mm: return void from folio_start_writeback() and related functions

Nobody now checks the return value from any of these functions, so
add an assertion at the beginning of the function and return void.

Link: https://lkml.kernel.org/r/20231108204605.745109-5-willy@infradead.orgSigned-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Cc: David Howells <dhowells@redhat.com>
Cc: Steve French <sfrench@samba.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

include/linux/page-flags.h

@@ -772,8 +772,8 @@ static __always_inline void SetPageUptodate(struct page *page)
 
 CLEARPAGEFLAG(Uptodate, uptodate, PF_NO_TAIL)
 
-bool __folio_start_writeback(struct folio *folio, bool keep_write);
-bool set_page_writeback(struct page *page);
+void __folio_start_writeback(struct folio *folio, bool keep_write);
+void set_page_writeback(struct page *page);
 
 #define folio_start_writeback(folio)			\
 	__folio_start_writeback(folio, false)

mm/folio-compat.c

@@ -46,9 +46,9 @@ void mark_page_accessed(struct page *page)
 }
 EXPORT_SYMBOL(mark_page_accessed);
 
-bool set_page_writeback(struct page *page)
+void set_page_writeback(struct page *page)
 {
-	return folio_start_writeback(page_folio(page));
+	folio_start_writeback(page_folio(page));
 }
 EXPORT_SYMBOL(set_page_writeback);
 

mm/page-writeback.c

@@ -2982,28 +2982,27 @@ bool __folio_end_writeback(struct folio *folio)
 	return ret;
 }
 
-bool __folio_start_writeback(struct folio *folio, bool keep_write)
+void __folio_start_writeback(struct folio *folio, bool keep_write)
 {
 	long nr = folio_nr_pages(folio);
 	struct address_space *mapping = folio_mapping(folio);
-	bool ret;
 	int access_ret;
 
+	VM_BUG_ON_FOLIO(folio_test_writeback(folio), folio);
+
 	folio_memcg_lock(folio);
 	if (mapping && mapping_use_writeback_tags(mapping)) {
 		XA_STATE(xas, &mapping->i_pages, folio_index(folio));
 		struct inode *inode = mapping->host;
 		struct backing_dev_info *bdi = inode_to_bdi(inode);
 		unsigned long flags;
+		bool on_wblist;
 
 		xas_lock_irqsave(&xas, flags);
 		xas_load(&xas);
-		ret = folio_test_set_writeback(folio);
-		if (!ret) {
-			bool on_wblist;
+		folio_test_set_writeback(folio);
 
-			on_wblist = mapping_tagged(mapping,
-						   PAGECACHE_TAG_WRITEBACK);
+		on_wblist = mapping_tagged(mapping, PAGECACHE_TAG_WRITEBACK);
 
 		xas_set_mark(&xas, PAGECACHE_TAG_WRITEBACK);
 		if (bdi->capabilities & BDI_CAP_WRITEBACK_ACCT) {
@@ -3015,34 +3014,31 @@ bool __folio_start_writeback(struct folio *folio, bool keep_write)
 		}
 
 		/*
-			 * We can come through here when swapping
-			 * anonymous folios, so we don't necessarily
-			 * have an inode to track for sync.
+		 * We can come through here when swapping anonymous
+		 * folios, so we don't necessarily have an inode to
+		 * track for sync.
 		 */
 		if (mapping->host && !on_wblist)
 			sb_mark_inode_writeback(mapping->host);
-		}
 		if (!folio_test_dirty(folio))
 			xas_clear_mark(&xas, PAGECACHE_TAG_DIRTY);
 		if (!keep_write)
 			xas_clear_mark(&xas, PAGECACHE_TAG_TOWRITE);
 		xas_unlock_irqrestore(&xas, flags);
 	} else {
-		ret = folio_test_set_writeback(folio);
+		folio_test_set_writeback(folio);
 	}
-	if (!ret) {
+
 	lruvec_stat_mod_folio(folio, NR_WRITEBACK, nr);
 	zone_stat_mod_folio(folio, NR_ZONE_WRITE_PENDING, nr);
-	}
 	folio_memcg_unlock(folio);
+
 	access_ret = arch_make_folio_accessible(folio);
 	/*
 	 * If writeback has been triggered on a page that cannot be made
 	 * accessible, it is too late to recover here.
 	 */
 	VM_BUG_ON_FOLIO(access_ret != 0, folio);
-
-	return ret;
 }
 EXPORT_SYMBOL(__folio_start_writeback);