Re: [PATCH v3 7/8] mm: drop VMA lock before waiting for migration

[Suren Baghdasaryan]

On Tue, Jun 27, 2023 at 8:49 AM Peter Xu <peterx@xxxxxxxxxx> wrote:
>
> On Mon, Jun 26, 2023 at 09:23:20PM -0700, Suren Baghdasaryan wrote:
> > migration_entry_wait does not need VMA lock, therefore it can be
> > dropped before waiting.
>
> Hmm, I'm not sure..
>
> Note that we're still dereferencing *vmf->pmd when waiting, while *pmd is
> on the page table and IIUC only be guaranteed if the vma is still there.
> If without both mmap / vma lock I don't see what makes sure the pgtable is
> always there.  E.g. IIUC a race can happen where unmap() runs right after
> vma_end_read() below but before pmdp_get_lockless() (inside
> migration_entry_wait()), then pmdp_get_lockless() can read some random
> things if the pgtable is freed.

That sounds correct. I thought ptl would keep pmd stable but there is
time between vma_end_read() and spin_lock(ptl) when it can be freed
from under us. I think it would work if we do vma_end_read() after
spin_lock(ptl) but that requires code refactoring. I'll probably drop
this optimization from the patchset for now to keep things simple and
will get back to it later.

>
> >
> > Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>
> > ---
> >  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 = 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 = 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 |= VM_FAULT_COMPLETED;
> > +                     }
> > +                     migration_entry_wait(mm, vmf->pmd, vmf->address);
> >               } else if (is_device_exclusive_entry(entry)) {
> >                       vmf->page = pfn_swap_entry_to_page(entry);
> >                       ret = remove_device_exclusive_entry(vmf);
> > --
> > 2.41.0.178.g377b9f9a00-goog
> >
>
> --
> Peter Xu
>