Message-ID: <20070507121115.GE7012@amitarora.in.ibm.com>
Date:	Mon, 7 May 2007 17:41:15 +0530
From:	"Amit K. Arora" <aarora@...ux.vnet.ibm.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	torvalds@...ux-foundation.org, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-ext4@...r.kernel.org,
	xfs@....sgi.com, suparna@...ibm.com, cmm@...ibm.com
Subject: Re: [PATCH 5/5] ext4: write support for preallocated blocks/extents

On Thu, May 03, 2007 at 09:32:38PM -0700, Andrew Morton wrote:
> On Thu, 26 Apr 2007 23:46:23 +0530 "Amit K. Arora" <aarora@...ux.vnet.ibm.com> wrote:
> > + */
> > +int ext4_ext_try_to_merge(struct inode *inode,
> > +				struct ext4_ext_path *path,
> > +				struct ext4_extent *ex)
> > +{
> > +	struct ext4_extent_header *eh;
> > +	unsigned int depth, len;
> > +	int merge_done=0, uninitialized = 0;
> 
> space around "=", please.
> 
> Many people prefer not to do the multiple-definitions-per-line, btw:
> 
> 	int merge_done = 0;
> 	int uninitialized = 0;

Ok. Will make the change.

> 
> reasons:
> 
> - If gives you some space for a nice comment
> 
> - It makes patches much more readable, and it makes rejects easier to fix
> 
> - standardisation.
> 
> > +	depth = ext_depth(inode);
> > +	BUG_ON(path[depth].p_hdr == NULL);
> > +	eh = path[depth].p_hdr;
> > +
> > +	while (ex < EXT_LAST_EXTENT(eh)) {
> > +		if (!ext4_can_extents_be_merged(inode, ex, ex + 1))
> > +			break;
> > +		/* merge with next extent! */
> > +		if (ext4_ext_is_uninitialized(ex))
> > +			uninitialized = 1;
> > +		ex->ee_len = cpu_to_le16(ext4_ext_get_actual_len(ex)
> > +					+ ext4_ext_get_actual_len(ex + 1));
> > +		if (uninitialized)
> > +			ext4_ext_mark_uninitialized(ex);
> > +
> > +		if (ex + 1 < EXT_LAST_EXTENT(eh)) {
> > +			len = (EXT_LAST_EXTENT(eh) - ex - 1)
> > +					* sizeof(struct ext4_extent);
> > +			memmove(ex + 1, ex + 2, len);
> > +		}
> > +		eh->eh_entries = cpu_to_le16(le16_to_cpu(eh->eh_entries)-1);
> 
> Kenrel convention is to put spaces around "-"

Will fix this.

> 
> > +		merge_done = 1;
> > +		BUG_ON(eh->eh_entries == 0);
> 
> eek, scary BUG_ON.  Do we really need to be that severe?  Would it be
> better to warn and run ext4_error() here?
Ok.
> 
> > +	}
> > +
> > +	return merge_done;
> > +}
> > +
> > +
> >
> > ...
> >
> > +/*
> > + * ext4_ext_convert_to_initialized:
> > + * 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
> > + * extent into multiple extents (upto three). Atleast one initialized extent
> > + * and atmost two uninitialized extents can result.
> 
> There are some typos here
> 
> > + * There are three possibilities:
> > + *   a> No split required: Entire extent should be initialized.
> > + *   b> Split into two extents: Only one end of the extent is being written to.
> > + *   c> Split into three extents: Somone is writing in middle of the extent.
> 
> and here
> 
Ok. Will fix them.
> > + */
> > +int ext4_ext_convert_to_initialized(handle_t *handle, struct inode *inode,
> > +					struct ext4_ext_path *path,
> > +					ext4_fsblk_t iblock,
> > +					unsigned long max_blocks)
> > +{
> > +	struct ext4_extent *ex, *ex1 = NULL, *ex2 = NULL, *ex3 = NULL, newex;
> > +	struct ext4_extent_header *eh;
> > +	unsigned int allocated, ee_block, ee_len, depth;
> > +	ext4_fsblk_t newblock;
> > +	int err = 0, ret = 0;
> > +
> > +	depth = ext_depth(inode);
> > +	eh = path[depth].p_hdr;
> > +	ex = path[depth].p_ext;
> > +	ee_block = le32_to_cpu(ex->ee_block);
> > +	ee_len = ext4_ext_get_actual_len(ex);
> > +	allocated = ee_len - (iblock - ee_block);
> > +	newblock = iblock - ee_block + ext_pblock(ex);
> > +	ex2 = ex;
> > +
> > +	/* ex1: ee_block to iblock - 1 : uninitialized */
> > +	if (iblock > ee_block) {
> > +		ex1 = ex;
> > +		ex1->ee_len = cpu_to_le16(iblock - ee_block);
> > +		ext4_ext_mark_uninitialized(ex1);
> > +		ex2 = &newex;
> > +	}
> > +	/* for sanity, update the length of the ex2 extent before
> > +	 * we insert ex3, if ex1 is NULL. This is to avoid temporary
> > +	 * overlap of blocks.
> > +	 */
> > +	if (!ex1 && allocated > max_blocks)
> > +		ex2->ee_len = cpu_to_le16(max_blocks);
> > +	/* ex3: to ee_block + ee_len : uninitialised */
> > +	if (allocated > max_blocks) {
> > +		unsigned int newdepth;
> > +		ex3 = &newex;
> > +		ex3->ee_block = cpu_to_le32(iblock + max_blocks);
> > +		ext4_ext_store_pblock(ex3, newblock + max_blocks);
> > +		ex3->ee_len = cpu_to_le16(allocated - max_blocks);
> > +		ext4_ext_mark_uninitialized(ex3);
> > +		err = ext4_ext_insert_extent(handle, inode, path, ex3);
> > +		if (err)
> > +			goto out;
> > +		/* The depth, and hence eh & ex might change
> > +		 * as part of the insert above.
> > +		 */
> > +		newdepth = ext_depth(inode);
> > +		if (newdepth != depth)
> > +		{
> 
> Use
> 
> 		if (newdepth != depth) {

Ok.
> 
> > +			depth=newdepth;
> 
> spaces
Ok.
> 
> > +			path = ext4_ext_find_extent(inode, iblock, NULL);
> > +			if (IS_ERR(path)) {
> > +				err = PTR_ERR(path);
> > +				path = NULL;
> > +				goto out;
> > +			}
> > +			eh = path[depth].p_hdr;
> > +			ex = path[depth].p_ext;
> > +			if (ex2 != &newex)
> > +				ex2 = ex;
> > +		}
> > +		allocated = max_blocks;
> > +	}
> > +	/* If there was a change of depth as part of the
> > +	 * insertion of ex3 above, we need to update the length
> > +	 * of the ex1 extent again here
> > +	 */
> > +	if (ex1 && ex1 != ex) {
> > +		ex1 = ex;
> > +		ex1->ee_len = cpu_to_le16(iblock - ee_block);
> > +		ext4_ext_mark_uninitialized(ex1);
> > +		ex2 = &newex;
> > +	}
> > +	/* ex2: iblock to iblock + maxblocks-1 : initialised */
> > +	ex2->ee_block = cpu_to_le32(iblock);
> > +	ex2->ee_start = cpu_to_le32(newblock);
> > +	ext4_ext_store_pblock(ex2, newblock);
> > +	ex2->ee_len = cpu_to_le16(allocated);
> > +	if (ex2 != ex)
> > +		goto insert;
> > +	if ((err = ext4_ext_get_access(handle, inode, path + depth)))
> > +		goto out;
> 
> The preferred style is
> 
> 	err = ext4_ext_get_access(handle, inode, path + depth);
> 	if (err)
> 		goto out;

Right. Will change it.
 
> > +	/* New (initialized) extent starts from the first block
> > +	 * in the current extent. i.e., ex2 == ex
> > +	 * We have to see if it can be merged with the extent
> > +	 * on the left.
> > +	 */
> > +	if (ex2 > EXT_FIRST_EXTENT(eh)) {
> > +		/* To merge left, pass "ex2 - 1" to try_to_merge(),
> > +		 * since it merges towards right _only_.
> > +		 */
> > +		ret = ext4_ext_try_to_merge(inode, path, ex2 - 1);
> > +		if (ret) {
> > +			err = ext4_ext_correct_indexes(handle, inode, path);
> > +			if (err)
> > +				goto out;
> > +			depth = ext_depth(inode);
> > +			ex2--;
> > +		}
> > +	}
> > +	/* Try to Merge towards right. This might be required
> > +	 * only when the whole extent is being written to.
> > +	 * i.e. ex2==ex and ex3==NULL.
> > +	 */
> > +	if (!ex3) {
> > +		ret = ext4_ext_try_to_merge(inode, path, ex2);
> > +		if (ret) {
> > +			err = ext4_ext_correct_indexes(handle, inode, path);
> > +			if (err)
> > +				goto out;
> > +		}
> > +	}
> > +	/* Mark modified extent as dirty */
> > +	err = ext4_ext_dirty(handle, inode, path + depth);
> > +	goto out;
> > +insert:
> > +	err = ext4_ext_insert_extent(handle, inode, path, &newex);
> > +out:
> > +	return err ? err : allocated;
> > +}
> 
> Sigh.  I hope you guys know how all this works, because the extent code is
> a mystery to me.  Is the on-disk layout and the allocation strategy
> described anywhere?
> 
> > +extern int ext4_ext_try_to_merge(struct inode *, struct ext4_ext_path *, struct ext4_extent *);
> 
> Again, I do think that sticking the identifiers in there helps
> readability.  Although it is not as important in a boring old declaration
> as it is in, say, inode_operations, etc.
> 
> Please try to keep the code looking nice in an 80-column display.

Ok. Will make the required changes.

Thanks again for your comments!

--
Regards,
Amit Arora
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