[PATCH 4.19 022/149] iomap: get/put the page in iomap_page_create/release()

From: Greg Kroah-Hartman
Date: Tue Mar 12 2019 - 13:56:54 EST

4.19-stable review patch. If anyone has any objections, please let me know.


[ Upstream commit 8e47a457321ca1a74ad194ab5dcbca764bc70731 ]

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 commit 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().

It causes the move_pages() syscall to misbehave on memory mapped files
from xfs. It does not not move any pages, which I suppose is "just" a
perf issue, but it also ends up returning a positive number which is out
of spec for the syscall. Talking to Michal Hocko, it sounds like
returning positive numbers might be a necessary update to move_pages()
anyway though.

Fixes: 82cb14175e7d ("xfs: add support for sub-pagesize writeback without buffer_heads")
Signed-off-by: Piotr Jaroszynski <pjaroszynski@xxxxxxxxxx>
[hch: actually get/put the page iomap_migrate_page() to make it work
Signed-off-by: Christoph Hellwig <hch@xxxxxx>
Reviewed-by: Dave Chinner <dchinner@xxxxxxxxxx>
Reviewed-by: Darrick J. Wong <darrick.wong@xxxxxxxxxx>
Signed-off-by: Darrick J. Wong <darrick.wong@xxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
fs/iomap.c | 9 +++++++++
1 file changed, 9 insertions(+)

diff --git a/fs/iomap.c b/fs/iomap.c
index e57fb1e534c5..2e3e64012db7 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);
+ /*
+ * 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);
return iop;
@@ -133,6 +139,7 @@ iomap_page_release(struct page *page)
set_page_private(page, 0);
+ put_page(page);

@@ -565,8 +572,10 @@ iomap_migrate_page(struct address_space *mapping, struct page *newpage,

if (page_has_private(page)) {
+ get_page(newpage);
set_page_private(newpage, page_private(page));
set_page_private(page, 0);
+ put_page(page);