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 <[email protected]>
---
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 */
--
2.35.3
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 <[email protected]>
> ---
> 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?
Thanks,
--
Peter Xu
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 <[email protected]>
> > ---
> > 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