[PATCH 08/11] fs: add support for non-blocking timestamp updates

Christoph Hellwig
2025-12-23T00:38:49Z

Currently file_update_time_flags unconditionally returns -EAGAIN if any
timestamp needs to be updated and IOCB_NOWAIT is passed. This makes
non-blocking direct writes impossible on file systems with granular
enough timestamps.

Add a S_NOWAIT to ask for timestamps to not block, and return -EAGAIN in
all methods for now.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
---
fs/fat/misc.c | 3 +++
fs/gfs2/inode.c | 3 +++
fs/inode.c | 29 ++++++++++++++++++++++++++---
fs/overlayfs/inode.c | 2 ++
fs/ubifs/file.c | 3 +++
fs/xfs/xfs_iops.c | 3 +++
include/linux/fs.h | 21 +++++++++++++++++----
7 files changed, 57 insertions(+), 7 deletions(-)

diff --git a/fs/fat/misc.c b/fs/fat/misc.c
index 950da09f0961..5df3193c35f9 100644
--- a/fs/fat/misc.c
+++ b/fs/fat/misc.c
@@ -346,6 +346,9 @@ int fat_update_time(struct inode *inode, int flags)
if (inode->i_ino == MSDOS_ROOT_INO)
return 0;

+ if (flags & S_NOWAIT)
+ return -EAGAIN;
+
if (flags & (S_ATIME | S_CTIME | S_MTIME)) {
fat_truncate_time(inode, NULL, flags);
if (inode->i_sb->s_flags & SB_LAZYTIME)
diff --git a/fs/gfs2/inode.c b/fs/gfs2/inode.c
index e08eb419347c..0dce2af533b2 100644
--- a/fs/gfs2/inode.c
+++ b/fs/gfs2/inode.c
@@ -2249,6 +2249,9 @@ static int gfs2_update_time(struct inode *inode, int flags)
struct gfs2_holder *gh;
int error;

+ if (flags & S_NOWAIT)
+ return -EAGAIN;
+
gh = gfs2_glock_is_locked_by_me(gl);
if (gh && gl->gl_state != LM_ST_EXCLUSIVE) {
gfs2_glock_dq(gh);
diff --git a/fs/inode.c b/fs/inode.c
index 212dab5c65ad..98a878427ecb 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -2111,6 +2111,13 @@ int inode_update_timestamps(struct inode *inode, int flags, int *dirty_flags)

*dirty_flags = 0;

+ /*
+ * Non-blocking timestamp updates require an explicit opt-in from the
+ * file system.
+ */
+ if ((flags & S_NOWAIT) && !(flags & S_CAN_NOWAIT_LAZYTIME))
+ return -EAGAIN;
+
if (flags & (S_MTIME | S_CTIME | S_VERSION)) {
struct timespec64 ctime = inode_get_ctime(inode);
struct timespec64 mtime = inode_get_mtime(inode);
@@ -2120,8 +2127,24 @@ int inode_update_timestamps(struct inode *inode, int flags, int *dirty_flags)
updated |= S_CTIME;
if (!timespec64_equal(&now, &mtime))
updated |= S_MTIME;
- if (IS_I_VERSION(inode) && inode_maybe_inc_iversion(inode, updated))
- updated |= S_VERSION;
+
+ /*
+ * Pure timestamp updates can be recorded in the inode without
+ * blocking by not dirtying the inode. But when the file system
+ * requires i_version updates, actual i_version update may block
+ * despite that. Error out if we'd actually have to update
+ * i_version or don't support lazytime.
+ */
+ if (IS_I_VERSION(inode)) {
+ if (flags & S_NOWAIT) {
+ if (!(inode->i_sb->s_flags & SB_LAZYTIME) ||
+ inode_iversion_need_inc(inode))
+ return -EAGAIN;
+ } else {
+ if (inode_maybe_inc_iversion(inode, updated))
+ updated |= S_NOWAIT;
+ }
+ }
} else {
now = current_time(inode);
}
@@ -2391,7 +2414,7 @@ static int file_update_time_flags(struct file *file, unsigned int flags)
return 0;

if (flags & IOCB_NOWAIT)
- return -EAGAIN;
+ sync_mode |= S_NOWAIT;

if (mnt_get_write_access_file(file))
return 0;
diff --git a/fs/overlayfs/inode.c b/fs/overlayfs/inode.c
index bdbf86b56a9b..6d23cacbf776 100644
--- a/fs/overlayfs/inode.c
+++ b/fs/overlayfs/inode.c
@@ -565,6 +565,8 @@ int ovl_update_time(struct inode *inode, int flags)
};

if (upperpath.dentry) {
+ if (flags & S_NOWAIT)
+ return -EAGAIN;
touch_atime(&upperpath);
inode_set_atime_to_ts(inode,
inode_get_atime(d_inode(upperpath.dentry)));
diff --git a/fs/ubifs/file.c b/fs/ubifs/file.c
index fe236886484c..b74dd4d21330 100644
--- a/fs/ubifs/file.c
+++ b/fs/ubifs/file.c
@@ -1382,6 +1382,9 @@ int ubifs_update_time(struct inode *inode, int flags)
if (!IS_ENABLED(CONFIG_UBIFS_ATIME_SUPPORT))
return generic_update_time(inode, flags);

+ if (flags & S_NOWAIT)
+ return -EAGAIN;
+
err = ubifs_budget_space(c, &req);
if (err)
return err;
diff --git a/fs/xfs/xfs_iops.c b/fs/xfs/xfs_iops.c
index 9dedb54e3cb0..626a541b247b 100644
--- a/fs/xfs/xfs_iops.c
+++ b/fs/xfs/xfs_iops.c
@@ -1195,6 +1195,9 @@ xfs_vn_update_time(

trace_xfs_update_time(ip);

+ if (flags & S_NOWAIT)
+ return -EAGAIN;
+
if (inode->i_sb->s_flags & SB_LAZYTIME) {
if (!((flags & S_VERSION) &&
inode_maybe_inc_iversion(inode, false)))
diff --git a/include/linux/fs.h b/include/linux/fs.h
index d1d57149aa93..0ea175e19a8b 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -2239,10 +2239,23 @@ static inline void inode_dec_link_count(struct inode *inode)
}

enum file_time_flags {
- S_ATIME = 1,
- S_MTIME = 2,
- S_CTIME = 4,
- S_VERSION = 8,
+ /* update atime: */
+ S_ATIME = 1U << 0,
+
+ /* update mtime */
+ S_MTIME = 1U << 1,
+
+ /* update ctime */
+ S_CTIME = 1U << 2,
+
+ /* force update i_version even if no timestamp changes */
+ S_VERSION = 1U << 3,
+
+ /* only update timestamps or i_version if it doesn't require blocking */
+ S_NOWAIT = 1U << 14,
+
+ /* support S_NOWAIT for SB_LAZYTIME mounts in inode_update_timestamps */
+ S_CAN_NOWAIT_LAZYTIME = 1U << 15,
};

extern bool atime_needs_update(const struct path *, struct inode *);
--
2.47.3