Date:	Mon, 19 Mar 2012 19:24:32 +0100 (CET)
From:	Lukas Czerner <lczerner@...hat.com>
To:	Lukas Czerner <lczerner@...hat.com>
cc:	linux-ext4@...r.kernel.org, tytso@....edu,
	achender@...ux.vnet.ibm.com
Subject: Re: [PATCH 1/3 v2] ext4: Rewrite punch hole to use
 ext4_ext_remove_space()

Hi Allison,

I just want to let you know that I have rebased your patches to
allow punching hole past i_size, but I am going to send it as a separate
series hopefully this week. I need to solve the problem with cleaning
EOFBLOCKS_FL flag properly first. So just you know that you do not need
to worry about those.

Thanks!
-Lukas

On Mon, 19 Mar 2012, Lukas Czerner wrote:

> This commit rewrites ext4 punch hole implementation to use
> ext4_ext_remove_space() instead of its home gown way of doing this via
> ext4_ext_map_blocks(). There are several reasons for changing this.
> 
> Firstly it is quite non obvious that punching hole needs to
> ext4_ext_map_blocks() to punch a hole, especially given that this
> function should map blocks, not unmap it. It also required a lot of new
> code in ext4_ext_map_blocks().
> 
> Secondly the design of it is not very effective. The reason is that we
> are trying to punch out blocks in ext4_ext_punch_hole() in opposite
> direction than in ext4_ext_rm_leaf() which causes the ext4_ext_rm_leaf()
> to iterate through the whole tree from the end to the start to find the
> requested extent for every extent we are going to punch out.
> 
> And finally the current implementation does not use the existing code,
> but bring a lot of new code, which is IMO unnecessary since there
> already is some infrastructure we can use. Specifically
> ext4_ext_remove_space().
> 
> This commit changes ext4_ext_remove_space() to accept 'end' parameter so
> we can not only truncate to the end of file, but also remove the space
> in the middle of the file (punch a hole). Moreover, because the last
> block to punch out, might be in the middle of the extent, we have to
> split the extent at 'end + 1' so ext4_ext_rm_leaf() can easily either
> remove the whole fist part of split extent, or change its size.
> 
> ext4_ext_remove_space() is then used to actually remove the space
> (extents) from within the hole, instead of ext4_ext_map_blocks().
> 
> Note that this also fix the issue with punch hole, where we would forget
> to remove empty index blocks from the extent tree, resulting in double
> free block error and file system corruption. This is simply because we
> now use different code path, where this problem does not exist.
> 
> This has been tested with fsx running for several days and xfstests,
> plus xfstest #251 with '-o discard' run on the loop image (which
> converts discard requestes into punch hole to the backing file). All of
> it on 1K and 4K file system block size.
> 
> Signed-off-by: Lukas Czerner <lczerner@...hat.com>
> ---
> v2: Change condition to distinct punch_hole from truncate in
>     ext4_ext_remove_space()
> 
>  fs/ext4/extents.c |  170 +++++++++++++++++++++++++++-------------------------
>  1 files changed, 88 insertions(+), 82 deletions(-)
> 
> diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
> index 74f23c2..522f429 100644
> --- a/fs/ext4/extents.c
> +++ b/fs/ext4/extents.c
> @@ -44,6 +44,14 @@
>  
>  #include <trace/events/ext4.h>
>  
> +/*
> + * used by extent splitting.
> + */
> +#define EXT4_EXT_MAY_ZEROOUT	0x1  /* safe to zeroout if split fails \
> +					due to ENOSPC */
> +#define EXT4_EXT_MARK_UNINIT1	0x2  /* mark first half uninitialized */
> +#define EXT4_EXT_MARK_UNINIT2	0x4  /* mark second half uninitialized */
> +
>  static int ext4_split_extent(handle_t *handle,
>  				struct inode *inode,
>  				struct ext4_ext_path *path,
> @@ -51,6 +59,13 @@ static int ext4_split_extent(handle_t *handle,
>  				int split_flag,
>  				int flags);
>  
> +static int ext4_split_extent_at(handle_t *handle,
> +			     struct inode *inode,
> +			     struct ext4_ext_path *path,
> +			     ext4_lblk_t split,
> +			     int split_flag,
> +			     int flags);
> +
>  static int ext4_ext_truncate_extend_restart(handle_t *handle,
>  					    struct inode *inode,
>  					    int needed)
> @@ -2308,7 +2323,7 @@ ext4_ext_rm_leaf(handle_t *handle, struct inode *inode,
>  	struct ext4_extent *ex;
>  
>  	/* the header must be checked already in ext4_ext_remove_space() */
> -	ext_debug("truncate since %u in leaf\n", start);
> +	ext_debug("truncate since %u in leaf to %u\n", start, end);
>  	if (!path[depth].p_hdr)
>  		path[depth].p_hdr = ext_block_hdr(path[depth].p_bh);
>  	eh = path[depth].p_hdr;
> @@ -2343,7 +2358,7 @@ ext4_ext_rm_leaf(handle_t *handle, struct inode *inode,
>  		ext_debug("  border %u:%u\n", a, b);
>  
>  		/* If this extent is beyond the end of the hole, skip it */
> -		if (end <= ex_ee_block) {
> +		if (end < ex_ee_block) {
>  			ex--;
>  			ex_ee_block = le32_to_cpu(ex->ee_block);
>  			ex_ee_len = ext4_ext_get_actual_len(ex);
> @@ -2482,7 +2497,8 @@ ext4_ext_more_to_rm(struct ext4_ext_path *path)
>  	return 1;
>  }
>  
> -static int ext4_ext_remove_space(struct inode *inode, ext4_lblk_t start)
> +static int ext4_ext_remove_space(struct inode *inode, ext4_lblk_t start,
> +				 ext4_lblk_t end)
>  {
>  	struct super_block *sb = inode->i_sb;
>  	int depth = ext_depth(inode);
> @@ -2491,7 +2507,7 @@ static int ext4_ext_remove_space(struct inode *inode, ext4_lblk_t start)
>  	handle_t *handle;
>  	int i, err;
>  
> -	ext_debug("truncate since %u\n", start);
> +	ext_debug("truncate since %u to %u\n", start, end);
>  
>  	/* probably first extent we're gonna free will be last in block */
>  	handle = ext4_journal_start(inode, depth + 1);
> @@ -2504,6 +2520,61 @@ again:
>  	trace_ext4_ext_remove_space(inode, start, depth);
>  
>  	/*
> +	 * Check if we are removing extents inside the extent tree. If that
> +	 * is the case, we are going to punch a hole inside the extent tree
> +	 * so we have to check whether we need to split the extent covering
> +	 * the last block to remove so we can easily remove the part of it
> +	 * in ext4_ext_rm_leaf().
> +	 */
> +	if (end < EXT_MAX_BLOCKS - 1) {
> +		struct ext4_extent *ex;
> +		ext4_lblk_t ee_block;
> +
> +		/* find extent for this block */
> +		path = ext4_ext_find_extent(inode, end, NULL);
> +		if (IS_ERR(path)) {
> +			ext4_journal_stop(handle);
> +			return PTR_ERR(path);
> +		}
> +		depth = ext_depth(inode);
> +		ex = path[depth].p_ext;
> +		if (!ex)
> +			goto cont;
> +
> +		ee_block = le32_to_cpu(ex->ee_block);
> +
> +		/*
> +		 * See if the last block is inside the extent, if so split
> +		 * the extent at 'end' block so we can easily remove the
> +		 * tail of the first part of the split extent in
> +		 * ext4_ext_rm_leaf().
> +		 */
> +		if (end >= ee_block &&
> +		    end < ee_block + ext4_ext_get_actual_len(ex) - 1) {
> +			int split_flag = 0;
> +
> +			if (ext4_ext_is_uninitialized(ex))
> +				split_flag = EXT4_EXT_MARK_UNINIT1 |
> +					     EXT4_EXT_MARK_UNINIT2;
> +
> +			/*
> +			 * Split the extent in two so that 'end' is the last
> +			 * block in the first new extent
> +			 */
> +			err = ext4_split_extent_at(handle, inode, path,
> +						end + 1, split_flag,
> +						EXT4_GET_BLOCKS_PRE_IO |
> +						EXT4_GET_BLOCKS_PUNCH_OUT_EXT);
> +
> +			if (err < 0)
> +				goto out;
> +		}
> +		ext4_ext_drop_refs(path);
> +		kfree(path);
> +	}
> +cont:
> +
> +	/*
>  	 * We start scanning from right side, freeing all the blocks
>  	 * after i_size and walking into the tree depth-wise.
>  	 */
> @@ -2515,6 +2586,7 @@ again:
>  	}
>  	path[0].p_depth = depth;
>  	path[0].p_hdr = ext_inode_hdr(inode);
> +
>  	if (ext4_ext_check(inode, path[0].p_hdr, depth)) {
>  		err = -EIO;
>  		goto out;
> @@ -2526,7 +2598,7 @@ again:
>  			/* this is leaf block */
>  			err = ext4_ext_rm_leaf(handle, inode, path,
>  					       &partial_cluster, start,
> -					       EXT_MAX_BLOCKS - 1);
> +					       end);
>  			/* root level has p_bh == NULL, brelse() eats this */
>  			brelse(path[i].p_bh);
>  			path[i].p_bh = NULL;
> @@ -2709,14 +2781,6 @@ static int ext4_ext_zeroout(struct inode *inode, struct ext4_extent *ex)
>  }
>  
>  /*
> - * used by extent splitting.
> - */
> -#define EXT4_EXT_MAY_ZEROOUT	0x1  /* safe to zeroout if split fails \
> -					due to ENOSPC */
> -#define EXT4_EXT_MARK_UNINIT1	0x2  /* mark first half uninitialized */
> -#define EXT4_EXT_MARK_UNINIT2	0x4  /* mark second half uninitialized */
> -
> -/*
>   * ext4_split_extent_at() splits an extent at given block.
>   *
>   * @handle: the journal handle
> @@ -4228,7 +4292,7 @@ void ext4_ext_truncate(struct inode *inode)
>  
>  	last_block = (inode->i_size + sb->s_blocksize - 1)
>  			>> EXT4_BLOCK_SIZE_BITS(sb);
> -	err = ext4_ext_remove_space(inode, last_block);
> +	err = ext4_ext_remove_space(inode, last_block, EXT_MAX_BLOCKS - 1);
>  
>  	/* In a multi-transaction truncate, we only make the final
>  	 * transaction synchronous.
> @@ -4705,14 +4769,12 @@ int ext4_ext_punch_hole(struct file *file, loff_t offset, loff_t length)
>  {
>  	struct inode *inode = file->f_path.dentry->d_inode;
>  	struct super_block *sb = inode->i_sb;
> -	struct ext4_ext_cache cache_ex;
> -	ext4_lblk_t first_block, last_block, num_blocks, iblock, max_blocks;
> +	ext4_lblk_t first_block, stop_block;
>  	struct address_space *mapping = inode->i_mapping;
> -	struct ext4_map_blocks map;
>  	handle_t *handle;
>  	loff_t first_page, last_page, page_len;
>  	loff_t first_page_offset, last_page_offset;
> -	int ret, credits, blocks_released, err = 0;
> +	int credits, err = 0;
>  
>  	/* No need to punch hole beyond i_size */
>  	if (offset >= inode->i_size)
> @@ -4728,10 +4790,6 @@ int ext4_ext_punch_hole(struct file *file, loff_t offset, loff_t length)
>  		   offset;
>  	}
>  
> -	first_block = (offset + sb->s_blocksize - 1) >>
> -		EXT4_BLOCK_SIZE_BITS(sb);
> -	last_block = (offset + length) >> EXT4_BLOCK_SIZE_BITS(sb);
> -
>  	first_page = (offset + PAGE_CACHE_SIZE - 1) >> PAGE_CACHE_SHIFT;
>  	last_page = (offset + length) >> PAGE_CACHE_SHIFT;
>  
> @@ -4810,7 +4868,6 @@ int ext4_ext_punch_hole(struct file *file, loff_t offset, loff_t length)
>  		}
>  	}
>  
> -
>  	/*
>  	 * If i_size is contained in the last page, we need to
>  	 * unmap and zero the partial page after i_size
> @@ -4830,73 +4887,22 @@ int ext4_ext_punch_hole(struct file *file, loff_t offset, loff_t length)
>  		}
>  	}
>  
> +	first_block = (offset + sb->s_blocksize - 1) >>
> +		EXT4_BLOCK_SIZE_BITS(sb);
> +	stop_block = (offset + length) >> EXT4_BLOCK_SIZE_BITS(sb);
> +
>  	/* If there are no blocks to remove, return now */
> -	if (first_block >= last_block)
> +	if (first_block >= stop_block)
>  		goto out;
>  
>  	down_write(&EXT4_I(inode)->i_data_sem);
>  	ext4_ext_invalidate_cache(inode);
>  	ext4_discard_preallocations(inode);
>  
> -	/*
> -	 * Loop over all the blocks and identify blocks
> -	 * that need to be punched out
> -	 */
> -	iblock = first_block;
> -	blocks_released = 0;
> -	while (iblock < last_block) {
> -		max_blocks = last_block - iblock;
> -		num_blocks = 1;
> -		memset(&map, 0, sizeof(map));
> -		map.m_lblk = iblock;
> -		map.m_len = max_blocks;
> -		ret = ext4_ext_map_blocks(handle, inode, &map,
> -			EXT4_GET_BLOCKS_PUNCH_OUT_EXT);
> -
> -		if (ret > 0) {
> -			blocks_released += ret;
> -			num_blocks = ret;
> -		} else if (ret == 0) {
> -			/*
> -			 * If map blocks could not find the block,
> -			 * then it is in a hole.  If the hole was
> -			 * not already cached, then map blocks should
> -			 * put it in the cache.  So we can get the hole
> -			 * out of the cache
> -			 */
> -			memset(&cache_ex, 0, sizeof(cache_ex));
> -			if ((ext4_ext_check_cache(inode, iblock, &cache_ex)) &&
> -				!cache_ex.ec_start) {
> +	err = ext4_ext_remove_space(inode, first_block, stop_block - 1);
>  
> -				/* The hole is cached */
> -				num_blocks = cache_ex.ec_block +
> -				cache_ex.ec_len - iblock;
> -
> -			} else {
> -				/* The block could not be identified */
> -				err = -EIO;
> -				break;
> -			}
> -		} else {
> -			/* Map blocks error */
> -			err = ret;
> -			break;
> -		}
> -
> -		if (num_blocks == 0) {
> -			/* This condition should never happen */
> -			ext_debug("Block lookup failed");
> -			err = -EIO;
> -			break;
> -		}
> -
> -		iblock += num_blocks;
> -	}
> -
> -	if (blocks_released > 0) {
> -		ext4_ext_invalidate_cache(inode);
> -		ext4_discard_preallocations(inode);
> -	}
> +	ext4_ext_invalidate_cache(inode);
> +	ext4_discard_preallocations(inode);
>  
>  	if (IS_SYNC(inode))
>  		ext4_handle_sync(handle);
> 

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