linux-ext4 - [RFC/PATCH 0/3] Implement XFS's GOINGDOWN ioctl for ext4

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Date:   Thu,  2 Feb 2017 17:59:21 -0500
From:   Theodore Ts'o <tytso@....edu>
To:     Ext4 Developers List <linux-ext4@...r.kernel.org>
Cc:     Theodore Ts'o <tytso@....edu>, fstests@...r.kernel.org
Subject: [RFC/PATCH 0/3] Implement XFS's GOINGDOWN ioctl for ext4

This is a proof of concept implementation of XFS's GOINGDOWN ioctl for
ext4.

I've tried to replicate XFS's semantics (as much as they can translate
to ext4).  This test series is currently *not* passing xfstests.
Specifically, the following tests are failing:

	generic/042 generic/044 generic/045 generic/046

As near as I can tell, these tests are sensitive to how the file
system implements and handles delayed allocation.  In particular,
generic/04[456] assumes that if you do a delayed allocation write of
64k, and then truncate the file to 64k or 32k, the file will either be
zero length, or i_size is non-zero, it MUST NOT have no extents.

It's not clear to me why this needs to be true.  The test description
says "test for NULL files problem".  But since POSIX states that how
truncate will handle truncates beyond i_size is unspecified, and what
happens after a crash w/o an fsync() is similarly unspecified, it's
not clear what is the best way to deal with this.

One is to simply use a different ioctl code point, to avoid enabling
the xfstests tests.  Another to modify the tests to skip them for
ext4.  Or I can teach kvm-xfstests and gce-xfstests to ignore these
test failures by skipping the tests in my test framework.

Comments, thoughts?

						- Ted

P.S.  So I'm not implementing this just for increased xfstests
coverage; I have an operational need for this functionality on
production systems.  The short version is if you are tearing down a
container, and you don't care about its scratch space, being able to
drop all of the writes from being sent to the storage device (which
might be over the network, say using iSCSI), is a Good Thing.

Theodore Ts'o (3):
  ext4: rename s_resize_flags to s_ext4_flags
  ext4: add shutdown bit and check for it
  ext4: add EXT4_IOC_GOINGDOWN ioctl

 fs/ext4/ext4.h      | 26 +++++++++++++++++++++++---
 fs/ext4/ext4_jbd2.c |  2 ++
 fs/ext4/file.c      | 12 ++++++++++++
 fs/ext4/fsync.c     |  3 +++
 fs/ext4/ialloc.c    |  3 +++
 fs/ext4/inode.c     | 25 +++++++++++++++++++++++++
 fs/ext4/ioctl.c     | 42 ++++++++++++++++++++++++++++++++++++++++++
 fs/ext4/namei.c     | 12 ++++++++++++
 fs/ext4/resize.c    |  5 +++--
 fs/ext4/super.c     |  2 +-
 fs/ext4/xattr.c     |  3 +++
 11 files changed, 129 insertions(+), 6 deletions(-)

-- 
2.11.0.rc0.7.gbe5a750