Date:	Wed, 21 Mar 2012 08:25:45 +0100 (CET)
From:	Lukas Czerner <lczerner@...hat.com>
To:	Allison Henderson <achender@...ux.vnet.ibm.com>
cc:	Lukas Czerner <lczerner@...hat.com>, linux-ext4@...r.kernel.org,
	tytso@....edu
Subject: Re: [PATCH 1/3 v2] ext4: Rewrite punch hole to use
 ext4_ext_remove_space()

On Mon, 19 Mar 2012, Allison Henderson wrote:

> On 03/19/2012 11:24 AM, Lukas Czerner wrote:
> > 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
> 
> Alrighty then, sounds good.  Also, I looked through the v2 set and it looks
> good to me.  Thx for the all the clean up :)
> 
> Allison Henderson

no problem, I just sent it out to the list so please let me know if
you're ok with it.

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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