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

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@nvidia.com>
[hch: actually get/put the page iomap_migrate_page() to make it work
      properly]
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>

diff --git a/fs/iomap.c b/fs/iomap.c
index a3088fa..cb184ff 100644 (file)
--- a/fs/iomap.c
+++ b/fs/iomap.c
@@ -116,6 +116,12 @@ 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);
        SetPagePrivate(page);
        return iop;
@@ -132,6 +138,7 @@ 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);
 }
 
@@ -569,8 +576,10 @@ iomap_migrate_page(struct address_space *mapping, struct page *newpage,
 
        if (page_has_private(page)) {
                ClearPagePrivate(page);
+               get_page(newpage);
                set_page_private(newpage, page_private(page));
                set_page_private(page, 0);
+               put_page(page);
                SetPagePrivate(newpage);
        }