Date:	Thu, 21 Feb 2013 23:05:44 -0500
From:	Theodore Ts'o <tytso@....edu>
To:	Lukáš Czerner <lczerner@...hat.com>,
	linux-ext4@...r.kernel.org
Subject: Re: [PATCH] ext4: fix overhead calculation in bigalloc filesystem
 (Re: ... )

On Fri, Feb 22, 2013 at 11:03:27AM +0800, Zheng Liu wrote:
> 
> I agree with you that we should forbid user to use bigalloc feature with
> block size = 1k or 2k because I guess no one really use it, at least in
> Taobao we always use bigalloc feature with block size = 4k.

The main reason why file systems with 1k or 2k (with or without
bigalloc) is that it's useful is that it's a good way of testing what
happens on an architecture with a 16k page size and a 4k blocksize.  I
am regularly testing with a 1k blocksize because it catches problems
that would otherwise only show up on PowerPC or Intanium platforms.
I'm not testing with bigalloc plus 1k block size, though, I admit.

> FWIW, recently I am considering whether we could remove 'data=journal'
> and 'data=writeback' mode.  'data=journal' mode hurts performance
> dramatically.  Further, 'data=writeback' seems also useless, especially
> after we have 'no journal' feature.  TBH, these modes are lack of tests.
> Maybe this is a crazy idea in my mind.

As far as data=writeback and data=ordered are concerned, the only
difference is a single call to ext4_jbd2_file_end() in
ext4_ordered_write_end().  In fact, it looks like we could do
something like the following attached patch to simplify the code and
improve code coverage from a testing perspective.  (Note: patch not
yet tested!)

As far as "data=journalled", it's a bit more complicated but I do have
a sentimental attachment to it, since it's something which no other
file system has.  I have been regularly testing it, and it's something
which has been pretty stable for quite a while now.

If there was a mode that I'd be tempted to get rid of, it would be the
combined data=ordered/data=writeback modes.  The main reason why we
keep it is because of a concern of buggy applications that depend on
the implied fsync() of data=ordered mode at each commit.  However,
ext4 has been around for long enough that I think most of the buggy
applications have been fixed by now.  And of course, those buggy
applications will lose in the same way when they are using btrfs and
xfs.  Something to consider....

						- Ted

commit d075b5c3031752a4c41642ff505c3302e5321940
Author: Theodore Ts'o <tytso@....edu>
Date:   Thu Feb 21 23:04:59 2013 -0500

    ext4: collapse handling of data=ordered and data=writeback codepaths
    
    The only difference between how we handle data=ordered and
    data=writeback is a single call to ext4_jbd2_file_inode().  Eliminate
    code duplication by factoring out redundant the code paths.
    
    Signed-off-by: "Theodore Ts'o" <tytso@....edu>

diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
index 6e16c18..85dfd49 100644
--- a/fs/ext4/ext4.h
+++ b/fs/ext4/ext4.h
@@ -1373,6 +1373,7 @@ enum {
 	EXT4_STATE_DIOREAD_LOCK,	/* Disable support for dio read
 					   nolocking */
 	EXT4_STATE_MAY_INLINE_DATA,	/* may have in-inode data */
+	EXT4_STATE_ORDERED_MODE,	/* data=ordered mode */
 };
 
 #define EXT4_INODE_BIT_FNS(name, field, offset)				\
diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 95a0c62..2e338a6 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -1055,77 +1055,36 @@ static int ext4_generic_write_end(struct file *file,
  * ext4 never places buffers on inode->i_mapping->private_list.  metadata
  * buffers are managed internally.
  */
-static int ext4_ordered_write_end(struct file *file,
-				  struct address_space *mapping,
-				  loff_t pos, unsigned len, unsigned copied,
-				  struct page *page, void *fsdata)
+static int ext4_write_end(struct file *file,
+			  struct address_space *mapping,
+			  loff_t pos, unsigned len, unsigned copied,
+			  struct page *page, void *fsdata)
 {
 	handle_t *handle = ext4_journal_current_handle();
 	struct inode *inode = mapping->host;
 	int ret = 0, ret2;
 
 	trace_ext4_ordered_write_end(inode, pos, len, copied);
-	ret = ext4_jbd2_file_inode(handle, inode);
-
-	if (ret == 0) {
-		ret2 = ext4_generic_write_end(file, mapping, pos, len, copied,
-							page, fsdata);
-		copied = ret2;
-		if (pos + len > inode->i_size && ext4_can_truncate(inode))
-			/* if we have allocated more blocks and copied
-			 * less. We will have blocks allocated outside
-			 * inode->i_size. So truncate them
-			 */
-			ext4_orphan_add(handle, inode);
-		if (ret2 < 0)
-			ret = ret2;
-	} else {
-		unlock_page(page);
-		page_cache_release(page);
-	}
-
-	ret2 = ext4_journal_stop(handle);
-	if (!ret)
-		ret = ret2;
-
-	if (pos + len > inode->i_size) {
-		ext4_truncate_failed_write(inode);
-		/*
-		 * If truncate failed early the inode might still be
-		 * on the orphan list; we need to make sure the inode
-		 * is removed from the orphan list in that case.
-		 */
-		if (inode->i_nlink)
-			ext4_orphan_del(NULL, inode);
+	if (ext4_test_inode_state(inode, EXT4_STATE_ORDERED_MODE)) {
+		ret = ext4_jbd2_file_inode(handle, inode);
+		if (ret) {
+			unlock_page(page);
+			page_cache_release(page);
+			goto errout;
+		}
 	}
 
-
-	return ret ? ret : copied;
-}
-
-static int ext4_writeback_write_end(struct file *file,
-				    struct address_space *mapping,
-				    loff_t pos, unsigned len, unsigned copied,
-				    struct page *page, void *fsdata)
-{
-	handle_t *handle = ext4_journal_current_handle();
-	struct inode *inode = mapping->host;
-	int ret = 0, ret2;
-
-	trace_ext4_writeback_write_end(inode, pos, len, copied);
-	ret2 = ext4_generic_write_end(file, mapping, pos, len, copied,
-							page, fsdata);
-	copied = ret2;
+	copied = ext4_generic_write_end(file, mapping, pos, len, copied,
+					page, fsdata);
+	if (copied < 0)
+		ret = copied;
 	if (pos + len > inode->i_size && ext4_can_truncate(inode))
 		/* if we have allocated more blocks and copied
 		 * less. We will have blocks allocated outside
 		 * inode->i_size. So truncate them
 		 */
 		ext4_orphan_add(handle, inode);
-
-	if (ret2 < 0)
-		ret = ret2;
-
+errout:
 	ret2 = ext4_journal_stop(handle);
 	if (!ret)
 		ret = ret2;
@@ -2656,18 +2615,9 @@ static int ext4_da_write_end(struct file *file,
 	unsigned long start, end;
 	int write_mode = (int)(unsigned long)fsdata;
 
-	if (write_mode == FALL_BACK_TO_NONDELALLOC) {
-		switch (ext4_inode_journal_mode(inode)) {
-		case EXT4_INODE_ORDERED_DATA_MODE:
-			return ext4_ordered_write_end(file, mapping, pos,
-					len, copied, page, fsdata);
-		case EXT4_INODE_WRITEBACK_DATA_MODE:
-			return ext4_writeback_write_end(file, mapping, pos,
-					len, copied, page, fsdata);
-		default:
-			BUG();
-		}
-	}
+	if (write_mode == FALL_BACK_TO_NONDELALLOC)
+		return ext4_write_end(file, mapping, pos,
+				      len, copied, page, fsdata);
 
 	trace_ext4_da_write_end(inode, pos, len, copied);
 	start = pos & (PAGE_CACHE_SIZE - 1);
@@ -3172,27 +3122,12 @@ static int ext4_journalled_set_page_dirty(struct page *page)
 	return __set_page_dirty_nobuffers(page);
 }
 
-static const struct address_space_operations ext4_ordered_aops = {
+static const struct address_space_operations ext4_aops = {
 	.readpage		= ext4_readpage,
 	.readpages		= ext4_readpages,
 	.writepage		= ext4_writepage,
 	.write_begin		= ext4_write_begin,
-	.write_end		= ext4_ordered_write_end,
-	.bmap			= ext4_bmap,
-	.invalidatepage		= ext4_invalidatepage,
-	.releasepage		= ext4_releasepage,
-	.direct_IO		= ext4_direct_IO,
-	.migratepage		= buffer_migrate_page,
-	.is_partially_uptodate  = block_is_partially_uptodate,
-	.error_remove_page	= generic_error_remove_page,
-};
-
-static const struct address_space_operations ext4_writeback_aops = {
-	.readpage		= ext4_readpage,
-	.readpages		= ext4_readpages,
-	.writepage		= ext4_writepage,
-	.write_begin		= ext4_write_begin,
-	.write_end		= ext4_writeback_write_end,
+	.write_end		= ext4_write_end,
 	.bmap			= ext4_bmap,
 	.invalidatepage		= ext4_invalidatepage,
 	.releasepage		= ext4_releasepage,
@@ -3237,23 +3172,21 @@ void ext4_set_aops(struct inode *inode)
 {
 	switch (ext4_inode_journal_mode(inode)) {
 	case EXT4_INODE_ORDERED_DATA_MODE:
-		if (test_opt(inode->i_sb, DELALLOC))
-			inode->i_mapping->a_ops = &ext4_da_aops;
-		else
-			inode->i_mapping->a_ops = &ext4_ordered_aops;
+		ext4_set_inode_state(inode, EXT4_STATE_ORDERED_MODE);
 		break;
 	case EXT4_INODE_WRITEBACK_DATA_MODE:
-		if (test_opt(inode->i_sb, DELALLOC))
-			inode->i_mapping->a_ops = &ext4_da_aops;
-		else
-			inode->i_mapping->a_ops = &ext4_writeback_aops;
+		ext4_clear_inode_state(inode, EXT4_STATE_ORDERED_MODE);
 		break;
 	case EXT4_INODE_JOURNAL_DATA_MODE:
 		inode->i_mapping->a_ops = &ext4_journalled_aops;
-		break;
+		return;
 	default:
 		BUG();
 	}
+	if (test_opt(inode->i_sb, DELALLOC))
+		inode->i_mapping->a_ops = &ext4_da_aops;
+	else
+		inode->i_mapping->a_ops = &ext4_aops;
 }
 
 
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