linux-ext4 - Re: [Bug 32972] New: EXT4 causes corrupt BitTorrent downloads

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Message-ID: <20110411014625.GA5802@thunk.org>
Date:	Sun, 10 Apr 2011 21:46:25 -0400
From:	Ted Ts'o <tytso@....edu>
To:	Yongqiang Yang <xiaoqiangnk@...il.com>
Cc:	bugzilla-daemon@...zilla.kernel.org, damien@...ssart.com,
	feng.tang@...el.com, linux-ext4@...r.kernel.org
Subject: Re: [Bug 32972] New: EXT4 causes corrupt BitTorrent downloads

On Sun, Apr 10, 2011 at 10:30:13PM +0800, Yongqiang Yang wrote:
> So the right code should be：
> 
> 	bh->b_state = (bh->b_state & ~EXT4_MAP_FLAGS) | map.m_flags;
> 	map_bh(bh, inode->i_sb, map.m_pblk);
> 	if (buffer_unwritten(bh)) {
> 		/* A delayed write to unwritten bh should be marked
> 		 * new and mapped.  Mapped ensures that we don't do
> 		 * get_block multiple times when we write to the same
> 		 * offset and new ensures that we do proper zero out
> 		 * for partial write.
> 		 */
> 		set_buffer_new(bh);
> 	}

Actually, I'm much more comfortable backing out commit 6de9843da
entirely.  The above is *not* equivalent to what we had before ---
consider the case where ext4_map_blocks returns !EXT4_MAP_MAPPED && 
!EXT4_MAP_UNWRITTEN.

I don't *think* this should happen in the case where ext4_map_blocks
returns a value > 0, but the fact that it's not obvious, means I'd
much rather keep things the way that they are.  It's not like dropping
the set_buffer_mapped(bh) was saving anything measurable anyway....

						- Ted
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