Re: [PATCH 3/5] mm/migrate.c: fix potential indeterminate pte entry in migrate_vma_insert_page()

Alistair Popple
2021-03-23T11:07:32Z

On Tuesday, 23 March 2021 9:26:43 PM AEDT David Hildenbrand wrote:
> On 20.03.21 10:36, Miaohe Lin wrote:
> > If the zone device page does not belong to un-addressable device memory,
> > the variable entry will be uninitialized and lead to indeterminate pte
> > entry ultimately. Fix this unexpectant case and warn about it.
>
> s/unexpectant/unexpected/
>
> >
> > Fixes: df6ad69838fc ("mm/device-public-memory: device memory cache
coherent with CPU")
> > Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
> > ---
> > mm/migrate.c | 7 +++++++
> > 1 file changed, 7 insertions(+)
> >
> > diff --git a/mm/migrate.c b/mm/migrate.c
> > index 20a3bf75270a..271081b014cb 100644
> > --- a/mm/migrate.c
> > +++ b/mm/migrate.c
> > @@ -2972,6 +2972,13 @@ static void migrate_vma_insert_page(struct
migrate_vma *migrate,
> >
> > swp_entry = make_device_private_entry(page, vma->vm_flags &
VM_WRITE);
> > entry = swp_entry_to_pte(swp_entry);
> > + } else {
> > + /*
> > + * For now we only support migrating to un-addressable
> > + * device memory.
> > + */
> > + WARN_ON(1);
> > + goto abort;
>
> Fix it by crashing the kernel with panic_on_warn? :)
>
> If this case can actual happen, than no WARN_ON() - rather a
> pr_warn_once(). If this case cannot happen, why do we even care (it's
> not a fix then)?

There is also already a check for this case in migrate_vma_pages(). The
problem is it happens after the call to migrate_vma_insert_page(). I wonder if
instead it would be better just to move the existing check to before that
call?