From: Mingming Cao <cmm@us.ibm.com>
Subject: Re: [PATCH] ext4: Need clear buffer_delay after page writeout for
	delayed allocation
Date: Mon, 02 Jun 2008 00:04:07 -0700
Message-ID: <1212390247.4368.107.camel@localhost.localdomain>
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Cc: jack@suse.cz, linux-ext4@vger.kernel.org
To: "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>
In-Reply-To: <20080602063519.GA26379@skywalker>
Sender: linux-ext4-owner@vger.kernel.org

On Mon, 2008-06-02 at 12:05 +0530, Aneesh Kumar K.V wrote:
> On Sun, Jun 01, 2008 at 10:38:35PM -0700, Mingming Cao wrote:
> > On Mon, 2008-06-02 at 09:39 +0530, Aneesh Kumar K.V wrote:
> > > On Sun, Jun 01, 2008 at 08:50:32PM -0700, Mingming Cao wrote:
> > > > On Mon, 2008-06-02 at 08:44 +0530, Aneesh Kumar K.V wrote:
> > > > > On Sun, Jun 01, 2008 at 02:10:02PM -0700, Mingming Cao wrote:
> > > > > > ext4:  Need clear buffer_delay after page writeout for delayed allocation
> > > > > > 
> > > > > > From: Mingming Cao <cmm@us.ibm.com>
> > > > > > 
> > > > > > Need clear buffer_delay in ext4_da_writepage() after page has been writeout
> > > > > > 
> > > > > > Signed-off-by: Mingming Cao <cmm@us.ibm.com>
> > > > > > 
> > > > > > ---
> > > > > 
> > > > > We do that in mpage_put_bnr_to_bhs.
> > > > > 
> > > > Normally delayed buffer could be cleared in that case, but if allocation
> > > > failed in __mapge_da_writepages(), it will keep buffer_delay marked and
> > > > deferring to later ext4_da_writepage() to do block allocation. This
> > > > patch handles clear bh delay bit in this case.
> > > > 
> > > 
> > > Why not do it in ext4_da_get_block_write then.
> > 
> > The buffer head passed to ext4_da_get_block_write() calling from
> > mpage_da_map_blocks is a dummy one, to store the allocated extent, not
> > the bh that need map.
> 
> 
> ie true when ext4_da_get_block_write is called via writepages. In
> that case mpage_put_bnr_to_bhs clears the delay bit properly. How about
> the changes below.
> 
I see your patch below is trying to address how to detect and assign
blocks with your suggestion(i.e clear delayed bit in get_block). But I
don;t think it's needed. 

My last email I mean the buffer head new in mpage_da_map_blocks() is a
dummy bh, the real buffer head lbh is not passed to get_block. We could
clear the delayed bit on successful return of get_block,
mpage_put_bnr_to_bhs() ignore that dummy bh anyway. But that seems
twisted, unccessary.

I still think clear the bit in the ext4_da_write_page() is more clean
way. the original patch clears the delayed bit on success case. 

For the error case I think we could handle properly by only clear the
delayed bit if buffer is mapped.

Mingming
> 
> diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
> index 26f3626..9d25255 100644
> --- a/fs/ext4/extents.c
> +++ b/fs/ext4/extents.c
> @@ -2873,6 +2873,7 @@ int ext4_ext_get_blocks(handle_t *handle, struct inode *inode,
>  		allocated = max_blocks;
>  	ext4_ext_show_leaf(inode, path);
>  	__set_bit(BH_Mapped, &bh_result->b_state);
> +	__clear_bit(BH_Delay, &bh_result->b_state);
>  	bh_result->b_bdev = inode->i_sb->s_bdev;
>  	bh_result->b_blocknr = newblock;
>  out2:
> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> index 2194aa7..2a2f832 100644
> --- a/fs/ext4/inode.c
> +++ b/fs/ext4/inode.c
> @@ -917,6 +917,7 @@ int ext4_get_blocks_handle(handle_t *handle, struct inode *inode,
>  		goto cleanup;
> 
>  	set_buffer_new(bh_result);
> +	clear_buffer_delay(bh_result);
>  got_it:
>  	map_bh(bh_result, inode->i_sb, le32_to_cpu(chain[depth-1].key));
>  	if (count > blocks_to_boundary)
> diff --git a/fs/mpage.c b/fs/mpage.c
> index c107728..0d05d7a 100644
> --- a/fs/mpage.c
> +++ b/fs/mpage.c
> @@ -897,6 +897,14 @@ static void mpage_da_map_blocks(struct mpage_da_data *mpd)
>  	int err = 0, remain = lbh->b_size;
>  	sector_t next = lbh->b_blocknr;
>  	struct buffer_head new;
> +	int delay_buf = 0;
> +
> +	/*
> +	 * We clear the delay bit in get_block so remember
> +	 * whether we are doing a delay allocation
> +	 */
> +	if (buffer_delay(lbh))
> +		delay_buf = 1;
> 
>  	/*
>  	 * We consider only non-mapped and non-allocated blocks
> @@ -926,7 +934,7 @@ static void mpage_da_map_blocks(struct mpage_da_data *mpd)
>  		 * If blocks are delayed marked, we need to
>  		 * put actual blocknr and drop delayed bit
>  		 */
> -		if (buffer_delay(lbh))
> +		if (delay_buf)
>  			mpage_put_bnr_to_bhs(mpd, next, &new);
> 
>  		/* go for the remaining blocks */
>