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    Subject[PATCH 4.20 028/171] iomap: get/put the page in iomap_page_create/release()
    Date
    4.20-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@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>
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    fs/iomap.c | 9 +++++++++
    1 file changed, 9 insertions(+)

    diff --git a/fs/iomap.c b/fs/iomap.c
    index ce837d962d47..7793e9e42ad5 100644
    --- 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);
    }

    --
    2.19.1


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