Re: [PATCH V2 2/2] mm/highmem: Lift memcpy_[to|from]_page to core

[Dan Williams]

On Mon, Dec 7, 2020 at 3:27 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
>
> On Mon, Dec 07, 2020 at 02:57:03PM -0800, ira.weiny@xxxxxxxxx wrote:
> > +static inline void memcpy_page(struct page *dst_page, size_t dst_off,
> > +                            struct page *src_page, size_t src_off,
> > +                            size_t len)
> > +{
> > +     char *dst = kmap_local_page(dst_page);
> > +     char *src = kmap_local_page(src_page);
>
> I appreciate you've only moved these, but please add:
>
>         BUG_ON(dst_off + len > PAGE_SIZE || src_off + len > PAGE_SIZE);

I imagine it's not outside the realm of possibility that some driver
on CONFIG_HIGHMEM=n is violating this assumption and getting away with
it because kmap_atomic() of contiguous pages "just works (TM)".
Shouldn't this WARN rather than BUG so that the user can report the
buggy driver and not have a dead system?