Re: [PATCH] mm: Fix XFS oops due to dirty pages without buffers on s390

[Andrew Morton <akpm@linux-foundation.org>]

On Mon, 22 Oct 2012 17:06:46 +0200
Jan Kara <jack@suse.cz> 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 buffered 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 of mappings
> with mapping_cap_account_dirty(). This is safe because for such mappings 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.
> 
> Thanks to Hugh Dickins <hughd@google.com> for pointing out mapping has to have
> mapping_cap_account_dirty() for things to work and proposing a cleaned up
> variant of the patch.
> 
> The patch has survived about two hours of running fsx-linux on tmpfs while
> heavily swapping and several days of running on out build machines where the
> original problem was triggered.

That seems a fairly serious problem.  To which kernel version(s) should
we apply the fix?

> diff --git a/mm/rmap.c b/mm/rmap.c

It's a bit surprising that none of the added comments mention the s390
pte-dirtying oddity.  I don't see an obvious place to mention this, but
I for one didn't know about this and it would be good if we could
capture the info _somewhere_?

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