Re: [PATCH 1/3] mm: handle swap page faults under VMA lock if page is uncontended

[Matthew Wilcox]

On Mon, May 01, 2023 at 07:30:13PM -0700, Suren Baghdasaryan wrote:
> On Mon, May 1, 2023 at 7:02 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
> >
> > On Mon, May 01, 2023 at 10:50:23AM -0700, Suren Baghdasaryan wrote:
> > > +++ b/mm/memory.c
> > > @@ -3711,11 +3711,6 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> > >       if (!pte_unmap_same(vmf))
> > >               goto out;
> > >
> > > -     if (vmf->flags & FAULT_FLAG_VMA_LOCK) {
> > > -             ret = VM_FAULT_RETRY;
> > > -             goto out;
> > > -     }
> > > -
> > >       entry = pte_to_swp_entry(vmf->orig_pte);
> > >       if (unlikely(non_swap_entry(entry))) {
> > >               if (is_migration_entry(entry)) {
> >
> > You're missing the necessary fallback in the (!folio) case.
> > swap_readpage() is synchronous and will sleep.
> 
> True, but is it unsafe to do that under VMA lock and has to be done
> under mmap_lock?

... you were the one arguing that we didn't want to wait for I/O with
the VMA lock held?