fs: kill BDI_CAP_SWAP_BACKED

This bdi flag isn't too useful - we can determine that a vma is backed by
either swap or shmem trivially in the caller.

This also allows removing the backing_dev_info instaces for swap and shmem
in favor of noop_backing_dev_info.

Signed-off-by: Christoph Hellwig <[e-mail]>
Reviewed-by: Tejun Heo <[e-mail]>
Reviewed-by: Jan Kara <[e-mail]>
Signed-off-by: Jens Axboe <[e-mail]>

include/linux/backing-dev.h

···
  * BDI_CAP_WRITE_MAP:      Can be mapped for writing
  * BDI_CAP_EXEC_MAP:       Can be mapped for execution
  *
- * BDI_CAP_SWAP_BACKED:    Count shmem/tmpfs objects as swap-backed.
- *
  * BDI_CAP_STRICTLIMIT:    Keep number of dirty pages below bdi threshold.
  */
 #define BDI_CAP_NO_ACCT_DIRTY	0x00000001
···
 #define BDI_CAP_WRITE_MAP	0x00000020
 #define BDI_CAP_EXEC_MAP	0x00000040
 #define BDI_CAP_NO_ACCT_WB	0x00000080
-#define BDI_CAP_SWAP_BACKED	0x00000100
 #define BDI_CAP_STABLE_WRITES	0x00000200
 #define BDI_CAP_STRICTLIMIT	0x00000400
 
···
 				      BDI_CAP_NO_WRITEBACK));
 }
 
-static inline bool bdi_cap_swap_backed(struct backing_dev_info *bdi)
-{
-	return bdi->capabilities & BDI_CAP_SWAP_BACKED;
-}
-
 static inline bool mapping_cap_writeback_dirty(struct address_space *mapping)
 {
 	return bdi_cap_writeback_dirty(mapping->backing_dev_info);
···
 static inline bool mapping_cap_account_dirty(struct address_space *mapping)
 {
 	return bdi_cap_account_dirty(mapping->backing_dev_info);
-}
-
-static inline bool mapping_cap_swap_backed(struct address_space *mapping)
-{
-	return bdi_cap_swap_backed(mapping->backing_dev_info);
 }
 
 static inline int bdi_sched_wait(void *word)

mm/madvise.c

···
 	struct file *file = vma->vm_file;
 
 #ifdef CONFIG_SWAP
-	if (!file || mapping_cap_swap_backed(file->f_mapping)) {
+	if (!file) {
 		*prev = vma;
-		if (!file)
-			force_swapin_readahead(vma, start, end);
-		else
-			force_shm_swapin_readahead(vma, start, end,
-						file->f_mapping);
+		force_swapin_readahead(vma, start, end);
 		return 0;
 	}
-#endif
 
+	if (shmem_mapping(file->f_mapping)) {
+		*prev = vma;
+		force_shm_swapin_readahead(vma, start, end,
+					file->f_mapping);
+		return 0;
+	}
+#else
 	if (!file)
 		return -EBADF;
+#endif
 
 	if (file->f_mapping->a_ops->get_xip_mem) {
 		/* no bad return value, but ignore advice */

mm/shmem.c

···
 static const struct inode_operations shmem_special_inode_operations;
 static const struct vm_operations_struct shmem_vm_ops;
 
-static struct backing_dev_info shmem_backing_dev_info  __read_mostly = {
-	.ra_pages	= 0,	/* No readahead */
-	.capabilities	= BDI_CAP_NO_ACCT_AND_WRITEBACK | BDI_CAP_SWAP_BACKED,
-};
-
 static LIST_HEAD(shmem_swaplist);
 static DEFINE_MUTEX(shmem_swaplist_mutex);
 
···
 		goto redirty;
 
 	/*
-	 * shmem_backing_dev_info's capabilities prevent regular writeback or
-	 * sync from ever calling shmem_writepage; but a stacking filesystem
-	 * might use ->writepage of its underlying filesystem, in which case
-	 * tmpfs should write out to swap only in response to memory pressure,
-	 * and not for the writeback threads or sync.
+	 * Our capabilities prevent regular writeback or sync from ever calling
+	 * shmem_writepage; but a stacking filesystem might use ->writepage of
+	 * its underlying filesystem, in which case tmpfs should write out to
+	 * swap only in response to memory pressure, and not for the writeback
+	 * threads or sync.
 	 */
 	if (!wbc->for_reclaim) {
 		WARN_ON_ONCE(1);	/* Still happens? Tell us about it! */
···
 		inode->i_ino = get_next_ino();
 		inode_init_owner(inode, dir, mode);
 		inode->i_blocks = 0;
-		inode->i_mapping->backing_dev_info = &shmem_backing_dev_info;
+		inode->i_mapping->backing_dev_info = &noop_backing_dev_info;
 		inode->i_atime = inode->i_mtime = inode->i_ctime = CURRENT_TIME;
 		inode->i_generation = get_seconds();
 		info = SHMEM_I(inode);
···
 
 bool shmem_mapping(struct address_space *mapping)
 {
-	return mapping->backing_dev_info == &shmem_backing_dev_info;
+	return mapping->host->i_sb->s_op == &shmem_ops;
 }
 
 #ifdef CONFIG_TMPFS
···
 	if (shmem_inode_cachep)
 		return 0;
 
-	error = bdi_init(&shmem_backing_dev_info);
-	if (error)
-		goto out4;
-
 	error = shmem_init_inodecache();
 	if (error)
 		goto out3;
···
 out2:
 	shmem_destroy_inodecache();
 out3:
-	bdi_destroy(&shmem_backing_dev_info);
-out4:
 	shm_mnt = ERR_PTR(error);
 	return error;
 }

mm/swap.c

···
 #ifdef CONFIG_SWAP
 	int i;
 
-	if (bdi_init(swapper_spaces[0].backing_dev_info))
-		panic("Failed to init swap bdi");
 	for (i = 0; i < MAX_SWAPFILES; i++) {
 		spin_lock_init(&swapper_spaces[i].tree_lock);
 		INIT_LIST_HEAD(&swapper_spaces[i].i_mmap_nonlinear);

mm/swap_state.c

···
 #endif
 };
 
-static struct backing_dev_info swap_backing_dev_info = {
-	.name		= "swap",
-	.capabilities	= BDI_CAP_NO_ACCT_AND_WRITEBACK | BDI_CAP_SWAP_BACKED,
-};
-
 struct address_space swapper_spaces[MAX_SWAPFILES] = {
 	[0 ... MAX_SWAPFILES - 1] = {
 		.page_tree	= RADIX_TREE_INIT(GFP_ATOMIC|__GFP_NOWARN),
 		.i_mmap_writable = ATOMIC_INIT(0),
 		.a_ops		= &swap_aops,
-		.backing_dev_info = &swap_backing_dev_info,
+		.backing_dev_info = &noop_backing_dev_info,
 	}
 };