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Message-ID: <20141013183447.GG12009@birch.djwong.org>
Date: Mon, 13 Oct 2014 11:34:47 -0700
From: "Darrick J. Wong" <darrick.wong@...cle.com>
To: "Theodore Ts'o" <tytso@....edu>
Cc: linux-ext4@...r.kernel.org
Subject: Re: [PATCH 17/34] libext2fs: support allocating uninit blocks in
bmap2()
On Mon, Oct 13, 2014 at 09:56:53AM -0700, Darrick J. Wong wrote:
> On Mon, Oct 13, 2014 at 10:35:26AM -0400, Theodore Ts'o wrote:
> > On Sat, Sep 13, 2014 at 03:13:06PM -0700, Darrick J. Wong wrote:
> > > 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
> > > fallocate or 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 +
> > > 2 files changed, 23 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;
> > > + }
> >
> > What I think we should do is to have two separate new BMAP_ flags;
> > BMAP_UNINIT, which sets the uninit bit, and BMAP_ZERO, which requests
> > that the block be zeroed. I don't think it should follow that whe you
> > set the uninit bit via the libext2fs, the block wil automatically be
> > zeroed. After all, userspace can't assume that if the uninit bit is
> > set, that the block will be pre-zeroed, since files fallocated by the
> > kernel won't meet that guarantee.
>
> On an extent based file, we can record the BLOCK_UNINIT status in the extent
> flags so that subsequent reads return zeroes. On a block mapped file it's not
> possible to record the uninitialized status (short of unmapping the block), so
> here I was trying to emulate the read behavior you'd get with an extent file.
>
> Kernel fallocate() refuses to service non-extent files, so there's not much
> precedent there unless you want to block BMAP_UNINIT on such files.
>
> So... I agree that BMAP_ZERO would be a useful feature anyway. My question is,
> if we pass in a non-extent file with BMAP_UNINIT but not BMAP_ZERO, should we
> simply return -EINVAL?
Never mind, now that I thought more about the implementation, I think we can
trust callers to DTRT.
Alternately, this enables that fallocate-but-dont-ever-zero use case which
keeps popping up, so it would seem to have /some/ value to some people.
--D
> --D
>
> >
> > - Ted
> > --
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