[PATCH 15/24] xfs: handle directio copy-on-write for reflinked blocks

["Darrick J. Wong" <darrick.wong@xxxxxxxxxx>]

We hope that CoW writes will be rare and that directio CoW writes will
be even more rare.  Therefore, fall-back any such write to the
buffered path.

Signed-off-by: Darrick J. Wong <darrick.wong@xxxxxxxxxx>
---
 fs/xfs/xfs_aops.c    |    6 ++++++
 fs/xfs/xfs_file.c    |   12 ++++++++++--
 fs/xfs/xfs_reflink.c |   34 ++++++++++++++++++++++++++++++++++
 3 files changed, 50 insertions(+), 2 deletions(-)


diff --git a/fs/xfs/xfs_aops.c b/fs/xfs/xfs_aops.c
index 7332d72..bf4b408 100644
--- a/fs/xfs/xfs_aops.c
+++ b/fs/xfs/xfs_aops.c
@@ -1494,6 +1494,12 @@ __xfs_get_blocks(
        if (imap.br_startblock != HOLESTARTBLOCK &&
            imap.br_startblock != DELAYSTARTBLOCK &&
            (create || !ISUNWRITTEN(&imap))) {
+               if (create && direct) {
+                       error = xfs_reflink_redirect_directio_write(ip, &imap,
+                                       offset);
+                       if (error)
+                               return error;
+               }
                xfs_map_buffer(inode, bh_result, &imap, offset);
                if (ISUNWRITTEN(&imap))
                        set_buffer_unwritten(bh_result);
diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
index 981b028..211052a 100644
--- a/fs/xfs/xfs_file.c
+++ b/fs/xfs/xfs_file.c
@@ -869,10 +869,18 @@ xfs_file_write_iter(
        if (XFS_FORCED_SHUTDOWN(ip->i_mount))
                return -EIO;
 
-       if ((iocb->ki_flags & IOCB_DIRECT) || IS_DAX(inode))
+       /*
+        * Allow DIO to fall back to buffered *only* in the case that we're
+        * doing a reflink CoW.
+        */
+       if ((iocb->ki_flags & IOCB_DIRECT) || IS_DAX(inode)) {
                ret = xfs_file_dio_aio_write(iocb, from);
-       else
+               if (ret == -EREMCHG)
+                       goto buffered;
+       } else {
+buffered:
                ret = xfs_file_buffered_aio_write(iocb, from);
+       }
 
        if (ret > 0) {
                ssize_t err;
diff --git a/fs/xfs/xfs_reflink.c b/fs/xfs/xfs_reflink.c
index 263f30b..f841a1a 100644
--- a/fs/xfs/xfs_reflink.c
+++ b/fs/xfs/xfs_reflink.c
@@ -406,6 +406,40 @@ advloop:
 }
 
 /**
+ * xfs_reflink_redirect_directio_write() - bounce a directio write to a
+ *                                        reflinked region down to buffered
+ *                                        write mode.
+ *
+ * @ip: XFS inode object
+ * @imap: the fileoff:fsblock mapping that we might fork
+ * @offset: the file byte offset of the block we're examining
+ */
+int
+xfs_reflink_redirect_directio_write(
+       struct xfs_inode        *ip,
+       struct xfs_bmbt_irec    *imap,
+       xfs_off_t               offset)
+{
+       bool                    type = false;
+       int                     error;
+
+       error = xfs_reflink_should_fork_block(ip, imap, offset, &type);
+       if (error)
+               return error;
+       if (!type)
+               return 0;
+
+       /*
+        * Are we doing a DIO write to a reflinked block?  In the ideal world
+        * we at least would fork full blocks, but for now just fall back to
+        * buffered mode.  Yuck.  Use -EREMCHG ("remote address changed") to
+        * signal this, since in general XFS doesn't do this sort of fallback.
+        */
+       trace_xfs_reflink_bounce_direct_write(ip, imap);
+       return -EREMCHG;
+}
+
+/**
  * xfs_reflink_write_fork_block() -- find a remapping object and redirect the
  *                                  write.
  *