Date:	Tue, 6 Mar 2012 08:29:14 +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, Mingming Cao <cmm@...ux.vnet.ibm.com>
Subject: Re: [PATCH 1/3] ext4: Rewrite punch hole to use
 ext4_ext_remove_space()

On Mon, 5 Mar 2012, Allison Henderson wrote:

> On 03/02/2012 03:26 AM, 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>
> 
> Hi Lukas,
> 
> Thank you for taking the time to go back over the punch hole code, I do like
> the new set up, and I think it looks cleaner :).  There are some things though
> that are in the current solution that I do not see in this new solution, but I
> am hoping that maybe we dont need them since you seem to be getting through
> the tests ok.  But I thought that I should point out they are there just in
> case something happens, we are aware of them. Also xfstests 255 and 256 are
> good punch hole tests to run through, if you havent tried them out yet.

Hi Allison,

thank you for the review. I did run a lot of xfstests including the two
you mentioned.

> 
> Also, there's another unmerged patch out there that I will need to rebase on
> top of this one.  It's not a big change, but there is one thing in this patch
> that will need to change to make it work.  I go over that too in the comments
> below.

I'll try to see what it is all about, but since I just realized that
indeed we do not hold imutex, then we should definitely handle end >
isize case. We would probably need a xfstest for that :). Thanks!

> 
> 
> > ---
> >   fs/ext4/extents.c |  174
> > ++++++++++++++++++++++++++++-------------------------
> >   1 files changed, 92 insertions(+), 82 deletions(-)
> > 
> > diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
> > index 74f23c2..04dd188 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);
> > @@ -2482,16 +2497,18 @@ 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);
> >   	struct ext4_ext_path *path;
> >   	ext4_fsblk_t partial_cluster = 0;
> >   	handle_t *handle;
> > +	ext4_lblk_t size;
> >   	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);
> > @@ -2503,6 +2520,64 @@ again:
> > 
> >   	trace_ext4_ext_remove_space(inode, start, depth);
> > 
> 
> So in this snippet below it looks like you do the splitting only if the hole
> is contained with in i_size.  I think though that we need to allow punching
> beyond i_size.  A while ago Hugh found this bug (this email: "punch-hole
> should go beyond i_size" sent  around 1/11/2012).  So, I sent out a patch for
> it ("[PATCH 0/3] ext4: punch hole beyond i_size" sent around 1/14/2012 ), but
> I think it just got lost in the traffic, because I dont think it got merged.
> I've been meaning to resend but have just been tied up.  I try as much as I
> can to keep up with ext4 on my own time, but I do not get much time allotted
> to it anymore :( .  The set actually changes code up in ext4_ext_punch_hole,
> and I dont think your patch touches the same code, so I should be able to
> rebase it on top of your patch fairly easily.
> I will send an update based on your new set, but it looks like the line below
> will need to change too.  In this case, I think what what you're meaning to do
> is split extents if we are punching holes (right?).  Maybe instead of using
> i_size, we could just check "if (end != EXT_MAX_BLOCKS - 1)", since truncate
> will always use "EXT_MAX_BLOCKS - 1"

Yes, in that case "if (end != EXT_MAX_BLOCKS - 1)" would probably make
more sense. I'll try to come up with the test case to trigger the
problem. Thanks for pointing it out.

> 
> > +	size = (inode->i_size + sb->s_blocksize - 1)
> > +		>>  EXT4_BLOCK_SIZE_BITS(sb);
> > +
> > +	/*
> > +	 * 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 (size>  end) {
> > +		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);
> > +
> 
> Some comments on the split logic here:  When my earlier versions of the punch
> hole patches were getting reviewed, I remember one of the things that came up
> was that the extents need to be marked uninitialized before being removed.  In
> this code, it looks like you mark them only if they are already uninitialized,
> and at the end of the hole.
> 
> The reason this changes the split logic is because initialized extents can be
> larger than uninitialized extents.  So you cant just mark it uninitialized
> with out splitting.  Even if it's in the middle of the hole, you may have to
> split it anyway to make it fit in an uninitialized extent.
> 
> In the previous solution, this worked because the split logic, being inside
> map_blocks, was in the body of the loop (called from ext4_ext_punch_hole), and
> the search started from the beginning each time we looked up an extent.  In
> this case though, it might get tricky because this loop is trying to walk the
> extent tree.
> 
> As I recall I think the reason we were marking them uninitialized was because
> we wanted them to read back zeros while the punch hole was in progress.  Punch
> hole does not take i_mutex in fallocate, and since I was moved, I havnt been
> able to get much time in on extent locks.  I suppose i_mutex would be a quick
> fix, but I realize Ted wanted to avoid further use of i_mutex since we really
> shouldn't be using it at all. Maybe we can get some more folks in on this
> discussion here, because if we really dont need them to be uninitialized, this
> solution is really simple and clean.  :)

Hmm, I have not realized that we actually need to read zeroes from the
punched out extents before they are actually punched out. I need to take
a closer look at this but, in this case we'll have to use imutex anyway
to make marking the extents as uninitialized atomic operation. If it is
the case, then I am not sure what we gain here as opposed to just remove
the extents under imutex. But I guess I need to look at this problem
more closely. Thanks for pointing it out.

Anyway, the extent locks might be a help here, maybe we can cooperate on
this to come up with something sooner rather than later ?


Thanks for review Allison.

-Lukas

> 
> > +		/*
> > +		 * 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 +2590,7 @@ 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 +2602,7 @@ 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 +2785,6 @@ 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 +4296,7 @@ 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 +4773,12 @@ 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;
> > 
> 
> This thing right here disappears with that other unmerged patch.  It looks
> like you hop over it though, so hopefully it wont be a problem
> >   	/* No need to punch hole beyond i_size */
> >   	if (offset>= inode->i_size)
> > @@ -4728,10 +4794,6 @@ 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 +4872,6 @@ 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 +4891,22 @@ 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);
> 
> That's all my comments for now, the rest of it looks really nice.  Thank you
> for taking the time to go through it!
> 
> Allison Henderson
> 
> 

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