Re: [PATCH] ext4: zero out small extents when writing to prealloc area.

[Mingming Cao <cmm@us.ibm.com>]

On Tue, 2008-03-04 at 18:16 +0530, Aneesh Kumar K.V wrote:
> If the preallocated area is small zero out the full extent
> instead of splitting them. This should avoid the "write
> every alternate block" problem that could grow the number
> of extents dramatically.
> 
> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
> ---
>  fs/ext4/extents.c |   63 +++++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 files changed, 63 insertions(+), 0 deletions(-)
> 
> diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
> index 839caf2..dcdf92a 100644
> --- a/fs/ext4/extents.c
> +++ b/fs/ext4/extents.c
> @@ -2210,6 +2210,8 @@ static int ext4_ext_zeroout(struct inode *inode, struct ext4_extent *ex)
>  	return ret;
>  }
> 
> +#define EXT4_EXT_ZERO_LEN 7
> +
>  /*
>   * This function is called by ext4_ext_get_blocks() if someone tries to write
>   * to an uninitialized extent. It may result in splitting the uninitialized
> @@ -2252,6 +2254,18 @@ static int ext4_ext_convert_to_initialized(handle_t *handle,
>  	err = ext4_ext_get_access(handle, inode, path + depth);
>  	if (err)
>  		goto out;
> +	/* If extent has less than 2*EXT4_EXT_ZERO_LEN zerout directly */

Hmm, here this is range is extended to 2*EXT4_EXT_ZERO_LEN? I am a
little more biased to keep the threshold constant as EXT4_EXT_ZERO_LEN
around all places...

> +	if (ee_len <= 2*EXT4_EXT_ZERO_LEN) {
> +		err =  ext4_ext_zeroout(inode, &orig_ex);
> +		if (err)
> +			goto fix_extent_len;
> +		/* update the extent length and mark as initialized */
> +		ex->ee_block = orig_ex.ee_block;
> +		ex->ee_len   = orig_ex.ee_len;
> +		ext4_ext_store_pblock(ex, ext_pblock(&orig_ex));
> +		ext4_ext_dirty(handle, inode, path + depth);
> +		return le16_to_cpu(ex->ee_len);
> +	}
> 
>  	/* ex1: ee_block to iblock - 1 : uninitialized */
>  	if (iblock > ee_block) {
> @@ -2270,6 +2284,38 @@ static int ext4_ext_convert_to_initialized(handle_t *handle,
>  	/* ex3: to ee_block + ee_len : uninitialised */
>  	if (allocated > max_blocks) {
>  		unsigned int newdepth;
> +		/* If extent has less than EXT4_EXT_ZERO_LEN zerout directly */
> +		if (allocated <= EXT4_EXT_ZERO_LEN) {
> +			/* Mark first half uninitialized.
> +			 * Mark second half initialized and zero out the
> +			 * initialized extent
> +			 */
> +			ex->ee_block = orig_ex.ee_block;
> +			ex->ee_len   = cpu_to_le16(ee_len - allocated);
> +			ext4_ext_mark_uninitialized(ex);
> +			ext4_ext_store_pblock(ex, ext_pblock(&orig_ex));
> +			ext4_ext_dirty(handle, inode, path + depth);
> +
> +			ex3 = &newex;
> +			ex3->ee_block = cpu_to_le32(iblock);
> +			ext4_ext_store_pblock(ex3, newblock);
> +			ex3->ee_len = cpu_to_le16(allocated);
> +			err = ext4_ext_insert_extent(handle, inode, path, ex3);
> +			if (err == -ENOSPC) {
> +				err =  ext4_ext_zeroout(inode, &orig_ex);
> +				if (err)
> +					goto fix_extent_len;
> +				ex->ee_block = orig_ex.ee_block;
> +				ex->ee_len   = orig_ex.ee_len;
> +				ext4_ext_store_pblock(ex, ext_pblock(&orig_ex));
> +				ext4_ext_dirty(handle, inode, path + depth);
> +				return le16_to_cpu(ex->ee_len);
> +
> +			} else if (err)
> +				goto fix_extent_len;
> +
> +			return allocated;
> +		}
>  		ex3 = &newex;
>  		ex3->ee_block = cpu_to_le32(iblock + max_blocks);
>  		ext4_ext_store_pblock(ex3, newblock + max_blocks);
> @@ -2318,6 +2364,23 @@ static int ext4_ext_convert_to_initialized(handle_t *handle,
>  				goto out;
>  		}
>  		allocated = max_blocks;
> +
> +		/* If extent has less than EXT4_EXT_ZERO_LEN and we are trying
> +		 * to insert a extent in the middle zerout directly
> +		 * otherwise give the extent a chance to merge to left
> +		 */
> +		if (le16_to_cpu(orig_ex.ee_len) <= EXT4_EXT_ZERO_LEN &&
> +							iblock != ee_block) {
> +			err =  ext4_ext_zeroout(inode, &orig_ex);
> +			if (err)
> +				goto fix_extent_len;
> +			/* update the extent length and mark as initialized */
> +			ex->ee_block = orig_ex.ee_block;
> +			ex->ee_len   = orig_ex.ee_len;
> +			ext4_ext_store_pblock(ex, ext_pblock(&orig_ex));
> +			ext4_ext_dirty(handle, inode, path + depth);
> +			return le16_to_cpu(ex->ee_len);
> +		}
>  	}
>  	/*
>  	 * If there was a change of depth as part of the