cf1edbd 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1474,7 +1474,7 @@ 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);

/*
* Snapshotting support.
@@ -2212,6 +2212,7 @@ struct file_system_type {
#define FS_USERNS_MOUNT 8 /* Can be mounted by userns root */
#define FS_DISALLOW_NOTIFY_PERM 16 /* Disable fanotify permission events */
#define FS_ALLOW_IDMAP 32 /* FS has been updated to handle vfs idmappings. */
+#define FS_MULTIGRAIN_TS 64 /* Filesystem uses multigrain timestamps */
#define FS_RENAME_DOES_D_MOVE 32768 /* FS will handle d_move() during rename() internally. */
int (*init_fs_context)(struct fs_context *);
const struct fs_parameter_spec *parameters;
@@ -2235,6 +2236,67 @@ struct file_system_type {

#define MODULE_ALIAS_FS(NAME) MODULE_ALIAS("fs-" NAME)

+/*
+ * Multigrain timestamps
+ *
+ * Conditionally use fine-grained ctime and mtime timestamps when there
+ * are users actively observing them via getattr. The primary use-case
+ * for this is NFS clients that use the ctime to distinguish between
+ * different states of the file, and that are often fooled by multiple
+ * operations that occur in the same coarse-grained timer tick.
+ */
+static inline bool is_multigrain_ts(const struct inode *inode)
+{
+ return inode->i_sb->s_type->fs_flags & FS_MULTIGRAIN_TS;
+}
+
+/*
+ * The kernel always keeps normalized struct timespec64 values in the ctime,
+ * which means that only the first 30 bits of the value are used. Use the
+ * 31st bit of the ctime's tv_nsec field as a flag to indicate that the value
+ * has been queried since it was last updated.
+ */
+#define I_CTIME_QUERIED (1L<<30)
+
+/**
+ * ctime_nsec_peek - peek at (but don't query) the ctime tv_nsec field
+ * @inode: inode to fetch the ctime from
+ *
+ * Grab the current ctime tv_nsec field from the inode, mask off the
+ * I_CTIME_QUERIED flag and return it. This is mostly intended for use by
+ * internal consumers of the ctime that aren't concerned with ensuring a
+ * fine-grained update on the next change (e.g. when preparing to store
+ * the value in the backing store for later retrieval).
+ *
+ * This is safe to call regardless of whether the underlying filesystem
+ * is using multigrain timestamps.
+ */
+static inline long ctime_nsec_peek(const struct inode *inode)
+{
+ return inode->i_ctime.tv_nsec &~ I_CTIME_QUERIED;
+}
+
+/**
+ * ctime_peek - peek at (but don't query) the ctime
+ * @inode: inode to fetch the ctime from
+ *
+ * Grab the current ctime from the inode, sans I_CTIME_QUERIED flag. For
+ * use by internal consumers that don't require a fine-grained update on
+ * the next change.
+ *
+ * This is safe to call regardless of whether the underlying filesystem
+ * is using multigrain timestamps.
+ */
+static inline struct timespec64 ctime_peek(const struct inode *inode)
+{
+ struct timespec64 ctime;
+
+ ctime.tv_sec = inode->i_ctime.tv_sec;
+ ctime.tv_nsec = ctime_nsec_peek(inode);
+
+ return ctime;
+}
+
extern struct dentry *mount_bdev(struct file_system_type *fs_type,
int flags, const char *dev_name, void *data,
int (*fill_super)(struct super_block *, void *, int));
@@ -2857,6 +2919,8 @@ extern void page_put_link(void *);
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 fill_multigrain_cmtime(u32 request_mask, struct inode *inode,
+ struct kstat *stat);
void generic_fillattr(struct mnt_idmap *, u32, struct inode *, struct kstat *);
void generic_fill_statx_attr(struct inode *inode, struct kstat *stat);
extern int vfs_getattr_nosec(const struct path *, struct kstat *, u32, unsigned int);
--
2.40.1