[ofa-general] Re: [PATCH] KVM swapping with MMU Notifiers V7

Robin Holt
2008-02-16T11:51:38Z

On Sat, Feb 16, 2008 at 11:48:27AM +0100, Andrea Arcangeli wrote:
> Those below two patches enable KVM to swap the guest physical memory
> through Christoph's V7.
>
> There's one last _purely_theoretical_ race condition I figured out and
> that I'm wondering how to best fix. The race condition worst case is
> that a few guest physical pages could remain pinned by sptes. The race
> can materialize if the linux pte is zapped after get_user_pages
> returns but before the page is mapped by the spte and tracked by
> rmap. The invalidate_ calls can also likely be optimized further but
> it's not a fast path so it's not urgent.

I am doing this in xpmem with a stack-based structure in the function
calling get_user_pages. That structure describes the start and
end address of the range we are doing the get_user_pages on. If an
invalidate_range_begin comes in while we are off to the kernel doing
the get_user_pages, the invalidate_range_begin marks that structure
indicating an invalidate came in. When the get_user_pages gets the
structures relocked, it checks that flag (really a generation counter)
and if it is set, retries the get_user_pages. After 3 retries, it
returns -EAGAIN and the fault is started over from the remote side.

Thanks,
Robin