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From:   Suren Baghdasaryan <surenb@google.com>
Date:   Tue, 27 Jun 2023 08:35:35 -0700
Message-ID: <CAJuCfpHSe8+h+wG6RxepiqxZiGDoyQJcMnZ7kkSeNbAzgEYROQ@mail.gmail.com>
Subject: Re: [PATCH v3 7/8] mm: drop VMA lock before waiting for migration
To:     Alistair Popple <apopple@nvidia.com>
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On Tue, Jun 27, 2023 at 1:06=E2=80=AFAM Alistair Popple <apopple@nvidia.com=
> wrote:
>
>
> Suren Baghdasaryan <surenb@google.com> writes:
>
> > migration_entry_wait does not need VMA lock, therefore it can be
> > dropped before waiting.
> >
> > Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> > ---
> >  mm/memory.c | 14 ++++++++++++--
> >  1 file changed, 12 insertions(+), 2 deletions(-)
> >
> > diff --git a/mm/memory.c b/mm/memory.c
> > index 5caaa4c66ea2..bdf46fdc58d6 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -3715,8 +3715,18 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> >       entry =3D pte_to_swp_entry(vmf->orig_pte);
> >       if (unlikely(non_swap_entry(entry))) {
> >               if (is_migration_entry(entry)) {
> > -                     migration_entry_wait(vma->vm_mm, vmf->pmd,
> > -                                          vmf->address);
> > +                     /* Save mm in case VMA lock is dropped */
> > +                     struct mm_struct *mm =3D vma->vm_mm;
> > +
> > +                     if (vmf->flags & FAULT_FLAG_VMA_LOCK) {
> > +                             /*
> > +                              * No need to hold VMA lock for migration=
.
> > +                              * WARNING: vma can't be used after this!
> > +                              */
> > +                             vma_end_read(vma);
> > +                             ret |=3D VM_FAULT_COMPLETED;
>
> Doesn't this need to also set FAULT_FLAG_LOCK_DROPPED to ensure we don't
> call vma_end_read() again in __handle_mm_fault()?

Uh, right. Got lost during the last refactoring. Thanks for flagging!

>
> > +                     }
> > +                     migration_entry_wait(mm, vmf->pmd, vmf->address);
> >               } else if (is_device_exclusive_entry(entry)) {
> >                       vmf->page =3D pfn_swap_entry_to_page(entry);
> >                       ret =3D remove_device_exclusive_entry(vmf);
>