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Message-ID: <alpine.LFD.2.00.1405061736390.2255@localhost.localdomain>
Date: Tue, 6 May 2014 17:45:01 +0200 (CEST)
From: Lukáš Czerner <lczerner@...hat.com>
To: "Darrick J. Wong" <darrick.wong@...cle.com>
cc: tytso@....edu, linux-ext4@...r.kernel.org
Subject: Re: [PATCH 16/37] libext2fs: support allocating uninit blocks in
bmap2()
On Thu, 1 May 2014, Darrick J. Wong wrote:
> Date: Thu, 01 May 2014 16:14:07 -0700
> From: Darrick J. Wong <darrick.wong@...cle.com>
> To: tytso@....edu, darrick.wong@...cle.com
> Cc: linux-ext4@...r.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@...cle.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) {
>
> --
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