Re: [PATCH 3/5] ext4_zero_range: fix incorect journal credits reservation

[Dmitry Monakhov <dmonakhov@openvz.org>]

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On Sat, 23 Aug 2014 15:08:35 -0400, "Theodore Ts'o" <tytso@mit.edu> wrote:
> On Fri, Aug 22, 2014 at 03:32:25PM +0400, Dmitry Monakhov wrote:
> > -	handle = ext4_journal_start(inode, EXT4_HT_MISC, 4);
> > +	/* In worst case we have to writeout two nonadjacent unwritten blocks */
> > +	credits = ext4_chunk_trans_blocks(inode, 1) * 2 -
> > +		EXT4_META_TRANS_BLOCKS(inode->i_sb);
> 
> This looks like it would be a massive over-estimate, since it includes
> the block group allocation bitmaps, which we wouldn't need to update,
> no?
  Oh. You right. groups metadata/quota should not be affected.
> 
> Wouldn't
> 
> 	credts = ext4_index_trans_blocks(inode, 1, 1) * 2;
Yes, but ext4_index_trans_blocks is statically defined in inode.c,
but since zero_range works only for extent based
ext4_ext_index_trans_blocks is sufficient. Also we must reserve 2 block
for data. So final calculation looks like follows:
credits =  ext4_ext_index_trans_blocks(inode, 2) + 2


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>From ec7e1926e8bb99ccd0eb525efc6666cb8ea36a5e Mon Sep 17 00:00:00 2001
From: Dmitry Monakhov <dmonakhov@openvz.org>
Date: Mon, 25 Aug 2014 11:12:35 +0400
Subject: [PATCH] ext4_zero_range: fix incorect journal credits reservation v2

Currently we reserve only 4 blocks but in worst case scenario
ext4_zero_partial_blocks() may want to zeroout and convert two
non adjacent blocks.

Signed-off-by: Dmitry Monakhov <dmonakhov@openvz.org>
---
 fs/ext4/extents.c |    6 ++++--
 1 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
index 5bf718b..f09dbe7 100644
--- a/fs/ext4/extents.c
+++ b/fs/ext4/extents.c
@@ -4740,6 +4740,7 @@ static long ext4_zero_range(struct file *file, loff_t offset,
 	loff_t new_size = 0;
 	int ret = 0;
 	int flags;
+	int credits;
 	int partial;
 	loff_t start, end;
 	ext4_lblk_t lblk;
@@ -4839,8 +4840,9 @@ static long ext4_zero_range(struct file *file, loff_t offset,
 		if (ret)
 			goto out_dio;
 	}