xfs: call dax_fault on read page faults for DAX

Orabug: 22913653

When modifying the patch series to handle the XFS MMAP_LOCK nesting
of page faults, I botched the conversion of the read page fault
path, and so it is only every calling through the page cache. Re-add
the necessary __dax_fault() call for such files.

Because the get_blocks callback on read faults may not set up the
mapping buffer correctly to allow unwritten extent completion to be
run, we need to allow callers of __dax_fault() to pass a null
complete_unwritten() callback. The DAX code always zeros the
unwritten page when it is read faulted so there are no stale data
exposure issues with not doing the conversion. The only downside
will be the potential for increased CPU overhead on repeated read
faults of the same page. If this proves to be a problem, then the
filesystem needs to fix it's get_block callback and provide a
convert_unwritten() callback to the read fault path.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Matthew Wilcox <willy@linux.intel.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>
(cherry picked from commit b2442c5a7fe92cca08437070c8a45a7aa0d1703e)
Signed-off-by: Dan Duval <dan.duval@oracle.com>

diff --git a/fs/dax.c b/fs/dax.c
index c3e21ccfc358b2da1170c15f09a5946afad0ff16..a7f77e1fa18c25e62e8de5f809389d041e59266e 100644 (file)
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -319,6 +319,12 @@ static int dax_insert_mapping(struct inode *inode, struct buffer_head *bh,
  * @vma: The virtual memory area where the fault occurred
  * @vmf: The description of the fault
  * @get_block: The filesystem method used to translate file offsets to blocks
+ * @complete_unwritten: The filesystem method used to convert unwritten blocks
+ *     to written so the data written to them is exposed. This is required for
+ *     required by write faults for filesystems that will return unwritten
+ *     extent mappings from @get_block, but it is optional for reads as
+ *     dax_insert_mapping() will always zero unwritten blocks. If the fs does
+ *     not support unwritten extents, the it should pass NULL.
  *
  * When a page fault occurs, filesystems may call this helper in their
  * fault handler for DAX files. __dax_fault() assumes the caller has done all
@@ -437,8 +443,12 @@ int __dax_fault(struct vm_area_struct *vma, struct vm_fault *vmf,
         * as for normal BH based IO completions.
         */
        error = dax_insert_mapping(inode, &bh, vma, vmf);
-       if (buffer_unwritten(&bh))
-               complete_unwritten(&bh, !error);
+       if (buffer_unwritten(&bh)) {
+               if (complete_unwritten)
+                       complete_unwritten(&bh, !error);
+               else
+                       WARN_ON_ONCE(!(vmf->flags & FAULT_FLAG_WRITE));
+       }
 
  out:
        if (error == -ENOMEM)

diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
index a77d39a6fd221d14518c42534d82f82dbdce5865..891c0dcbbecdc7bd758bf90a0bb7b60e65090f8a 100644 (file)
--- a/fs/xfs/xfs_file.c
+++ b/fs/xfs/xfs_file.c
@@ -1507,18 +1507,27 @@ xfs_filemap_fault(
        struct vm_area_struct   *vma,
        struct vm_fault         *vmf)
 {
-       struct xfs_inode        *ip = XFS_I(file_inode(vma->vm_file));
+       struct inode            *inode = file_inode(vma->vm_file);
        int                     ret;
 
-       trace_xfs_filemap_fault(ip);
+       trace_xfs_filemap_fault(XFS_I(inode));
 
        /* DAX can shortcut the normal fault path on write faults! */
-       if ((vmf->flags & FAULT_FLAG_WRITE) && IS_DAX(VFS_I(ip)))
+       if ((vmf->flags & FAULT_FLAG_WRITE) && IS_DAX(inode))
                return xfs_filemap_page_mkwrite(vma, vmf);
 
-       xfs_ilock(ip, XFS_MMAPLOCK_SHARED);
-       ret = filemap_fault(vma, vmf);
-       xfs_iunlock(ip, XFS_MMAPLOCK_SHARED);
+       xfs_ilock(XFS_I(inode), XFS_MMAPLOCK_SHARED);
+       if (IS_DAX(inode)) {
+               /*
+                * we do not want to trigger unwritten extent conversion on read
+                * faults - that is unnecessary overhead and would also require
+                * changes to xfs_get_blocks_direct() to map unwritten extent
+                * ioend for conversion on read-only mappings.
+                */
+               ret = __dax_fault(vma, vmf, xfs_get_blocks_direct, NULL);
+       } else
+               ret = filemap_fault(vma, vmf);
+       xfs_iunlock(XFS_I(inode), XFS_MMAPLOCK_SHARED);
 
        return ret;
 }