[PATCH v8 3/5] fs: have setattr_copy handle multigrain timestamps appropriately

[Jeff Layton]

The setattr codepath is still using coarse-grained timestamps, even on
multigrain filesystems. To fix this, we need to fetch the timestamp for
ctime updates later, at the point where the assignment occurs in
setattr_copy.

On a multigrain inode, ignore the ia_ctime in the attrs, and always
update the ctime to the current clock value. Update the atime and mtime
with the same value (if needed) unless they are being set to other
specific values, a'la utimes().

Note that we don't want to do this universally however, as some
filesystems (e.g. most networked fs) want to do an explicit update
elsewhere before updating the local inode.

Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxx>
---
 fs/attr.c | 52 ++++++++++++++++++++++++++++++++++++++++++++++------
 1 file changed, 46 insertions(+), 6 deletions(-)

diff --git a/fs/attr.c b/fs/attr.c
index a8ae5f6d9b16..8ba330e6a582 100644
--- a/fs/attr.c
+++ b/fs/attr.c
@@ -275,6 +275,42 @@ int inode_newsize_ok(const struct inode *inode, loff_t offset)
 }
 EXPORT_SYMBOL(inode_newsize_ok);
 
+/**
+ * setattr_copy_mgtime - update timestamps for mgtime inodes
+ * @inode: inode timestamps to be updated
+ * @attr: attrs for the update
+ *
+ * With multigrain timestamps, we need to take more care to prevent races
+ * when updating the ctime. Always update the ctime to the very latest
+ * using the standard mechanism, and use that to populate the atime and
+ * mtime appropriately (unless we're setting those to specific values).
+ */
+static void setattr_copy_mgtime(struct inode *inode, const struct iattr *attr)
+{
+	unsigned int ia_valid = attr->ia_valid;
+	struct timespec64 now;
+
+	/*
+	 * If the ctime isn't being updated then nothing else should be
+	 * either.
+	 */
+	if (!(ia_valid & ATTR_CTIME)) {
+		WARN_ON_ONCE(ia_valid & (ATTR_ATIME|ATTR_MTIME));
+		return;
+	}
+
+	now = inode_set_ctime_current(inode);
+	if (ia_valid & ATTR_ATIME_SET)
+		inode->i_atime = attr->ia_atime;
+	else if (ia_valid & ATTR_ATIME)
+		inode->i_atime = now;
+
+	if (ia_valid & ATTR_MTIME_SET)
+		inode->i_mtime = attr->ia_mtime;
+	else if (ia_valid & ATTR_MTIME)
+		inode->i_mtime = now;
+}
+
 /**
  * setattr_copy - copy simple metadata updates into the generic inode
  * @idmap:	idmap of the mount the inode was found from
@@ -307,12 +343,6 @@ void setattr_copy(struct mnt_idmap *idmap, struct inode *inode,
 
 	i_uid_update(idmap, attr, inode);
 	i_gid_update(idmap, attr, inode);
-	if (ia_valid & ATTR_ATIME)
-		inode->i_atime = attr->ia_atime;
-	if (ia_valid & ATTR_MTIME)
-		inode->i_mtime = attr->ia_mtime;
-	if (ia_valid & ATTR_CTIME)
-		inode_set_ctime_to_ts(inode, attr->ia_ctime);
 	if (ia_valid & ATTR_MODE) {
 		umode_t mode = attr->ia_mode;
 		if (!in_group_or_capable(idmap, inode,
@@ -320,6 +350,16 @@ void setattr_copy(struct mnt_idmap *idmap, struct inode *inode,
 			mode &= ~S_ISGID;
 		inode->i_mode = mode;
 	}
+
+	if (is_mgtime(inode))
+		return setattr_copy_mgtime(inode, attr);
+
+	if (ia_valid & ATTR_ATIME)
+		inode->i_atime = attr->ia_atime;
+	if (ia_valid & ATTR_MTIME)
+		inode->i_mtime = attr->ia_mtime;
+	if (ia_valid & ATTR_CTIME)
+		inode_set_ctime_to_ts(inode, attr->ia_ctime);
 }
 EXPORT_SYMBOL(setattr_copy);
 

-- 
2.41.0