mm: move more expensive part of XA setup out of mapping check

The fast path here is not needing any writeback, yet we spend time
setting up the xarray lookup data upfront.  Move the part that actually
needs to iterate the address space mapping into a separate helper,
saving ~30% of the time here.

Link: https://lkml.kernel.org/r/49f67983-b802-8929-edab-d807f745c9ca@kernel.dkSigned-off-by: Jens Axboe <axboe@kernel.dk>
Cc: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/filemap.c

@@ -639,6 +639,30 @@ static bool mapping_needs_writeback(struct address_space *mapping)
 	return mapping->nrpages;
 }
 
+static bool filemap_range_has_writeback(struct address_space *mapping,
+					loff_t start_byte, loff_t end_byte)
+{
+	XA_STATE(xas, &mapping->i_pages, start_byte >> PAGE_SHIFT);
+	pgoff_t max = end_byte >> PAGE_SHIFT;
+	struct page *page;
+
+	if (end_byte < start_byte)
+		return false;
+
+	rcu_read_lock();
+	xas_for_each(&xas, page, max) {
+		if (xas_retry(&xas, page))
+			continue;
+		if (xa_is_value(page))
+			continue;
+		if (PageDirty(page) || PageLocked(page) || PageWriteback(page))
+			break;
+	}
+	rcu_read_unlock();
+	return page != NULL;
+
+}
+
 /**
  * filemap_range_needs_writeback - check if range potentially needs writeback
  * @mapping:           address space within which to check
@@ -656,29 +680,12 @@ static bool mapping_needs_writeback(struct address_space *mapping)
 bool filemap_range_needs_writeback(struct address_space *mapping,
 				   loff_t start_byte, loff_t end_byte)
 {
-	XA_STATE(xas, &mapping->i_pages, start_byte >> PAGE_SHIFT);
-	pgoff_t max = end_byte >> PAGE_SHIFT;
-	struct page *page;
-
 	if (!mapping_needs_writeback(mapping))
 		return false;
 	if (!mapping_tagged(mapping, PAGECACHE_TAG_DIRTY) &&
 	    !mapping_tagged(mapping, PAGECACHE_TAG_WRITEBACK))
 		return false;
-	if (end_byte < start_byte)
-		return false;
-
-	rcu_read_lock();
-	xas_for_each(&xas, page, max) {
-		if (xas_retry(&xas, page))
-			continue;
-		if (xa_is_value(page))
-			continue;
-		if (PageDirty(page) || PageLocked(page) || PageWriteback(page))
-			break;
-	}
-	rcu_read_unlock();
-	return page != NULL;
+	return filemap_range_has_writeback(mapping, start_byte, end_byte);
 }
 EXPORT_SYMBOL_GPL(filemap_range_needs_writeback);