ext4: enable scoped NOFS when starting a handle in nojournal mode

The jbd2 layer enables NOFS mode using memalloc_nofs_{save,restore}()
while a handle is active.  We need to do the same in nojournal mode so
that it is safe to remove GFP_NOFS flags while a jbd2 handle is
active.

This will require that we actually allocate a real handle, but with an
h_invalid flag set, so there is a place to put the saved memalloc
context.

Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Andreas Dilger <adilger@dilger.ca>

fs/ext4/ext4_jbd2.c

@@ -33,14 +33,22 @@ int ext4_inode_journal_mode(struct inode *inode)
 static handle_t *ext4_get_nojournal(void)
 {
 	handle_t *handle = current->journal_info;
-	unsigned long ref_cnt = (unsigned long)handle;
 
-	BUG_ON(ref_cnt >= EXT4_NOJOURNAL_MAX_REF_COUNT);
+	BUG_ON(handle && !handle->h_invalid);
 
-	ref_cnt++;
-	handle = (handle_t *)ref_cnt;
-
-	current->journal_info = handle;
+	if (!handle) {
+		handle = jbd2_alloc_handle(GFP_NOFS);
+		if (!handle)
+			return ERR_PTR(-ENOMEM);
+		handle->h_invalid = 1;
+		/*
+		 * This is done by start_this_handle() if journalling
+		 * is enabled.
+		 */
+		handle->saved_alloc_context = memalloc_nofs_save();
+		current->journal_info = handle;
+	}
+	handle->h_ref++;
 	return handle;
 }
 
@@ -48,14 +56,14 @@ static handle_t *ext4_get_nojournal(void)
 /* Decrement the non-pointer handle value */
 static void ext4_put_nojournal(handle_t *handle)
 {
-	unsigned long ref_cnt = (unsigned long)handle;
+	BUG_ON(handle->h_ref == 0);
 
-	BUG_ON(ref_cnt == 0);
-
-	ref_cnt--;
-	handle = (handle_t *)ref_cnt;
-
-	current->journal_info = handle;
+	handle->h_ref--;
+	if (handle->h_ref == 0) {
+		memalloc_nofs_restore(handle->saved_alloc_context);
+		jbd2_free_handle(handle);
+		current->journal_info = NULL;
+	}
 }
 
 /*

fs/ext4/ext4_jbd2.h

@@ -182,15 +182,11 @@ handle_t *__ext4_journal_start_sb(struct inode *inode, struct super_block *sb,
 				  int rsv_blocks, int revoke_creds);
 int __ext4_journal_stop(const char *where, unsigned int line, handle_t *handle);
 
-#define EXT4_NOJOURNAL_MAX_REF_COUNT ((unsigned long) 4096)
-
 /* Note:  Do not use this for NULL handles.  This is only to determine if
  * a properly allocated handle is using a journal or not. */
 static inline int ext4_handle_valid(handle_t *handle)
 {
-	if ((unsigned long)handle < EXT4_NOJOURNAL_MAX_REF_COUNT)
-		return 0;
-	return 1;
+	return (handle && !handle->h_invalid);
 }
 
 static inline void ext4_handle_sync(handle_t *handle)

fs/jbd2/journal.c

@@ -94,6 +94,7 @@ EXPORT_SYMBOL(jbd2_journal_init_jbd_inode);
 EXPORT_SYMBOL(jbd2_journal_release_jbd_inode);
 EXPORT_SYMBOL(jbd2_journal_begin_ordered_truncate);
 EXPORT_SYMBOL(jbd2_inode_cache);
+EXPORT_SYMBOL(jbd2_handle_cache);
 
 #ifdef CONFIG_JBD2_DEBUG
 void __jbd2_debug(int level, const char *file, const char *func,

include/linux/jbd2.h

@@ -513,6 +513,7 @@ struct jbd2_journal_handle
 	unsigned int	h_sync:		1;
 	unsigned int	h_reserved:	1;
 	unsigned int	h_aborted:	1;
+	unsigned int	h_invalid:	1;
 	unsigned int	h_type:		8;
 	unsigned int	h_line_no:	16;