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Date:	Thu, 01 Mar 2012 14:21:12 -0600
From:	Eric Sandeen <sandeen@...hat.com>
To:	Saugata Das <saugata.das@...ricsson.com>
CC:	linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	saugata.das@...aro.org, arnd.bergmann@...aro.org, tytso@....edu,
	adilger@...mcloud.com
Subject: Re: [PATCH] REQ_META set during meta data write

On 3/1/12 12:11 PM, Saugata Das wrote:
> From: Saugata Das <saugata.das@...aro.org>
> 
> Today, storage devices like eMMC has special features like data tagging
> (introduced in MMC-4.5 version) in order to improve performance of some
> specific writes. On MMC stack, data tagging is used for all writes which has
> REQ_META flag set. On EXT4, however, currently REQ_META is set only for read.
> This patch adds the capability to add REQ_META flag during meta data write.
> This patch adds new function set_buffer_meta, which is used to set a bit
> BH_Meta in the b_state field when meta data and super block is made dirty.
> 
> Signed-off-by: Saugata Das <saugata.das@...aro.org>
> Acked-by: Arnd Bergmann <arnd.bergmann@...aro.org>

I like this idea too, partly because I've had a project in the back of my
mind to teach seekwatcher to optionally distinguish between metadata &
data IO on its charts, but I had noticed that ext4 didn't always tag
things.

-Eric

> ---
>  fs/buffer.c                 |    3 +++
>  fs/ext4/ext4_jbd2.c         |    4 ++++
>  fs/ext4/inode.c             |    4 +++-
>  include/linux/buffer_head.h |    2 ++
>  4 files changed, 12 insertions(+), 1 deletions(-)
> 
> diff --git a/fs/buffer.c b/fs/buffer.c
> index 19d8eb7..71c8c74 100644
> --- a/fs/buffer.c
> +++ b/fs/buffer.c
> @@ -2926,6 +2926,9 @@ int submit_bh(int rw, struct buffer_head * bh)
>  	if (test_set_buffer_req(bh) && (rw & WRITE))
>  		clear_buffer_write_io_error(bh);
>  
> +	if (buffer_meta(bh) && (rw & WRITE))
> +		rw |= REQ_META;
> +
>  	/*
>  	 * from here on down, it's all bio -- do the initial mapping,
>  	 * submit_bio -> generic_make_request may further map this bio around
> diff --git a/fs/ext4/ext4_jbd2.c b/fs/ext4/ext4_jbd2.c
> index aca1790..097c062 100644
> --- a/fs/ext4/ext4_jbd2.c
> +++ b/fs/ext4/ext4_jbd2.c
> @@ -107,6 +107,8 @@ int __ext4_handle_dirty_metadata(const char *where, unsigned int line,
>  {
>  	int err = 0;
>  
> +	set_buffer_meta(bh);
> +
>  	if (ext4_handle_valid(handle)) {
>  		err = jbd2_journal_dirty_metadata(handle, bh);
>  		if (err) {
> @@ -143,6 +145,8 @@ int __ext4_handle_dirty_super(const char *where, unsigned int line,
>  	struct buffer_head *bh = EXT4_SB(sb)->s_sbh;
>  	int err = 0;
>  
> +	set_buffer_meta(bh);
> +
>  	if (ext4_handle_valid(handle)) {
>  		err = jbd2_journal_dirty_metadata(handle, bh);
>  		if (err)
> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> index 92655fd..105e1d0 100644
> --- a/fs/ext4/inode.c
> +++ b/fs/ext4/inode.c
> @@ -4144,8 +4144,10 @@ int ext4_write_inode(struct inode *inode, struct writeback_control *wbc)
>  		err = __ext4_get_inode_loc(inode, &iloc, 0);
>  		if (err)
>  			return err;
> -		if (wbc->sync_mode == WB_SYNC_ALL)
> +		if (wbc->sync_mode == WB_SYNC_ALL) {
> +			set_buffer_meta(iloc.bh);
>  			sync_dirty_buffer(iloc.bh);
> +		}
>  		if (buffer_req(iloc.bh) && !buffer_uptodate(iloc.bh)) {
>  			EXT4_ERROR_INODE_BLOCK(inode, iloc.bh->b_blocknr,
>  					 "IO error syncing inode");
> diff --git a/include/linux/buffer_head.h b/include/linux/buffer_head.h
> index 458f497..13bba17 100644
> --- a/include/linux/buffer_head.h
> +++ b/include/linux/buffer_head.h
> @@ -34,6 +34,7 @@ enum bh_state_bits {
>  	BH_Write_EIO,	/* I/O error on write */
>  	BH_Unwritten,	/* Buffer is allocated on disk but not written */
>  	BH_Quiet,	/* Buffer Error Prinks to be quiet */
> +	BH_Meta,	/* Is meta */
>  
>  	BH_PrivateStart,/* not a state bit, but the first bit available
>  			 * for private allocation by other entities
> @@ -124,6 +125,7 @@ BUFFER_FNS(Delay, delay)
>  BUFFER_FNS(Boundary, boundary)
>  BUFFER_FNS(Write_EIO, write_io_error)
>  BUFFER_FNS(Unwritten, unwritten)
> +BUFFER_FNS(Meta, meta)
>  
>  #define bh_offset(bh)		((unsigned long)(bh)->b_data & ~PAGE_MASK)
>  #define touch_buffer(bh)	mark_page_accessed(bh->b_page)

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