Commit 61c6de66 authored Dec 14, 2018 by Piotr Jaroszynski Committed by Linus Torvalds Dec 14, 2018

fs/iomap.c: 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
(https://lkml.kernel.org/r/20181116114955.GJ14706@dhcp22.suse.cz).

I only hit this in tests that verify that move_pages() actually moved
the pages.  The test also got confused by the positive return from
move_pages() (it got treated as a success as positive numbers were not
expected and not handled) making it a bit harder to track down what's
going on.

Link: http://lkml.kernel.org/r/20181115184140.1388751-1-pjaroszynski@nvidia.com


Fixes: 82cb1417 ("xfs: add support for sub-pagesize writeback without buffer_heads")
Signed-off-by: Piotr Jaroszynski <pjaroszynski@nvidia.com>
Reviewed-by: 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>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent 8ace22bc

fs/iomap.c

+7 −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);
		}

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