fs/writeback: skip AS_NO_DATA_INTEGRITY mappings in wait_sb_inodes()

Above the while() loop in wait_sb_inodes(), we document that we must wait
for all pages under writeback for data integrity. Consequently, if a
mapping, like fuse, traditionally does not have data integrity semantics,
there is no need to wait at all; we can simply skip these inodes.

This restores fuse back to prior behavior where syncs are no-ops. This
fixes a user regression where if a system is running a faulty fuse server
that does not reply to issued write requests, this causes wait_sb_inodes()
to wait forever.

Link: https://lkml.kernel.org/r/[e-mail]
Fixes: 0c58a97f919c ("fuse: remove tmp folio for writebacks and internal rb tree")
Signed-off-by: Joanne Koong <[e-mail]>
Reported-by: Athul Krishna <[e-mail]>
Reported-by: J. Neuschäfer <[e-mail]>
Reviewed-by: Bernd Schubert <[e-mail]>
Tested-by: J. Neuschäfer <[e-mail]>
Cc: Alexander Viro <[e-mail]>
Cc: Bernd Schubert <[e-mail]>
Cc: Bonaccorso Salvatore <[e-mail]>
Cc: Christian Brauner <[e-mail]>
Cc: David Hildenbrand <[e-mail]>
Cc: Jan Kara <[e-mail]>
Cc: "Liam R. Howlett" <[e-mail]>
Cc: Lorenzo Stoakes <[e-mail]>
Cc: "Matthew Wilcox (Oracle)" <[e-mail]>
Cc: Michal Hocko <[e-mail]>
Cc: Mike Rapoport <[e-mail]>
Cc: Miklos Szeredi <[e-mail]>
Cc: Suren Baghdasaryan <[e-mail]>
Cc: Vlastimil Babka <[e-mail]>
Cc: <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

fs/fs-writeback.c

···
 		 * The mapping can appear untagged while still on-list since we
 		 * do not have the mapping lock. Skip it here, wb completion
 		 * will remove it.
+		 *
+		 * If the mapping does not have data integrity semantics,
+		 * there's no need to wait for the writeout to complete, as the
+		 * mapping cannot guarantee that data is persistently stored.
 		 */
-		if (!mapping_tagged(mapping, PAGECACHE_TAG_WRITEBACK))
+		if (!mapping_tagged(mapping, PAGECACHE_TAG_WRITEBACK) ||
+		    mapping_no_data_integrity(mapping))
 			continue;
 
 		spin_unlock_irq(&sb->s_inode_wblist_lock);

fs/fuse/file.c

···
 
 	inode->i_fop = &fuse_file_operations;
 	inode->i_data.a_ops = &fuse_file_aops;
-	if (fc->writeback_cache)
+	if (fc->writeback_cache) {
 		mapping_set_writeback_may_deadlock_on_reclaim(&inode->i_data);
+		mapping_set_no_data_integrity(&inode->i_data);
+	}
 
 	INIT_LIST_HEAD(&fi->write_files);
 	INIT_LIST_HEAD(&fi->queued_writes);

include/linux/pagemap.h

···
 	AS_WRITEBACK_MAY_DEADLOCK_ON_RECLAIM = 9,
 	AS_KERNEL_FILE = 10,	/* mapping for a fake kernel file that shouldn't
 				   account usage to user cgroups */
+	AS_NO_DATA_INTEGRITY = 11, /* no data integrity guarantees */
 	/* Bits 16-25 are used for FOLIO_ORDER */
 	AS_FOLIO_ORDER_BITS = 5,
 	AS_FOLIO_ORDER_MIN = 16,
···
 static inline bool mapping_writeback_may_deadlock_on_reclaim(const struct address_space *mapping)
 {
 	return test_bit(AS_WRITEBACK_MAY_DEADLOCK_ON_RECLAIM, &mapping->flags);
+}
+
+static inline void mapping_set_no_data_integrity(struct address_space *mapping)
+{
+	set_bit(AS_NO_DATA_INTEGRITY, &mapping->flags);
+}
+
+static inline bool mapping_no_data_integrity(const struct address_space *mapping)
+{
+	return test_bit(AS_NO_DATA_INTEGRITY, &mapping->flags);
 }
 
 static inline gfp_t mapping_gfp_mask(const struct address_space *mapping)