Commit 8e47a457 authored Jan 27, 2019 by Piotr Jaroszynski Committed by Darrick J. Wong Jan 27, 2019

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 82cb1417
("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: 82cb1417 ("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>

parent 1c7fc5cb

fs/iomap.c

+9 −0

Original line number	Diff line number	Diff line
		@@ -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);
		}