Re: [PATCH] Ext4: Fix delalloc enospace handling counter update race

["Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>]

On Mon, Jul 14, 2008 at 09:28:17AM -0700, Mingming Cao wrote:
> 
> Ext4: Fix delalloc enospace handling counter update race
> 
> From: Mingming Cao <cmm@us.ibm.com>
> 
> Ext4 delalloc reserve meta blocks ahead of time so later when real block
> allocation fs will not short of free blocks for allocating meta blocks.
> It keeps track of the real number of new allocated meta data blocks, so
> after block allocation it will update how much meta data blocks still
> need to be reserved for that inode.
> 
> Both per inode reserved metadata blocks and per-allocation allocated metablocks
> are protected by the per inode delalloc reservation lock. The per-allocation
> allocated metablocks counter should be protected by the i_data_sem as well,
> so that it could avoid race with other block allocation to the same inode
> in parallel. The patch moves the code under the i_data_sem protection.
> 
> Also in the case of truncate, we should not clear the per-allocation allocated
> metablocks counter as that may be in-use by parallel allocation. The patch
> only clear the per-allocation allocated metablocks when allocation is successfully returned.
> 
> 
> Signed-off-by: Mingming Cao <cmm@us.ibm.com>

Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>

> 
> ---
>  fs/ext4/inode.c |   19 +++++++++++++------
>  1 file changed, 13 insertions(+), 6 deletions(-)
> 
> Index: linux-2.6.26-rc9/fs/ext4/inode.c
> ===================================================================
> --- linux-2.6.26-rc9.orig/fs/ext4/inode.c	2008-07-14 08:54:25.000000000 -0700
> +++ linux-2.6.26-rc9/fs/ext4/inode.c	2008-07-14 09:23:29.000000000 -0700
> @@ -1060,8 +1060,18 @@ int ext4_get_blocks_wrap(handle_t *handl
>  							~EXT4_EXT_MIGRATE;
>  		}
>  	}
> -	if (flag)
> +
> +	if (flag) {
>  		EXT4_I(inode)->i_delalloc_reserved_flag = 0;
> +		/*
> +		 * Update reserved blocks/metadata blocks
> +		 * after successful block allocation
> +		 * which were deferred till now
> +		 */
> +		if ((retval > 0) && buffer_delay(bh))
> +			ext4_da_release_space(inode, retval, 0);
> +	}
> +
>  	up_write((&EXT4_I(inode)->i_data_sem));
>  	return retval;
>  }
> @@ -1519,7 +1529,8 @@ void ext4_da_release_space(struct inode 
> 
>  	BUG_ON(mdb > EXT4_I(inode)->i_reserved_meta_blocks);
>  	EXT4_I(inode)->i_reserved_meta_blocks = mdb;
> -	EXT4_I(inode)->i_allocated_meta_blocks = 0;
> +	if (used)
> +		EXT4_I(inode)->i_allocated_meta_blocks = 0;
>  	spin_unlock(&EXT4_I(inode)->i_block_reservation_lock);
>  }
> 
> @@ -2005,10 +2016,6 @@ static int ext4_da_get_block_write(struc
>  	if (ret > 0) {
>  		bh_result->b_size = (ret << inode->i_blkbits);
> 
> -		/* release reserved-but-unused meta blocks */
> -		if (buffer_delay(bh_result))
> -			ext4_da_release_space(inode, ret, 0);
> -
>  		/*
>  		 * Update on-disk size along with block allocation
>  		 * we don't use 'extend_disksize' as size may change
> 
>