Re: [PATCH 16/37] libext2fs: support allocating uninit blocks in bmap2()

["Lukáš Czerner" <lczerner@redhat.com>]

On Thu, 1 May 2014, Darrick J. Wong wrote:

> Date: Thu, 01 May 2014 16:14:07 -0700
> From: Darrick J. Wong <darrick.wong@oracle.com>
> To: tytso@mit.edu, darrick.wong@oracle.com
> Cc: linux-ext4@vger.kernel.org
> Subject: [PATCH 16/37] libext2fs: support allocating uninit blocks in bmap2()
> 
> In order to support fallocate, we need to be able to have
> ext2fs_bmap2() allocate blocks and put them into uninitialized
> extents.  There's a flag to do this in the extent code, but it's not
> exposed to the bmap2 interface, so plumb that in.  Eventually fuse2fs
> or somebody will use it.
> 
> Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
> ---
>  lib/ext2fs/bmap.c      |   24 ++++++++++++++++++++++--
>  lib/ext2fs/ext2fs.h    |    1 +
>  lib/ext2fs/mkjournal.c |   17 +++++++++++++++++
>  3 files changed, 40 insertions(+), 2 deletions(-)
> 
> 
> diff --git a/lib/ext2fs/bmap.c b/lib/ext2fs/bmap.c
> index c1d0e6f..a4dc8ef 100644
> --- a/lib/ext2fs/bmap.c
> +++ b/lib/ext2fs/bmap.c
> @@ -72,6 +72,11 @@ static _BMAP_INLINE_ errcode_t block_ind_bmap(ext2_filsys fs, int flags,
>  					    block_buf + fs->blocksize, &b);
>  		if (retval)
>  			return retval;
> +		if (flags & BMAP_UNINIT) {
> +			retval = ext2fs_zero_blocks2(fs, b, 1, NULL, NULL);
> +			if (retval)
> +				return retval;
> +		}
>  
>  #ifdef WORDS_BIGENDIAN
>  		((blk_t *) block_buf)[nr] = ext2fs_swab32(b);
> @@ -214,10 +219,13 @@ static errcode_t extent_bmap(ext2_filsys fs, ext2_ino_t ino,
>  	errcode_t		retval = 0;
>  	blk64_t			blk64 = 0;
>  	int			alloc = 0;
> +	int			set_flags;
> +
> +	set_flags = bmap_flags & BMAP_UNINIT ? EXT2_EXTENT_SET_BMAP_UNINIT : 0;
>  
>  	if (bmap_flags & BMAP_SET) {
>  		retval = ext2fs_extent_set_bmap(handle, block,
> -						*phys_blk, 0);
> +						*phys_blk, set_flags);
>  		return retval;
>  	}
>  	retval = ext2fs_extent_goto(handle, block);
> @@ -254,7 +262,7 @@ got_block:
>  		alloc++;
>  	set_extent:
>  		retval = ext2fs_extent_set_bmap(handle, block,
> -						blk64, 0);
> +						blk64, set_flags);
>  		if (retval) {
>  			ext2fs_block_alloc_stats2(fs, blk64, -1);
>  			return retval;
> @@ -345,6 +353,12 @@ errcode_t ext2fs_bmap2(ext2_filsys fs, ext2_ino_t ino, struct ext2_inode *inode,
>  		goto done;
>  	}
>  
> +	if ((bmap_flags & BMAP_SET) && (bmap_flags & BMAP_UNINIT)) {
> +		retval = ext2fs_zero_blocks2(fs, *phys_blk, 1, NULL, NULL);
> +		if (retval)
> +			goto done;
> +	}
> +
>  	if (block < EXT2_NDIR_BLOCKS) {
>  		if (bmap_flags & BMAP_SET) {
>  			b = *phys_blk;
> @@ -360,6 +374,12 @@ errcode_t ext2fs_bmap2(ext2_filsys fs, ext2_ino_t ino, struct ext2_inode *inode,
>  			retval = ext2fs_alloc_block(fs, b, block_buf, &b);
>  			if (retval)
>  				goto done;
> +			if (bmap_flags & BMAP_UNINIT) {
> +				retval = ext2fs_zero_blocks2(fs, b, 1, NULL,
> +							     NULL);
> +				if (retval)
> +					goto done;
> +			}
>  			inode_bmap(inode, block) = b;
>  			blocks_alloc++;
>  			*phys_blk = b;
> diff --git a/lib/ext2fs/ext2fs.h b/lib/ext2fs/ext2fs.h
> index 599c972..819a14a 100644
> --- a/lib/ext2fs/ext2fs.h
> +++ b/lib/ext2fs/ext2fs.h
> @@ -527,6 +527,7 @@ typedef struct ext2_icount *ext2_icount_t;
>   */
>  #define BMAP_ALLOC	0x0001
>  #define BMAP_SET	0x0002
> +#define BMAP_UNINIT	0x0004
>  
>  /*
>   * Returned flags from ext2fs_bmap
> diff --git a/lib/ext2fs/mkjournal.c b/lib/ext2fs/mkjournal.c
> index 884d9c0..ecc3912 100644
> --- a/lib/ext2fs/mkjournal.c
> +++ b/lib/ext2fs/mkjournal.c
> @@ -174,6 +174,23 @@ errcode_t ext2fs_zero_blocks2(ext2_filsys fs, blk64_t blk, int num,
>  			return ENOMEM;
>  		memset(buf, 0, fs->blocksize * STRIDE_LENGTH);
>  	}
> +
> +	/* Try discard, if it zeroes data... */
> +	if (io_channel_discard_zeroes_data(fs->io)) {
> +		memset(buf + fs->blocksize, 0, fs->blocksize);
> +		retval = io_channel_discard(fs->io, blk, num);
> +		if (retval)
> +			goto skip_discard;
> +		retval = io_channel_read_blk64(fs->io, blk, 1, buf);
> +		if (retval)
> +			goto skip_discard;
> +		if (memcmp(buf, buf + fs->blocksize, fs->blocksize) == 0)
> +			return 0;
> +		/* Hah!  Discard doesn't zero! */
> +		fs->io->flags &= ~CHANNEL_FLAGS_DISCARD_ZEROES;
> +	}
> +skip_discard:

You did not mention that in the description, but this is actually a
problem. The reason is that discard might not be reliable on some
devices. This has been discussed several times and I am not the only
one who've seen that even if the device itself says that it will
return zeroes from discarded regions sometimes it might return data.

I would rather avoid this kind of optimization. However if the
underlying "device" is a loop device then it will be reliable if
it's supported. Also if then underlying "device" is a image then we
can just simply use punch hole.

Thanks!
-Lukas

> +
>  	/* OK, do the write loop */
>  	j=0;
>  	while (j < num) {
> 
> --
> To unsubscribe from this list: send the line "unsubscribe linux-ext4" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
>