[RFC PATCH 1/3] fs: dax.c: move fs hole signifier from DAX_ZERO_PAGE to XA_ZERO_ENTRY

[Amy Parker <enbyamy@xxxxxxxxx>]

DAX uses the DAX_ZERO_PAGE bit to represent holes in files. It could also use
a single entry, such as XArray's XA_ZERO_ENTRY. This distinguishes zero pages
and allows us to shift DAX_EMPTY down (see patch 2/3).

Signed-off-by: Amy Parker <enbyamy@xxxxxxxxx>
---
 fs/dax.c | 15 ++++++++++-----
 1 file changed, 10 insertions(+), 5 deletions(-)

diff --git a/fs/dax.c b/fs/dax.c
index 5b47834f2e1b..fa8ca1a71bbd 100644
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -77,9 +77,14 @@ fs_initcall(init_dax_wait_table);
 #define DAX_SHIFT    (4)
 #define DAX_LOCKED    (1UL << 0)
 #define DAX_PMD        (1UL << 1)
-#define DAX_ZERO_PAGE    (1UL << 2)
 #define DAX_EMPTY    (1UL << 3)

+/*
+ * A zero entry, XA_ZERO_ENTRY, is used to represent a zero page. This
+ * definition helps with checking if an entry is a PMD size.
+ */
+#define XA_ZERO_PMD_ENTRY DAX_PMD | (unsigned long)XA_ZERO_ENTRY
+
 static unsigned long dax_to_pfn(void *entry)
 {
     return xa_to_value(entry) >> DAX_SHIFT;
@@ -114,7 +119,7 @@ static bool dax_is_pte_entry(void *entry)

 static int dax_is_zero_entry(void *entry)
 {
-    return xa_to_value(entry) & DAX_ZERO_PAGE;
+    return xa_to_value(entry) & (unsigned long)XA_ZERO_ENTRY;
 }

 static int dax_is_empty_entry(void *entry)
@@ -738,7 +743,7 @@ static void *dax_insert_entry(struct xa_state *xas,
     if (dirty)
         __mark_inode_dirty(mapping->host, I_DIRTY_PAGES);

-    if (dax_is_zero_entry(entry) && !(flags & DAX_ZERO_PAGE)) {
+    if (dax_is_zero_entry(entry) && !(flags & (unsigned long)XA_ZERO_ENTRY)) {
         unsigned long index = xas->xa_index;
         /* we are replacing a zero page with block mapping */
         if (dax_is_pmd_entry(entry))
@@ -1047,7 +1052,7 @@ static vm_fault_t dax_load_hole(struct xa_state *xas,
     vm_fault_t ret;

     *entry = dax_insert_entry(xas, mapping, vmf, *entry, pfn,
-            DAX_ZERO_PAGE, false);
+            XA_ZERO_ENTRY, false);

     ret = vmf_insert_mixed(vmf->vma, vaddr, pfn);
     trace_dax_load_hole(inode, vmf, ret);
@@ -1434,7 +1439,7 @@ static vm_fault_t dax_pmd_load_hole(struct
xa_state *xas, struct vm_fault *vmf,

     pfn = page_to_pfn_t(zero_page);
     *entry = dax_insert_entry(xas, mapping, vmf, *entry, pfn,
-            DAX_PMD | DAX_ZERO_PAGE, false);
+            XA_ZERO_PMD_ENTRY, false);

     if (arch_needs_pgtable_deposit()) {
         pgtable = pte_alloc_one(vma->vm_mm);
-- 
2.29.2