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

[Suren Baghdasaryan]

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?