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: Jan Kara <jack@suse.cz>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Cc: Ross Zwisler <ross.zwisler@linux.intel.com>
Reported-by: Dave Chinner <david@fromorbit.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>

1 files changed, 13 insertions, 2 deletions

diff --git a/fs/dax.c b/fs/dax.c
index 78233c716757..27ba300660ff 100644
--- a/fs/dax.c
+++ b/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: