Use GFP_ATOMIC for allocations that are required for swap on zvols

dmu_bio_clone(), __vdev_disk_physio() and vdev_disk_io_flush() are used
when doing swap to a zvol. Failing to use GFP_ATOMIC permits them to
block under memory pressure, which can prevent swap operations and cause
deadlocks. We use GFP_ATOMIC | __GFP_HIGH for these allocations because
they are required for swap operations on zvols.

Signed-off-by: Richard Yao <ryao@cs.stonybrook.edu>
Call zvol_create_minors() in spa_open_common() when initializing pool

There is an extremely odd bug that causes zvols to fail to appear on
some systems, but not others. Recently, I was able to consistently
reproduce this issue over a period of 1 month. The issue disappeared
after I applied this change from FreeBSD.

This is from FreeBSD's pool version 28 import, which occurred in
revision 219089.

Ported-by: Richard Yao <ryao@cs.stonybrook.edu>
Cleanup zvol initialization code

The following error will occur on some (possibly all) kernels because
blk_init_queue() will try to take the spinlock before we initialize it.

[    5.538871] BUG: spinlock bad magic on CPU#0, zpool/4054
[    5.538885]  lock: 0xffff88021a73de60, .magic: 00000000, .owner:
<none>/-1, .owner_cpu: 0
[    5.538888] Pid: 4054, comm: zpool Not tainted 3.9.3 #11
[    5.538890] Call Trace:
[    5.538898]  [<ffffffff81478ef8>] spin_dump+0x8c/0x91
[    5.538902]  [<ffffffff81478f1e>] spin_bug+0x21/0x26
[    5.538906]  [<ffffffff812da097>] do_raw_spin_lock+0x127/0x130
[    5.538911]  [<ffffffff81253301>] ? zvol_probe+0x91/0xf0
[    5.538914]  [<ffffffff8147d851>] _raw_spin_lock_irq+0x21/0x30
[    5.538919]  [<ffffffff812c2c1e>] cfq_init_queue+0x1fe/0x350
[    5.538922]  [<ffffffff81253360>] ? zvol_probe+0xf0/0xf0
[    5.538926]  [<ffffffff812aacb8>] elevator_init+0x78/0x140
[    5.538930]  [<ffffffff812b2677>] blk_init_allocated_queue+0x87/0xb0
[    5.538933]  [<ffffffff81253360>] ? zvol_probe+0xf0/0xf0
[    5.538937]  [<ffffffff812b26d5>] blk_init_queue_node+0x35/0x70
[    5.538941]  [<ffffffff812b271e>] blk_init_queue+0xe/0x10
[    5.538944]  [<ffffffff8125211b>] __zvol_create_minor+0x24b/0x620
[    5.538947]  [<ffffffff81253264>] zvol_create_minors_cb+0x24/0x30
[    5.538952]  [<ffffffff811bd9ca>] dmu_objset_find_spa+0xea/0x510
[    5.538955]  [<ffffffff81253240>] ? zvol_free+0x60/0x60
[    5.538958]  [<ffffffff811bda71>] dmu_objset_find_spa+0x191/0x510
[    5.538962]  [<ffffffff81253240>] ? zvol_free+0x60/0x60
[    5.538965]  [<ffffffff81253ea2>] zvol_create_minors+0x92/0x180
[    5.538969]  [<ffffffff811f8d80>] spa_open_common+0x250/0x380
[    5.538973]  [<ffffffff811f8ece>] spa_open+0xe/0x10
[    5.538977]  [<ffffffff8122817e>] pool_status_check.part.22+0x1e/0x80
[    5.538980]  [<ffffffff81228a55>] zfsdev_ioctl+0x155/0x190
[    5.538984]  [<ffffffff8116a695>] do_vfs_ioctl+0x325/0x5a0
[    5.538989]  [<ffffffff81163f1d>] ? final_putname+0x1d/0x40
[    5.538992]  [<ffffffff8116a950>] sys_ioctl+0x40/0x80
[    5.538996]  [<ffffffff814812c9>] ? do_page_fault+0x9/0x10
[    5.539000]  [<ffffffff81483929>] system_call_fastpath+0x16/0x1b
[    5.541118]  zd0: unknown partition table

We fix this by calling spin_lock_init before blk_init_queue.

The manner in which zvol_init() initializes structures is
suspectible to a race between initialization and a probe on a zvol. We
reorganize zvol_init() to prevent that.

Lastly, calling zvol_create_minors(NULL) in zvol_init() does nothing
because no pools are imported, so we remove it.

Signed-off-by: Richard Yao <ryao@gentoo.org>
Fixing a bug related to set_capacity() before add_disk().

See these references:
  http://osdir.com/ml/linux.enbd.general/2002-10/msg00176.html
  http://osdir.com/ml/linux.enbd.general/2002-10/msg00177.html
  http://stackoverflow.com/questions/13518404/add-disk-hangs-on-insmod
Illumos #3006

3006 VERIFY[S,U,P] and ASSERT[S,U,P] frequently check if first argument
     is zero

Reviewed by Matt Ahrens <matthew.ahrens@delphix.com>
Reviewed by George Wilson <george.wilson@delphix.com>
Approved by Eric Schrock <eric.schrock@delphix.com>

References:
  illumos/illumos-gate@fb09f5aad449c97fe309678f3f604982b563a96f
  https://illumos.org/issues/3006

Ported-by: Brian Behlendorf <behlendorf1@llnl.gov>
Use kmem_cache for zil_vdev_node_t objects

Signed-off-by: Richard Yao <ryao@gentoo.org>
Use bitfields for boolean values in znode_t

znode_t uses 1312 bytes in length on Linux 3.9.6 amd64. Switching to
bitfields for boolean values reduces the size to 1296 bytes. Further
reductions should be possible if padding were removed, but saving 16
bytes per struct can translate into megabytes of savings when hundreds
of thousands of znodes are cached. While the savings is relatively
small in comparison to memory used, the small efficiency gain is
practically free.

Signed-off-by: Richard Yao <ryao@gentoo.org>
Remove unused option -r from zpool

Signed-off-by: Richard Yao <ryao@gentoo.org>
Implement -t option to zpool import for temporary pool names

Specifying newname to a pool import currently changes the pool name. We
implement option -t. Specifying option -t in conjuction with newpool to
zpool import will make the name change temporary. In specific, the
original pool name will be used in all label updates and therefore is
retained upon export.

This is useful in any situation where maintenance on a system's root
pool is done from another system. A good example is environments using
virtualization, where it is more desireable to do rollback of the root
dataset of a guest's root pool than it is to rollback of the zvol
hosting the guest's root pool when an issue prevents the guest from
booting properly. Other examples include maintenance on root pools
hosted on iSCSI and operating system installations on USB keys that are
used to rescue non-bootable sytems.

Signed-off-by: Richard Yao <ryao@gentoo.org>