Re: [PATCH] KVM: x86: Use gfn_to_pfn_cache for steal_time

[Matthew Wilcox <willy@infradead.org>]

On Fri, Aug 02, 2024 at 01:03:16PM +0100, David Woodhouse wrote:
> On Fri, 2024-08-02 at 11:44 +0000, Carsten Stollmaier wrote:
> > handle_userfault uses TASK_INTERRUPTIBLE, so it is interruptible by
> > signals. do_user_addr_fault then busy-retries it if the pending signal
> > is non-fatal. This leads to contention of the mmap_lock.

Why does handle_userfault use TASK_INTERRUPTIBLE?  We really don't
want to stop handling a page fault just because somebody resized a
window or a timer went off.  TASK_KILLABLE, sure.

This goes all the way back to Andreas' terse "add new syscall"
patch, so there's no justification for it in the commit logs.

> The busy-loop causes so much contention on mmap_lock that post-copy
> live migration fails to make progress, and is leading to failures. Yes?
> 
> > This patch replaces the use of gfn_to_hva_cache with gfn_to_pfn_cache,
> > as gfn_to_pfn_cache ensures page presence for the memory access,
> > preventing the contention of the mmap_lock.
> > 
> > Signed-off-by: Carsten Stollmaier <stollmc@amazon.com>
> 
> Reviewed-by: David Woodhouse <dwmw@amazon.co.uk>
> 
> I think this makes sense on its own, as it addresses the specific case
> where KVM is *likely* to be touching a userfaulted (guest) page. And it
> allows us to ditch yet another explicit asm exception handler.
> 
> We should note, though, that in terms of the original problem described
> above, it's a bit of a workaround. It just means that by using
> kvm_gpc_refresh() to obtain the user page, we end up in
> handle_userfault() without the FAULT_FLAG_INTERRUPTIBLE flag.
> 
> (Note to self: should kvm_gpc_refresh() take fault flags, to allow
> interruptible and killable modes to be selected by its caller?)
> 
> 
> An alternative workaround (which perhaps we should *also* consider)
> looked like this (plus some suitable code comment, of course):
> 
> --- a/arch/x86/mm/fault.c
> +++ b/arch/x86/mm/fault.c
> @@ -1304,6 +1304,8 @@ void do_user_addr_fault(struct pt_regs *regs,
>          */
>         if (user_mode(regs))
>                 flags |= FAULT_FLAG_USER;
> +       else
> +               flags &= ~FAULT_FLAG_INTERRUPTIBLE;
>  
>  #ifdef CONFIG_X86_64
>         /*
> 
> 
> That would *also* handle arbitrary copy_to_user/copy_from_user() to
> userfault pages, which could theoretically hit the same busy loop.
> 
> I'm actually tempted to make user access *interruptible* though, and
> either add copy_{from,to}_user_interruptible() or change the semantics
> of the existing ones (which I believe are already killable).
> 
> That would require each architecture implementing interruptible
> exceptions, by doing an extable lookup before the retry. Not overly
> complex, but needs to be done for all architectures (although not at
> once; we could live with not-yet-done architectures just remaining
> killable).
> 
> Thoughts?
>