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

Darrick J. Wong
2014-05-06T19:59:46Z

On Tue, May 06, 2014 at 05:45:01PM +0200, Lukáš Czerner wrote:
> 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 agree that the storage not living up to the interface it advertises is a
problem, hence the verification step that will unset the io channel flag if it
finds that the device is lying.

On the other hand, I wonder if this ought to be abstracted away in an
io_channel_zero() call that takes care of figuring out if it can do a zeroing
discard or if it has to write a block of zeroes.

Or, are you worried that a discard and immediate re-read will appear to work,
but that a later re-read will return non-zero 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.

But static whitelisting is also problematic -- what if the storage device is an
AHCI (or virtio-scsi) disk in QEMU that's ultimately backed by a file that we
can punch_hole? How do we distinguish that from an SSD hooked up to SATA
hardware?

In the qemu emulated AHCI case we ought to be able to zeroing discard, if
advertised. I thought it was a reasonable compromise to trust that it works
and verify the results afterward.

--D
>
> Thanks!
> -Lukas
>
> > +
> > /* OK, do the write loop */
> > j=0;
> > while (j < num) {
> >
> > --
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