[PATCH 6/7] xfs: xfs_filemap_pmd_fault treats read faults as write faults

[Dave Chinner <david@xxxxxxxxxxxxx>]

From: Dave Chinner <dchinner@xxxxxxxxxx>

The code initially committed didn't have the same checks for write
faults as the dax_pmd_fault code and hence treats all faults as
write faults. We can get read faults through this path because they
is no pmd_mkwrite path for write faults similar to the normal page
fault path. Hence we need to ensure that we only do c/mtime updates
on write faults, and freeze protection is unnecessary for read
faults.

Signed-off-by: Dave Chinner <dchinner@xxxxxxxxxx>
Reviewed-by: Brian Foster <bfoster@xxxxxxxxxx>
Signed-off-by: Dave Chinner <david@xxxxxxxxxxxxx>
---
 fs/xfs/xfs_file.c | 20 ++++++++++++++++----
 1 file changed, 16 insertions(+), 4 deletions(-)

diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
index e7cf9ec..0045b0a 100644
--- a/fs/xfs/xfs_file.c
+++ b/fs/xfs/xfs_file.c
@@ -1482,7 +1482,7 @@ xfs_file_llseek(
  *
  * mmap_sem (MM)
  *   sb_start_pagefault(vfs, freeze)
- *     i_mmap_lock (XFS - truncate serialisation)
+ *     i_mmaplock (XFS - truncate serialisation)
  *       page_lock (MM)
  *         i_lock (XFS - extent map serialisation)
  */
@@ -1550,6 +1550,13 @@ xfs_filemap_fault(
        return ret;
 }
 
+/*
+ * Similar to xfs_filemap_fault(), the DAX fault path can call into here on
+ * both read and write faults. Hence we need to handle both cases. There is no
+ * ->pmd_mkwrite callout for huge pages, so we have a single function here to
+ * handle both cases here. @flags carries the information on the type of fault
+ * occuring.
+ */
 STATIC int
 xfs_filemap_pmd_fault(
        struct vm_area_struct   *vma,
@@ -1566,13 +1573,18 @@ xfs_filemap_pmd_fault(
 
        trace_xfs_filemap_pmd_fault(ip);
 
-       sb_start_pagefault(inode->i_sb);
-       file_update_time(vma->vm_file);
+       if (flags & FAULT_FLAG_WRITE) {
+               sb_start_pagefault(inode->i_sb);
+               file_update_time(vma->vm_file);
+       }
+
        xfs_ilock(XFS_I(inode), XFS_MMAPLOCK_SHARED);
        ret = __dax_pmd_fault(vma, addr, pmd, flags, xfs_get_blocks_dax_fault,
                              NULL);
        xfs_iunlock(XFS_I(inode), XFS_MMAPLOCK_SHARED);
-       sb_end_pagefault(inode->i_sb);
+
+       if (flags & FAULT_FLAG_WRITE)
+               sb_end_pagefault(inode->i_sb);
 
        return ret;
 }
-- 
2.5.0