Re: [PATCH] mm: Fix XFS oops due to dirty pages without buffers ons390

[Mel Gorman]

On Mon, Oct 01, 2012 at 06:26:36PM +0200, Jan Kara wrote:
> On s390 any write to a page (even from kernel itself) sets architecture
> specific page dirty bit. Thus when a page is written to via standard write, HW
> dirty bit gets set and when we later map and unmap the page, page_remove_rmap()
> finds the dirty bit and calls set_page_dirty().
> 
> Dirtying of a page which shouldn't be dirty can cause all sorts of problems to
> filesystems. The bug we observed in practice is that buffers from the page get
> freed, so when the page gets later marked as dirty and writeback writes it, XFS
> crashes due to an assertion BUG_ON(!PagePrivate(page)) in page_buffers() called
> from xfs_count_page_state().
> 
> Similar problem can also happen when zero_user_segment() call from
> xfs_vm_writepage() (or block_write_full_page() for that matter) set the
> hardware dirty bit during writeback, later buffers get freed, and then page
> unmapped.
> 
> Fix the issue by ignoring s390 HW dirty bit for page cache pages in
> page_mkclean() and page_remove_rmap(). This is safe because when a page gets
> marked as writeable in PTE it is also marked dirty in do_wp_page() or
> do_page_fault(). When the dirty bit is cleared by clear_page_dirty_for_io(),
> the page gets writeprotected in page_mkclean(). So pagecache page is writeable
> if and only if it is dirty.
> 
> CC: Martin Schwidefsky <schwidefsky@xxxxxxxxxx>
> CC: Mel Gorman <mgorman@xxxxxxx>
> CC: linux-s390@xxxxxxxxxxxxxxx
> Signed-off-by: Jan Kara <jack@xxxxxxx>

Acked-by: Mel Gorman <mgorman@xxxxxxx>

-- 
Mel Gorman
SUSE Labs
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