Merge tag 'v6.6-rc3.vfs.ctime.revert' of git://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs

Pull finegrained timestamp reverts from Christian Brauner:
 "Earlier this week we sent a few minor fixes for the multi-grained
  timestamp work in [1]. While we were polishing those up after Linus
  realized that there might be a nicer way to fix them we received a
  regression report in [2] that fine grained timestamps break gnulib
  tests and thus possibly other tools.

  The kernel will elide fine-grain timestamp updates when no one is
  actively querying for them to avoid performance impacts. So a sequence
  like write(f1) stat(f2) write(f2) stat(f2) write(f1) stat(f1) may
  result in timestamp f1 to be older than the final f2 timestamp even
  though f1 was last written too but the second write didn't update the
  timestamp.

  Such plotholes can lead to subtle bugs when programs compare
  timestamps. For example, the nap() function in [2] will estimate that
  it needs to wait one ns on a fine-grain timestamp enabled filesytem
  between subsequent calls to observe a timestamp change. But in general
  we don't update timestamps with more than one jiffie if we think that
  no one is actively querying for fine-grain timestamps to avoid
  performance impacts.

  While discussing various fixes the decision was to go back to the
  drawing board and ultimately to explore a solution that involves only
  exposing such fine-grained timestamps to nfs internally and never to
  userspace.

  As there are multiple solutions discussed the honest thing to do here
  is not to fix this up or disable it but to cleanly revert. The general
  infrastructure will probably come back but there is no reason to keep
  this code in mainline.

  The general changes to timestamp handling are valid and a good cleanup
  that will stay. The revert is fully bisectable"

Link: https://lore.kernel.org/all/20230918-hirte-neuzugang-4c2324e7bae3@brauner [1]
Link: https://lore.kernel.org/all/bf0524debb976627693e12ad23690094e4514303.camel@linuxfromscratch.org [2]

* tag 'v6.6-rc3.vfs.ctime.revert' of git://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs:
  Revert "fs: add infrastructure for multigrain timestamps"
  Revert "btrfs: convert to multigrain timestamps"
  Revert "ext4: switch to multigrain timestamps"
  Revert "xfs: switch to multigrain timestamps"
  Revert "tmpfs: add support for multigrain timestamps"

1 files changed, 2 insertions, 44 deletions

diff --git a/include/linux/fs.h b/include/linux/fs.h
index 4aeb3fa11927..b528f063e8ff 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1508,47 +1508,18 @@ static inline bool fsuidgid_has_mapping(struct super_block *sb,
 	       kgid_has_mapping(fs_userns, kgid);
 }
 
-struct timespec64 current_mgtime(struct inode *inode);
 struct timespec64 current_time(struct inode *inode);
 struct timespec64 inode_set_ctime_current(struct inode *inode);
 
-/*
- * 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.
- *
- * 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)
-
 /**
  * inode_get_ctime - fetch the current ctime from the inode
  * @inode: inode from which to fetch ctime
  *
- * 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.
+ * Grab the current ctime from the inode and return it.
  */
 static inline struct timespec64 inode_get_ctime(const struct inode *inode)
 {
-	struct timespec64 ctime;
-
-	ctime.tv_sec = inode->__i_ctime.tv_sec;
-	ctime.tv_nsec = inode->__i_ctime.tv_nsec & ~I_CTIME_QUERIED;
-
-	return ctime;
+	return inode->__i_ctime;
 }
 
 /**
@@ -2334,7 +2305,6 @@ 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_MGTIME		64	/* FS 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;
@@ -2358,17 +2328,6 @@ struct file_system_type {
 
 #define MODULE_ALIAS_FS(NAME) MODULE_ALIAS("fs-" NAME)
 
-/**
- * is_mgtime: is this inode using multigrain timestamps
- * @inode: inode to test for multigrain timestamps
- *
- * Return true if the inode uses multigrain timestamps, false otherwise.
- */
-static inline bool is_mgtime(const struct inode *inode)
-{
-	return inode->i_sb->s_type->fs_flags & FS_MGTIME;
-}
-
 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));
@@ -3054,7 +3013,6 @@ 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_mg_cmtime(struct kstat *stat, u32 request_mask, struct inode *inode);
 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);