[PATCH RFC] ext4: enable scoped NOFS when starting a handle in nojournal mode

["Theodore Ts'o" <tytso@mit.edu>]

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>
---
 fs/ext4/ext4_jbd2.c  | 35 ++++++++++++++++++++++-------------
 fs/ext4/ext4_jbd2.h  |  6 +-----
 include/linux/jbd2.h |  1 +
 3 files changed, 24 insertions(+), 18 deletions(-)

diff --git a/fs/ext4/ext4_jbd2.c b/fs/ext4/ext4_jbd2.c
index 9a8c225f2753..c2f09cf4b506 100644
--- a/fs/ext4/ext4_jbd2.c
+++ b/fs/ext4/ext4_jbd2.c
@@ -35,12 +35,21 @@ 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);
-
-	ref_cnt++;
-	handle = (handle_t *)ref_cnt;
-
-	current->journal_info = handle;
+	BUG_ON(handle && !handle->h_invalid);
+
+	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 +57,14 @@ 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;
+	}
 }
 
 /*
diff --git a/fs/ext4/ext4_jbd2.h b/fs/ext4/ext4_jbd2.h
index 63d17c5201b5..75d4670d389c 100644
--- a/fs/ext4/ext4_jbd2.h
+++ b/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->h_invalid;
 }
 
 static inline void ext4_handle_sync(handle_t *handle)
diff --git a/include/linux/jbd2.h b/include/linux/jbd2.h
index b68561187e90..7348fdadc810 100644
--- a/include/linux/jbd2.h
+++ b/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;
 
-- 
2.53.0