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Date:	Wed, 13 Aug 2008 17:50:56 -0700
From:	Mingming Cao <cmm@...ibm.com>
To:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
Cc:	tytso <tytso@....edu>, linux-ext4@...r.kernel.org,
	Andreas Dilger <adilger@....com>
Subject: Re: [PATCH 3/6 ]Ext4: journal credits reservation fixes for DIO,
	fallocate


在 2008-08-13三的 14:23 +0530,Aneesh Kumar K.V写道: 
> On Tue, Aug 12, 2008 at 09:29:50AM -0700, Mingming Cao wrote:
> > 
> ......
> ....
> 
> > 
> > ===================================================================
> > Index: linux-2.6.27-rc1/fs/ext4/extents.c
> > ===================================================================
> > --- linux-2.6.27-rc1.orig/fs/ext4/extents.c	2008-08-11 22:25:39.000000000 -0700
> > +++ linux-2.6.27-rc1/fs/ext4/extents.c	2008-08-11 22:25:55.000000000 -0700
> > @@ -2799,7 +2799,7 @@ void ext4_ext_truncate(struct inode *ino
> >  	/*
> >  	 * probably first extent we're gonna free will be last in block
> >  	 */
> > -	err = ext4_writepage_trans_blocks(inode) + 3;
> > +	err = ext4_writepage_trans_blocks(inode);
> >  	handle = ext4_journal_start(inode, err);
> >  	if (IS_ERR(handle))
> >  		return;
> > @@ -2951,10 +2951,9 @@ long ext4_fallocate(struct inode *inode,
> >  	max_blocks = (EXT4_BLOCK_ALIGN(len + offset, blkbits) >> blkbits)
> >  							- block;
> >  	/*
> > -	 * credits to insert 1 extent into extent tree + buffers to be able to
> > -	 * modify 1 super block, 1 block bitmap and 1 group descriptor.
> > +	 * credits to insert 1 extent into extent tree
> >  	 */
> > -	credits = EXT4_DATA_TRANS_BLOCKS(inode->i_sb) + 3;
> > +	credits = ext4_data_trans_blocks(inode, max_blocks);
> 
> 
> Why do we need to consider data=journaled mode here. We are not writing
> any data here. Instead we are just inserting an extent.
> 

Actually the change here is not mean to support data=journalled here.
The ext4_data_trans_blocks() is intended for calculate credits for a
chunk of allocation, used for DIO and fallocate, regardless of delalloc
or not. 
 We should remove the considering of data journal in the
ext4_data_trans_blocks(), I agree.

Now that I realize the data=journalled code doesn't work for delalloc
(or delalloc da writepages doesn' t support the journalled mode, due to
the lock ordering issue),  I am not sure if there is plan to do so (or
there is need to support journalled mode on delalloc).  We still need to
keep the data=journalled consideration for writepage/write_begin, just
to help user move from ext3 to ext4 I guess.

> 
> >  	mutex_lock(&inode->i_mutex);
> >  retry:
> >  	while (ret >= 0 && ret < max_blocks) {
> > Index: linux-2.6.27-rc1/fs/ext4/inode.c
> > ===================================================================
> > --- linux-2.6.27-rc1.orig/fs/ext4/inode.c	2008-08-11 22:18:31.000000000 -0700
> > +++ linux-2.6.27-rc1/fs/ext4/inode.c	2008-08-11 22:25:55.000000000 -0700
> > @@ -1041,18 +1041,6 @@ static void ext4_da_update_reserve_space
> >  	spin_unlock(&EXT4_I(inode)->i_block_reservation_lock);
> >  }
> > 
> > -/* Maximum number of blocks we map for direct IO at once. */
> > -#define DIO_MAX_BLOCKS 4096
> > -/*
> > - * Number of credits we need for writing DIO_MAX_BLOCKS:
> > - * We need sb + group descriptor + bitmap + inode -> 4
> > - * For B blocks with A block pointers per block we need:
> > - * 1 (triple ind.) + (B/A/A + 2) (doubly ind.) + (B/A + 2) (indirect).
> > - * If we plug in 4096 for B and 256 for A (for 1KB block size), we get 25.
> > - */
> > -#define DIO_CREDITS 25
> > -
> > -
> >  /*
> >   * The ext4_get_blocks_wrap() function try to look up the requested blocks,
> >   * and returns if the blocks are already mapped.
> > @@ -1164,19 +1152,23 @@ int ext4_get_blocks_wrap(handle_t *handl
> >  	return retval;
> >  }
> > 
> > +/* Maximum number of blocks we map for direct IO at once. */
> > +#define DIO_MAX_BLOCKS 4096
> > +
> >  static int ext4_get_block(struct inode *inode, sector_t iblock,
> >  			struct buffer_head *bh_result, int create)
> >  {
> >  	handle_t *handle = ext4_journal_current_handle();
> >  	int ret = 0, started = 0;
> >  	unsigned max_blocks = bh_result->b_size >> inode->i_blkbits;
> > +	int dio_credits;
> > 
> >  	if (create && !handle) {
> >  		/* Direct IO write... */
> >  		if (max_blocks > DIO_MAX_BLOCKS)
> >  			max_blocks = DIO_MAX_BLOCKS;
> > -		handle = ext4_journal_start(inode, DIO_CREDITS +
> > -			      2 * EXT4_QUOTA_TRANS_BLOCKS(inode->i_sb));
> > +		dio_credits = ext4_data_trans_blocks(inode, max_blocks);
> > +		handle = ext4_journal_start(inode, dio_credits);
> 
> Even in data=journal mode directIO will put the buffer_heads to journal
> right ? . So should we use ext4_data_trans_blocks here ?
> 
> 
> 
> >  		if (IS_ERR(handle)) {
> >  			ret = PTR_ERR(handle);
> >  			goto out;
> > @@ -2222,7 +2214,7 @@ static int ext4_da_writepage(struct page
> >   * for DIO, writepages, and truncate
> >   */
> >  #define EXT4_MAX_WRITEBACK_PAGES      DIO_MAX_BLOCKS
> > -#define EXT4_MAX_WRITEBACK_CREDITS    DIO_CREDITS
> > +#define EXT4_MAX_WRITEBACK_CREDITS    25
> > 
> >  static int ext4_da_writepages(struct address_space *mapping,
> >  				struct writeback_control *wbc)
> > @@ -4429,7 +4421,8 @@ static int ext4_writeblocks_trans_credit
> > 
> >  /*
> > 
> 
> ....
> ....
> 
> -aneesh

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