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

Darrick J. Wong
2014-05-07T21:37:12Z

On Wed, May 07, 2014 at 12:02:30PM +0200, Lukáš Czerner wrote:
> On Tue, 6 May 2014, Darrick J. Wong wrote:
>
> > Date: Tue, 6 May 2014 12:59:38 -0700
> > From: Darrick J. Wong <darrick.wong@oracle.com>
> > To: Lukáš Czerner <lczerner@redhat.com>
> > Cc: tytso@mit.edu, linux-ext4@vger.kernel.org
> > Subject: Re: [PATCH 16/37] libext2fs: support allocating uninit blocks in
> > bmap2()
> >
> > 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?
>
> Yes I am, because we know that it sometimes behaves unpredictably
> and this is one of the things that might just happen. Even though I
> have not seen this exact case I've seen the opposite where right
> after discard I've read non zero values but later it actually
> returned zeroes.
>
> So I would much rather not rely on discard here because you might
> expose stale data on indirect files and there is no way to turn this
> optimization off.

Fair enough.

> >
> > > 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?
>
> We do not. We can only do that if we know we're sitting on a file.
> It is really unfortunate, but I think that there is a limitation in
> how we can use discard.
>
> However we could use write same which should help on devices which
> supports it and on the fs images because QEMU will convert that to
> zero range (at least on xfs since ext4 implementation is quite new).
> However I have no idea what is the interface to do that.

Hrmm, I guess it would be the BLKZEROOUT ioctl for block devices? Inside the
kernel it appears to be wired up to WRITE_SAME with a zero buffer or just a
regular WRITE with a lot of zero pages attached. For regular files, punch hole
(or zero range) seems to be fine. I think.

This ought to get moved into a separate IO manager routine.

--D
>
> -Lukas
>
> >
> > 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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