Re: [patch] mm: bug in set_page_dirty_buffers

[Nick Piggin <npiggin@suse.de>]

On Mon, Oct 09, 2006 at 08:20:39PM -0700, Andrew Morton wrote:
> On Mon, 9 Oct 2006 20:06:05 -0700 (PDT)
> Linus Torvalds <torvalds@osdl.org> wrote:
> 
> > On Tue, 10 Oct 2006, Nick Piggin wrote:
> > >
> > > This was triggered, but not the fault of, the dirty page accounting
> > > patches. Suitable for -stable as well, after it goes upstream.
> > 
> > Applied. However, I wonder what protects "page_mapping()" here?
> 
> Nothing.  And I don't understand the (unchangelogged) switch from
> page->mapping to page_mapping().

I did the switch because that is that its callers and
the other spd functions are using to find the mapping.

> > I don't 
> > think we hold the page lock anywhere, so "page->mapping" can change at any 
> > time, no?
> 
> Yes.  The patch makes the race window a bit smaller.

It fixes the problem. The mapping is already pinned at this point,
the problem is that page_mapping is still free to go NULL at any
time, and __set_page_dirty_buffers wasn't checking for that.

If there is another race, then it must be because the buffer code
cannot cope with dirty buffers against a truncated page. It is
kind of spaghetti, though. What stops set_page_dirty_buffers from
racing with block_invalidatepage, for example?

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