Re: [PATCH -v2] rmap: make anon_vma_prepare link in all the anon_vmas of a mergeable VMA

[Rik van Riel <riel@redhat.com>]

On 04/12/2010 12:26 PM, Linus Torvalds wrote:

> But there is a _much_ more subtle case that involved swapping.
>
> So guys, here's my fairly simple theory on what happens:

That bug looks entirely possible.  Given that Borislav
has heavy swapping going on, it is quite possible that
this is the bug he has been triggering.

> The patch below is my largely mindless try at fixing this. It's untested.
> I'm not entirely sure that it actually works. But it makes some amount of
> conceptual sense. No?

The patch would help avoid the bug you described.

It does have the drawback of moving all the pages of
child processes back into the anon_vma of the parent
process after swapin, even if they are privately owned
pages by the child process.

I am guessing it may need a check to see whether the
page and swap slot are exclusively owned by the current
process.

Page or swap slot shared?      => oldest anon_vma
Page and swap slot exclusive?  => newest anon_vma

I suspect the easiest way to achieve this would be to
pass a flag in from do_swap_page, where we already
check this, a few lines above calling page_add_anon_rmap:

         if ((flags & FAULT_FLAG_WRITE) && reuse_swap_page(page)) {
                 pte = maybe_mkwrite(pte_mkdirty(pte), vma);
                 flags &= ~FAULT_FLAG_WRITE;
         }