Re: [PATCH] ext4: do not normalize block requests from fallocate.

[Lukas Czerner]

On Fri, 13 May 2011, Vivek Haldar wrote:

> Currently, an fallocate request of size slightly larger than a power of
> 2 is turned into two block requests, each a power of 2, with the extra
> blocks pre-allocated for future use. When an application calls
> fallocate, it already has an idea about how large the file may grow so
> there is usually little benefit to reserve extra blocks on the
> preallocation list. This reduces disk fragmentation.
> 
> Tested: fsstress. Also verified manually that fallocat'ed files are
> contiguously laid out with this change (whereas without it they begin at
> power-of-2 boundaries, leaving blocks in between). CPU usage of
> fallocate is not appreciably higher. In a tight fallocate loop, CPU
> usage hovers between 5%-8% with this change, and 5%-7% without it.

Hi Vivek,

the patch looks good, but I do not understand why you are introducing
new #define when you can simply use EXT4_MB_HINT_NOPREALLOC and then in
the ext4_ext_map_blocks() just do :

ar.flags |= (flags & EXT4_MB_HINT_NOPREALLOC)

and you do not need to introduce new condition. Am I missing something ?

Thanks!
-Lukas

> 
> Signed-off-by: Vivek Haldar <haldar@xxxxxxxxxx>
> ---
>  fs/ext4/ext4.h    |    2 ++
>  fs/ext4/extents.c |    5 ++++-
>  2 files changed, 6 insertions(+), 1 deletions(-)
> 
> diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
> index 076c5d2..e606d34 100644
> --- a/fs/ext4/ext4.h
> +++ b/fs/ext4/ext4.h
> @@ -512,6 +512,8 @@ struct ext4_new_group_data {
>  	/* Convert extent to initialized after IO complete */
>  #define EXT4_GET_BLOCKS_IO_CONVERT_EXT		(EXT4_GET_BLOCKS_CONVERT|\
>  					 EXT4_GET_BLOCKS_CREATE_UNINIT_EXT)
> +	/* Don't normalize when fallocat'ing */
> +#define EXT4_GET_BLOCKS_NO_NORMALIZE		0x0020
>  
>  /*
>   * Flags used by ext4_free_blocks
> diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
> index e363f21..f02dd52 100644
> --- a/fs/ext4/extents.c
> +++ b/fs/ext4/extents.c
> @@ -3304,6 +3304,8 @@ int ext4_ext_map_blocks(handle_t *handle, struct inode *inode,
>  	else
>  		/* disable in-core preallocation for non-regular files */
>  		ar.flags = 0;
> +	if (flags & EXT4_GET_BLOCKS_NO_NORMALIZE)
> +		ar.flags |= EXT4_MB_HINT_NOPREALLOC;
>  	newblock = ext4_mb_new_blocks(handle, &ar, &err);
>  	if (!newblock)
>  		goto out2;
> @@ -3549,7 +3551,8 @@ retry:
>  			break;
>  		}
>  		ret = ext4_map_blocks(handle, inode, &map,
> -				      EXT4_GET_BLOCKS_CREATE_UNINIT_EXT);
> +				      EXT4_GET_BLOCKS_CREATE_UNINIT_EXT |
> +				      EXT4_GET_BLOCKS_NO_NORMALIZE);
>  		if (ret <= 0) {
>  #ifdef EXT4FS_DEBUG
>  			WARN_ON(ret <= 0);
> 

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