On Mon, 22 Oct 2012 17:06:46 +0200
Jan Kara [off-list ref] 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 [off-list ref] 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?

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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