From: "Darrick J. Wong" Subject: Re: [PATCH 17/34] libext2fs: support allocating uninit blocks in bmap2() Date: Mon, 13 Oct 2014 11:34:47 -0700 Message-ID: <20141013183447.GG12009@birch.djwong.org> References: <20140913221112.13646.3873.stgit@birch.djwong.org> <20140913221306.13646.36911.stgit@birch.djwong.org> <20141013143526.GE9738@thunk.org> <20141013165653.GC12009@birch.djwong.org> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: linux-ext4@vger.kernel.org To: "Theodore Ts'o" Return-path: Received: from aserp1040.oracle.com ([141.146.126.69]:18467 "EHLO aserp1040.oracle.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753225AbaJMSew (ORCPT ); Mon, 13 Oct 2014 14:34:52 -0400 Content-Disposition: inline In-Reply-To: <20141013165653.GC12009@birch.djwong.org> Sender: linux-ext4-owner@vger.kernel.org List-ID: 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 > > > --- > > > 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 > > -- > > 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 > -- > 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