iomap: Revert "fs/iomap.c: get/put the page in iomap_page_create/release()"

This reverts commit 61c6de66.

The reverted commit added page reference counting to iomap page
structures that are used to track block size < page size state. This
was supposed to align the code with page migration page accounting
assumptions, but what it has done instead is break XFS filesystems.
Every fstests run I've done on sub-page block size XFS filesystems
has since picking up this commit 2 days ago has failed with bad page
state errors such as:

# ./run_check.sh "-m rmapbt=1,reflink=1 -i sparse=1 -b size=1k" "generic/038"
....
SECTION       -- xfs
FSTYP         -- xfs (debug)
PLATFORM      -- Linux/x86_64 test1 4.20.0-rc6-dgc+
MKFS_OPTIONS  -- -f -m rmapbt=1,reflink=1 -i sparse=1 -b size=1k /dev/sdc
MOUNT_OPTIONS -- /dev/sdc /mnt/scratch

generic/038 454s ...
 run fstests generic/038 at 2018-12-20 18:43:05
 XFS (sdc): Unmounting Filesystem
 XFS (sdc): Mounting V5 Filesystem
 XFS (sdc): Ending clean mount
 BUG: Bad page state in process kswapd0  pfn:3a7fa
 page:ffffea0000ccbeb0 count:0 mapcount:0 mapping:ffff88800d9b6360 index:0x1
 flags: 0xfffffc0000000()
 raw: 000fffffc0000000 dead000000000100 dead000000000200 ffff88800d9b6360
 raw: 0000000000000001 0000000000000000 00000000ffffffff
 page dumped because: non-NULL mapping
 CPU: 0 PID: 676 Comm: kswapd0 Not tainted 4.20.0-rc6-dgc+ #915
 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.11.1-1 04/01/2014
 Call Trace:
  dump_stack+0x67/0x90
  bad_page.cold.116+0x8a/0xbd
  free_pcppages_bulk+0x4bf/0x6a0
  free_unref_page_list+0x10f/0x1f0
  shrink_page_list+0x49d/0xf50
  shrink_inactive_list+0x19d/0x3b0
  shrink_node_memcg.constprop.77+0x398/0x690
  ? shrink_slab.constprop.81+0x278/0x3f0
  shrink_node+0x7a/0x2f0
  kswapd+0x34b/0x6d0
  ? node_reclaim+0x240/0x240
  kthread+0x11f/0x140
  ? __kthread_bind_mask+0x60/0x60
  ret_from_fork+0x24/0x30
 Disabling lock debugging due to kernel taint
....

The failures are from anyway that frees pages and empties the
per-cpu page magazines, so it's not a predictable failure or an easy
to debug failure.

generic/038 is a reliable reproducer of this problem - it has a 9 in
10 failure rate on one of my test machines. Failure on other
machines have been at random points in fstests runs but every run
has ended up tripping this problem. Hence generic/038 was used to
bisect the failure because it was the most reliable failure.

It is too close to the 4.20 release (not to mention holidays) to
try to diagnose, fix and test the underlying cause of the problem,
so reverting the commit is the only option we have right now. The
revert has been tested against a current tot 4.20-rc7+ kernel across
multiple machines running sub-page block size XFs filesystems and
none of the bad page state failures have been seen.
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Cc: Piotr Jaroszynski <pjaroszynski@nvidia.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: William Kucharski <william.kucharski@oracle.com>
Cc: Darrick J. Wong <darrick.wong@oracle.com>
Cc: Brian Foster <bfoster@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/iomap.c

@@ -116,12 +116,6 @@ iomap_page_create(struct inode *inode, struct page *page)
 	atomic_set(&iop->read_count, 0);
 	atomic_set(&iop->write_count, 0);
 	bitmap_zero(iop->uptodate, PAGE_SIZE / SECTOR_SIZE);
-
-	/*
-	 * migrate_page_move_mapping() assumes that pages with private data have
-	 * their count elevated by 1.
-	 */
-	get_page(page);
 	set_page_private(page, (unsigned long)iop);
 	SetPagePrivate(page);
 	return iop;
@@ -138,7 +132,6 @@ iomap_page_release(struct page *page)
 	WARN_ON_ONCE(atomic_read(&iop->write_count));
 	ClearPagePrivate(page);
 	set_page_private(page, 0);
-	put_page(page);
 	kfree(iop);
 }