Re: [patch 3/3] mm: fault handler to replace nopage and populate

Nick Piggin
2006-10-09T12:14:19Z

On Mon, Oct 09, 2006 at 10:07:50PM +1000, Benjamin Herrenschmidt wrote:
> On Mon, 2006-10-09 at 13:58 +0200, Nick Piggin wrote:
> >
> > The VM won't see that you have struct pages backing the ptes, and won't
> > do the right refcounting or rmap stuff... But for file backed mappings,
> > all the critical rmap stuff should be set up at mmap time, so you might
> > have another option to simply always do the nopfn thing, as far as the
> > VM is concerned (ie. even when you do have a struct page)
>
> Any reason why it wouldn't work to flip that bit on the first no_page()
> after a migration ? A migration always involves destroying all PTEs and
> is done with a per-object mutex held that no_page() takes too, so we can
> be pretty sure that the first nopage can set that bit before any PTE is
> actually inserted in the mapping after all the previous ones have been
> invalidated... That would avoid having to walk the vma's.

Ok I guess that would work. I was kind of thinking that one needs to
hold the mmap_sem for writing when changing the flags, but so long
as everyone *else* does, then I guess you can get exclusion from just
the read lock. And your per-object mutex would prevent concurrent
nopages from modifying it.