fs, dax: unify IOMAP_F_DIRTY read vs write handling policy in the dax core

While reviewing whether MAP_SYNC should strengthen its current guarantee
of syncing writes from the initiating process to also include
third-party readers observing dirty metadata, Dave pointed out that the
check of IOMAP_WRITE is misplaced.

The policy of what to with IOMAP_F_DIRTY should be separated from the
generic filesystem mechanism of reporting dirty metadata. Move this
policy to the fs-dax core to simplify the per-filesystem iomap handlers,
and further centralize code that implements the MAP_SYNC policy. This
otherwise should not change behavior, it just makes it easier to change
behavior in the future.
Reviewed-by: NJan Kara <jack@suse.cz>
Reviewed-by: NChristoph Hellwig <hch@lst.de>
Reviewed-by: NDarrick J. Wong <darrick.wong@oracle.com>
Cc: Ross Zwisler <ross.zwisler@linux.intel.com>
Reported-by: NDave Chinner <david@fromorbit.com>
Signed-off-by: NDan Williams <dan.j.williams@intel.com>

fs/dax.c

@@ -1079,6 +1079,17 @@ static int dax_fault_return(int error)
 	return VM_FAULT_SIGBUS;
 }
 
+/*
+ * MAP_SYNC on a dax mapping guarantees dirty metadata is
+ * flushed on write-faults (non-cow), but not read-faults.
+ */
+static bool dax_fault_is_synchronous(unsigned long flags,
+		struct vm_area_struct *vma, struct iomap *iomap)
+{
+	return (flags & IOMAP_WRITE) && (vma->vm_flags & VM_SYNC)
+		&& (iomap->flags & IOMAP_F_DIRTY);
+}
+
 static int dax_iomap_pte_fault(struct vm_fault *vmf, pfn_t *pfnp,
 			       const struct iomap_ops *ops)
 {
@@ -1170,7 +1181,7 @@ static int dax_iomap_pte_fault(struct vm_fault *vmf, pfn_t *pfnp,
 		goto finish_iomap;
 	}
 
-	sync = (vma->vm_flags & VM_SYNC) && (iomap.flags & IOMAP_F_DIRTY);
+	sync = dax_fault_is_synchronous(flags, vma, &iomap);
 
 	switch (iomap.type) {
 	case IOMAP_MAPPED:
@@ -1390,7 +1401,7 @@ static int dax_iomap_pmd_fault(struct vm_fault *vmf, pfn_t *pfnp,
 	if (iomap.offset + iomap.length < pos + PMD_SIZE)
 		goto finish_iomap;
 
-	sync = (vma->vm_flags & VM_SYNC) && (iomap.flags & IOMAP_F_DIRTY);
+	sync = dax_fault_is_synchronous(iomap_flags, vma, &iomap);
 
 	switch (iomap.type) {
 	case IOMAP_MAPPED:

fs/ext4/inode.c

@@ -3479,7 +3479,7 @@ static int ext4_iomap_begin(struct inode *inode, loff_t offset, loff_t length,
 	}
 
 	iomap->flags = 0;
-	if ((flags & IOMAP_WRITE) && ext4_inode_datasync_dirty(inode))
+	if (ext4_inode_datasync_dirty(inode))
 		iomap->flags |= IOMAP_F_DIRTY;
 	iomap->bdev = inode->i_sb->s_bdev;
 	iomap->dax_dev = sbi->s_daxdev;

fs/xfs/xfs_iomap.c

@@ -1087,8 +1087,8 @@ xfs_file_iomap_begin(
 		trace_xfs_iomap_found(ip, offset, length, 0, &imap);
 	}
 
-	if ((flags & IOMAP_WRITE) && xfs_ipincount(ip) &&
-	    (ip->i_itemp->ili_fsync_fields & ~XFS_ILOG_TIMESTAMP))
+	if (xfs_ipincount(ip) && (ip->i_itemp->ili_fsync_fields
+				& ~XFS_ILOG_TIMESTAMP))
 		iomap->flags |= IOMAP_F_DIRTY;
 
 	xfs_bmbt_to_iomap(ip, iomap, &imap);