Re: [PATCH v2 4/6] mm: drop VMA lock before waiting for migration

[Peter Xu <peterx@redhat.com>]

On Mon, Jun 12, 2023 at 09:07:38AM -0700, Suren Baghdasaryan wrote:
> On Mon, Jun 12, 2023 at 6:36 AM Peter Xu <peterx@redhat.com> wrote:
> >
> > On Fri, Jun 09, 2023 at 06:29:43PM -0700, Suren Baghdasaryan wrote:
> > > On Fri, Jun 9, 2023 at 3:30 PM Suren Baghdasaryan <surenb@google.com> wrote:
> > > >
> > > > On Fri, Jun 9, 2023 at 1:42 PM Peter Xu <peterx@redhat.com> wrote:
> > > > >
> > > > > On Thu, Jun 08, 2023 at 05:51:56PM -0700, Suren Baghdasaryan wrote:
> > > > > > migration_entry_wait does not need VMA lock, therefore it can be dropped
> > > > > > before waiting. Introduce VM_FAULT_VMA_UNLOCKED to indicate that VMA
> > > > > > lock was dropped while in handle_mm_fault().
> > > > > > Note that once VMA lock is dropped, the VMA reference can't be used as
> > > > > > there are no guarantees it was not freed.
> > > > >
> > > > > Then vma lock behaves differently from mmap read lock, am I right?  Can we
> > > > > still make them match on behaviors, or there's reason not to do so?
> > > >
> > > > I think we could match their behavior by also dropping mmap_lock here
> > > > when fault is handled under mmap_lock (!(fault->flags &
> > > > FAULT_FLAG_VMA_LOCK)).
> > > > I missed the fact that VM_FAULT_COMPLETED can be used to skip dropping
> > > > mmap_lock in do_page_fault(), so indeed, I might be able to use
> > > > VM_FAULT_COMPLETED to skip vma_end_read(vma) for per-vma locks as well
> > > > instead of introducing FAULT_FLAG_VMA_LOCK. I think that was your idea
> > > > of reusing existing flags?
> > > Sorry, I meant VM_FAULT_VMA_UNLOCKED, not FAULT_FLAG_VMA_LOCK in the
> > > above reply.
> > >
> > > I took a closer look into using VM_FAULT_COMPLETED instead of
> > > VM_FAULT_VMA_UNLOCKED but when we fall back from per-vma lock to
> > > mmap_lock we need to retry with an indication that the per-vma lock
> > > was dropped. Returning (VM_FAULT_RETRY | VM_FAULT_COMPLETE) to
> > > indicate such state seems strange to me ("retry" and "complete" seem
> >
> > Not relevant to this migration patch, but for the whole idea I was thinking
> > whether it should just work if we simply:
> >
> >         fault = handle_mm_fault(vma, address, flags | FAULT_FLAG_VMA_LOCK, regs);
> > -       vma_end_read(vma);
> > +       if (!(fault & (VM_FAULT_RETRY | VM_FAULT_COMPLETED)))
> > +               vma_end_read(vma);
> >
> > ?
> 
> Today when we can't handle a page fault under per-vma locks we return
> VM_FAULT_RETRY, in which case per-vma lock is dropped and the fault is

Oh I see what I missed.  I think it may not be a good idea to reuse
VM_FAULT_RETRY just for that, because it makes VM_FAULT_RETRY even more
complicated - now it adds one more case where the lock is not released,
that's when PER_VMA even if !NOWAIT.

> retried under mmap_lock. The condition you suggest above would not
> drop per-vma lock for VM_FAULT_RETRY case and would break the current
> fallback mechanism.
> However your suggestion gave me an idea. I could indicate that per-vma
> lock got dropped using vmf structure (like Matthew suggested before)
> and once handle_pte_fault(vmf) returns I could check if it returned
> VM_FAULT_RETRY but per-vma lock is still held.
> If that happens I can
> call vma_end_read() before returning from __handle_mm_fault(). That
> way any time handle_mm_fault() returns VM_FAULT_RETRY per-vma lock
> will be already released, so your condition in do_page_fault() will
> work correctly. That would eliminate the need for a new
> VM_FAULT_VMA_UNLOCKED flag. WDYT?

Sounds good.

So probably that's the major pain for now with the legacy fallback (I'll
have commented if I noticed it with the initial vma lock support..).  I
assume that'll go away as long as we recover the VM_FAULT_RETRY semantics
to before.

-- 
Peter Xu