dax: account DAX entries as nrpages

Simplify mapping_needs_writeback() by accounting DAX entries as pages
instead of exceptional entries.

Link: https://lkml.kernel.org/r/20201026151849.24232-4-willy@infradead.orgSigned-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Tested-by: Vishal Verma <vishal.l.verma@intel.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/dax.c

@@ -525,7 +525,7 @@ static void *grab_mapping_entry(struct xa_state *xas,
 		dax_disassociate_entry(entry, mapping, false);
 		xas_store(xas, NULL);	/* undo the PMD join */
 		dax_wake_entry(xas, entry, true);
-		mapping->nrexceptional--;
+		mapping->nrpages -= PG_PMD_NR;
 		entry = NULL;
 		xas_set(xas, index);
 	}
@@ -541,7 +541,7 @@ static void *grab_mapping_entry(struct xa_state *xas,
 		dax_lock_entry(xas, entry);
 		if (xas_error(xas))
 			goto out_unlock;
-		mapping->nrexceptional++;
+		mapping->nrpages += 1UL << order;
 	}
 
 out_unlock:
@@ -661,7 +661,7 @@ static int __dax_invalidate_entry(struct address_space *mapping,
 		goto out;
 	dax_disassociate_entry(entry, mapping, trunc);
 	xas_store(&xas, NULL);
-	mapping->nrexceptional--;
+	mapping->nrpages -= 1UL << dax_entry_order(entry);
 	ret = 1;
 out:
 	put_unlocked_entry(&xas, entry);

mm/filemap.c

@@ -618,9 +618,6 @@ EXPORT_SYMBOL(filemap_fdatawait_keep_errors);
 /* Returns true if writeback might be needed or already in progress. */
 static bool mapping_needs_writeback(struct address_space *mapping)
 {
-	if (dax_mapping(mapping))
-		return mapping->nrexceptional;
-
 	return mapping->nrpages;
 }