[PATCH] iomap: get/put the page in iomap_page_create/release()

[p.jaroszynski@gmail.com]

From: Piotr Jaroszynski <pjaroszynski@nvidia.com>

migrate_page_move_mapping() expects pages with private data set to have
a page_count elevated by 1. This is what used to happen for xfs through
the buffer_heads code before the switch to iomap in 82cb14175e7d ("xfs:
add support for sub-pagesize writeback without buffer_heads"). Not
having the count elevated causes move_pages() to fail on memory mapped
files coming from xfs.

Make iomap compatible with the migrate_page_move_mapping() assumption
by elevating the page count as part of iomap_page_create() and lowering
it in iomap_page_release().

Fixes: 82cb14175e7d ("xfs: add support for sub-pagesize writeback
                      without buffer_heads")
Signed-off-by: Piotr Jaroszynski <pjaroszynski@nvidia.com>
---
 fs/iomap.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/fs/iomap.c b/fs/iomap.c
index 90c2febc93ac..23977f9f23a2 100644
--- a/fs/iomap.c
+++ b/fs/iomap.c
@@ -117,6 +117,12 @@ 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);
+
+	/*
+	 * At least 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;
@@ -133,6 +139,7 @@ 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);
 }
 
-- 
2.11.0.262.g4b0a5b2.dirty