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Date:	Wed, 13 Aug 2008 15:16:37 +0530
From:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
To:	Mingming Cao <cmm@...ibm.com>
Cc:	tytso <tytso@....edu>, linux-ext4@...r.kernel.org,
	Andreas Dilger <adilger@....com>
Subject: Re: [PATCH 5/6 ]Ext4 journal credits reservation fixes

On Tue, Aug 12, 2008 at 09:35:38AM -0700, Mingming Cao wrote:
> Ext4: journal credit fix the delalloc writepages
> 
> From: Mingming Cao <cmm@...ibm.com>
> 
> Previous delalloc writepages implementation start a new transaction outside
> a loop call of get_block() to do the block allocation. Due to lack of information 
> of how many blocks to be allocated, the estimate of the journal credits is very
>  Conservative and caused many issues.
> 
> With the reworked delayed allocation, a new transaction is created for
> each get_block(), thus we don't need to guess how many credits for the multiple
> chunk of allocation. Start every transaction with credits for insert a single 
> extent is enough. But we still need to consider the journalled mode, where
> it need to account for the number of data blocks.  So we guess max number of
> data blocks for each allocation.


But we don't currently support data=journal with delalloc.


> Due to the current VFS implementation
> writepages() could only flush PAGEVEC of pages at a time, the max block
> allocation is limited and calculated based on that, an the total number
> of reserved delalloc datablocks, whichever is smaller.

That is not correct. Currently write_cache_pages do
while (!done && (index <= end) &&
		(nr_pages = pagevec_lookup_tag(&pvec, mapping, &index,
					       PAGECACHE_TAG_DIRTY,
					       min(end - index,
					      (pgoff_t)PAGEVEC_SIZE-1) + 1)))
{

and mpage_da_submit_io does
while (index <= end) {
	/* XXX: optimize tail */
	nr_pages = pagevec_lookup(&pvec, mapping, index, PAGEVEC_SIZE);



ie we iterate till index > end. So we can very well have more than
PAGEVEC number of pages in a single transaction.

> 
> Signed-off-by: Mingming Cao <cmm@...ibm.com>
> ---
>  fs/ext4/inode.c |   39 ++++++++++++++++++++++++---------------
>  1 file changed, 24 insertions(+), 15 deletions(-)
> 
> Index: linux-2.6.27-rc1/fs/ext4/inode.c
> ===================================================================
> --- linux-2.6.27-rc1.orig/fs/ext4/inode.c	2008-08-12 08:15:59.000000000 -0700
> +++ linux-2.6.27-rc1/fs/ext4/inode.c	2008-08-12 08:30:41.000000000 -0700
> @@ -2210,17 +2210,28 @@ static int ext4_da_writepage(struct page
>  }
> 
>  /*
> - * For now just follow the DIO way to estimate the max credits
> - * needed to write out EXT4_MAX_WRITEBACK_PAGES.
> - * todo: need to calculate the max credits need for
> - * extent based files, currently the DIO credits is based on
> - * indirect-blocks mapping way.
> - *
> - * Probably should have a generic way to calculate credits
> - * for DIO, writepages, and truncate
> + * This is called via ext4_da_writepages() to
> + * calulate the total number of credits to reserve to fit
> + * a single extent allocation into a single transaction,
> + * ext4_da_writpeages() will loop calling this before
> + * the block allocation.
> + *
> + * The page vector size limited the max number of pages could
> + * be writeout at a time. Based on this, the max blocks to pass to
> + * get_block is calculated
>   */
> -#define EXT4_MAX_WRITEBACK_PAGES      DIO_MAX_BLOCKS
> -#define EXT4_MAX_WRITEBACK_CREDITS    25
> +
> +#define		EXT4_MAX_WRITEPAGES_SIZE	PAGEVEC_SIZE
> +static int ext4_writepages_trans_blocks(struct inode *inode)
> +{
> +	int bpp = ext4_journal_blocks_per_page(inode);
> +	int max_blocks = EXT4_MAX_WRITEPAGES_SIZE * bpp;
> +
> +	if (max_blocks > EXT4_I(inode)->i_reserved_data_blocks)
> +		max_blocks =  EXT4_I(inode)->i_reserved_data_blocks;



Why are we limiting max_blocks to i_reserved_data_blocks ?


> +
> +	return ext4_data_trans_blocks(inode, max_blocks);
> +}
> 
>  static int ext4_da_writepages(struct address_space *mapping,
>                                  struct writeback_control *wbc)
> @@ -2262,7 +2273,7 @@ restart_loop:
>  		 * by delalloc
>  		 */
>  		BUG_ON(ext4_should_journal_data(inode));
> -		needed_blocks = EXT4_DATA_TRANS_BLOCKS(inode->i_sb);
> +		needed_blocks = ext4_writepages_trans_blocks(inode);
> 

The BUG_ON above is added to make sure we update this when start
supporting data=journal mode with delalloc.


>  		/* start a new transaction*/
>  		handle = ext4_journal_start(inode, needed_blocks);
> @@ -4449,11 +4460,9 @@ static int ext4_writeblocks_trans_credit
>   * the modification of a single pages into a single transaction,
>   * which may include multile chunk of block allocations.
>   *
> - * This could be called via ext4_write_begin() or later
> - * ext4_da_writepages() in delalyed allocation case.
> + * This could be called via ext4_write_begin()
>   *
> - * In both case it's possible that we could allocating multiple
> - * chunks of blocks. We need to consider the worse case, when
> + * We need to consider the worse case, when
>   * one new block per extent.
>   */
>  int ext4_writepage_trans_blocks(struct inode *inode)
> 
> 
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