On Fri, May 26, 2023 at 06:22:58PM -0400, Peter Xu wrote:
> On Sun, May 21, 2023 at 09:52:31PM -0700, Hugh Dickins wrote:
> > pte_offset_map() was still using kmap_atomic(): update it to the
> > preferred kmap_local_page() before making further changes there, in case
> > we need this as a bisection point; but I doubt it can cause any trouble.
> >
> > Signed-off-by: Hugh Dickins <hughd@google.com>
> > ---
> > include/linux/pgtable.h | 4 ++--
> > 1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
> > index 8ec27fe69dc8..94235ff2706e 100644
> > --- a/include/linux/pgtable.h
> > +++ b/include/linux/pgtable.h
> > @@ -96,9 +96,9 @@ static inline pte_t *pte_offset_kernel(pmd_t *pmd, unsigned long address)
> >
> > #if defined(CONFIG_HIGHPTE)
> > #define pte_offset_map(dir, address) \
> > - ((pte_t *)kmap_atomic(pmd_page(*(dir))) + \
> > + ((pte_t *)kmap_local_page(pmd_page(*(dir))) + \
> > pte_index((address)))
> > -#define pte_unmap(pte) kunmap_atomic((pte))
> > +#define pte_unmap(pte) kunmap_local((pte))
> > #else
> > #define pte_offset_map(dir, address) pte_offset_kernel((dir), (address))
> > #define pte_unmap(pte) ((void)(pte)) /* NOP */
>
> (I think this could be a dumb question if this patch has been running there
> for years downstream, but still..)
>
> I assume one major difference of using kmap_local() is page fault will not
> be disabled, while kmap_atomic() will.
>
> Meanwhile, pte_offset_map() is also used by pte_offset_map_lock(), which
> means before this patch CONFIG_HIGHPTE systems will disable pgfault before
> taking pgtable lock for it, while it will stop doing so after the change.
>
> Then what happens if a page fault happens on the same pgtable lock range
> that is already taken by the thread context? What stops the deadlock from
> happening?
Ah, stupid me. I think such a page fault just cannot happen when holding
the pgtable lock.. I believe the same applies to !HIGHPTE..
Sorry about the noise.
--
Peter Xu