Re: [PATCH v6 10/11] ext4: update ext4_sync_file() to not use __generic_file_fsync()

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On 10/28/19 4:23 PM, Matthew Bobrowski wrote:
When the filesystem is created without a journal, we eventually call
into __generic_file_fsync() in order to write out all the modified
in-core data to the permanent storage device. This function happens to
try and obtain an inode_lock() while synchronizing the files buffer
and it's associated metadata.

Generally, this is fine, however it becomes a problem when there is
higher level code that has already obtained an inode_lock() as this
leads to a recursive lock situation. This case is especially true when
porting across direct I/O to iomap infrastructure as we obtain an
inode_lock() early on in the I/O within ext4_dio_write_iter() and hold
it until the I/O has been completed. Consequently, to not run into
this specific issue, we move away from calling into
__generic_file_fsync() and perform the necessary synchronization tasks
within ext4_sync_file().

Signed-off-by: Matthew Bobrowski <mbobrowski@xxxxxxxxxxxxxx>


Thanks for the patch. Looks good to me.
You may add:
Reviewed-by: Ritesh Harjani <riteshh@xxxxxxxxxxxxx>


---

Thanks Jan and Christoph for the suggestion on this one, highly
appreciated.

  fs/ext4/fsync.c | 72 ++++++++++++++++++++++++++++++++-----------------
  1 file changed, 47 insertions(+), 25 deletions(-)

diff --git a/fs/ext4/fsync.c b/fs/ext4/fsync.c
index 5508baa11bb6..e10206e7f4bb 100644
--- a/fs/ext4/fsync.c
+++ b/fs/ext4/fsync.c
@@ -80,6 +80,43 @@ static int ext4_sync_parent(struct inode *inode)
  	return ret;
  }

+static int ext4_fsync_nojournal(struct inode *inode, bool datasync,
+				bool *needs_barrier)
+{
+	int ret, err;
+
+	ret = sync_mapping_buffers(inode->i_mapping);
+	if (!(inode->i_state & I_DIRTY_ALL))
+		return ret;
+	if (datasync && !(inode->i_state & I_DIRTY_DATASYNC))
+		return ret;
+
+	err = sync_inode_metadata(inode, 1);
+	if (!ret)
+		ret = err;
+
+	if (!ret)
+		ret = ext4_sync_parent(inode);
+	if (test_opt(inode->i_sb, BARRIER))
+		*needs_barrier = true;
+
+	return ret;
+}
+
+static int ext4_fsync_journal(struct inode *inode, bool datasync,
+			     bool *needs_barrier)
+{
+	struct ext4_inode_info *ei = EXT4_I(inode);
+	journal_t *journal = EXT4_SB(inode->i_sb)->s_journal;
+	tid_t commit_tid = datasync ? ei->i_datasync_tid : ei->i_sync_tid;
+
+	if (journal->j_flags & JBD2_BARRIER &&
+	    !jbd2_trans_will_send_data_barrier(journal, commit_tid))
+		*needs_barrier = true;
+
+	return jbd2_complete_transaction(journal, commit_tid);
+}
+
  /*
   * akpm: A new design for ext4_sync_file().
   *
@@ -91,17 +128,14 @@ static int ext4_sync_parent(struct inode *inode)
   * What we do is just kick off a commit and wait on it.  This will snapshot the
   * inode to disk.
   */
-
  int ext4_sync_file(struct file *file, loff_t start, loff_t end, int datasync)
  {
-	struct inode *inode = file->f_mapping->host;
-	struct ext4_inode_info *ei = EXT4_I(inode);
-	journal_t *journal = EXT4_SB(inode->i_sb)->s_journal;
  	int ret = 0, err;
-	tid_t commit_tid;
  	bool needs_barrier = false;
+	struct inode *inode = file->f_mapping->host;
+	struct ext4_sb_info *sbi = EXT4_SB(inode->i_sb);

-	if (unlikely(ext4_forced_shutdown(EXT4_SB(inode->i_sb))))
+	if (unlikely(ext4_forced_shutdown(sbi)))
  		return -EIO;

  	J_ASSERT(ext4_journal_current_handle() == NULL);
@@ -111,23 +145,15 @@ int ext4_sync_file(struct file *file, loff_t start, loff_t end, int datasync)
  	if (sb_rdonly(inode->i_sb)) {
  		/* Make sure that we read updated s_mount_flags value */
  		smp_rmb();
-		if (EXT4_SB(inode->i_sb)->s_mount_flags & EXT4_MF_FS_ABORTED)
+		if (sbi->s_mount_flags & EXT4_MF_FS_ABORTED)
  			ret = -EROFS;
  		goto out;
  	}

-	if (!journal) {
-		ret = __generic_file_fsync(file, start, end, datasync);
-		if (!ret)
-			ret = ext4_sync_parent(inode);
-		if (test_opt(inode->i_sb, BARRIER))
-			goto issue_flush;
-		goto out;
-	}
-
  	ret = file_write_and_wait_range(file, start, end);
  	if (ret)
  		return ret;
+
  	/*
  	 * data=writeback,ordered:
  	 *  The caller's filemap_fdatawrite()/wait will sync the data.
@@ -142,18 +168,14 @@ int ext4_sync_file(struct file *file, loff_t start, loff_t end, int datasync)
  	 *  (they were dirtied by commit).  But that's OK - the blocks are
  	 *  safe in-journal, which is all fsync() needs to ensure.
  	 */
-	if (ext4_should_journal_data(inode)) {
+	if (!sbi->s_journal)
+		ret = ext4_fsync_nojournal(inode, datasync, &needs_barrier);
+	else if (ext4_should_journal_data(inode))
  		ret = ext4_force_commit(inode->i_sb);
-		goto out;
-	}
+	else
+		ret = ext4_fsync_journal(inode, datasync, &needs_barrier);

-	commit_tid = datasync ? ei->i_datasync_tid : ei->i_sync_tid;
-	if (journal->j_flags & JBD2_BARRIER &&
-	    !jbd2_trans_will_send_data_barrier(journal, commit_tid))
-		needs_barrier = true;
-	ret = jbd2_complete_transaction(journal, commit_tid);
  	if (needs_barrier) {
-	issue_flush:
  		err = blkdev_issue_flush(inode->i_sb->s_bdev, GFP_KERNEL, NULL);
  		if (!ret)
  			ret = err;





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