VM: Fix nasty and subtle race in shared mmap'ed page writeback

The VM layer (on the face of it, fairly reasonably) expected that when
it does a ->writepage() call to the filesystem, it would write out the
full page at that point in time.  Especially since it had earlier marked
the whole page dirty with "set_page_dirty()".

But that isn't actually the case: ->writepage() does not actually write
a page, it writes the parts of the page that have been explicitly marked
dirty before, *and* that had not got written out for other reasons since
the last time we told it they were dirty.

That last caveat is the important one.

Which _most_ of the time ends up being the whole page (since we had
called "set_page_dirty()" on the page earlier), but if the filesystem
had done any dirty flushing of its own (for example, to honor some
internal write ordering guarantees), it might end up doing only a
partial page IO (or none at all) when ->writepage() is actually called.

That is the correct thing in general (since we actually often _want_
only the known-dirty parts of the page to be written out), but the
shared dirty page handling had implicitly forgotten about these details,
and had a number of cases where it was doing just the "->writepage()"
part, without telling the low-level filesystem that the whole page might
have been re-dirtied as part of being mapped writably into user space.

Since most of the time the FS did actually write out the full page, we
didn't notice this for a loong time, and this needed some really odd
patterns to trigger.  But it caused occasional corruption with rtorrent
and with the Debian "apt" database, because both use shared mmaps to
update the end result.

This fixes it. Finally. After way too much hair-pulling.

Acked-by: Nick Piggin <[email protected]>
Acked-by: Martin J. Bligh <[email protected]>
Acked-by: Martin Michlmayr <[email protected]>
Acked-by: Martin Johansson <[email protected]>
Acked-by: Ingo Molnar <[email protected]>
Acked-by: Andrei Popa <[email protected]>
Cc: High Dickins <[email protected]>
Cc: Andrew Morton <[email protected]>,
Cc: Peter Zijlstra <[email protected]>
Cc: Segher Boessenkool <[email protected]>
Cc: David Miller <[email protected]>
Cc: Arjan van de Ven <[email protected]>
Cc: Gordon Farquharson <[email protected]>
Cc: Guillaume Chazarain <[email protected]>
Cc: Theodore Tso <[email protected]>
Cc: Kenneth Cheng <[email protected]>
Cc: Tobias Diedrich <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

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