On 8/10/21 11:11 AM, Sebastian Andrzej Siewior wrote:
> kmap_atomic() disables preemption and pagefaults for historical
> reasons. The conversion to kmap_local(), which only disables
> migration, cannot be done wholesale because quite some call sites need
> to be updated to accommodate with the changed semantics.
>
> On PREEMPT_RT enabled kernels the kmap_atomic() semantics are
> problematic due to the implicit disabling of preemption which makes it
> impossible to acquire 'sleeping' spinlocks within the kmap atomic
> sections.
>
> PREEMPT_RT replaces the preempt_disable() with a migrate_disable() for
> more than a decade. It could be argued that this is a justification to
> do this unconditionally, but PREEMPT_RT covers only a limited number of
> architectures and it disables some functionality which limits the
> coverage further.
>
> Limit the replacement to PREEMPT_RT for now.
>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
Note I use the same pattern in my SLUB series, but for performance reasons
(migrate_disable() is an unconditional function call etc). But I can guess what
would be the answer if I suggested this pattern to get a common shared wrapper,
so I won't :P
> ---
> include/linux/highmem-internal.h | 27 ++++++++++++++++++++++-----
> 1 file changed, 22 insertions(+), 5 deletions(-)
>
> diff --git a/include/linux/highmem-internal.h b/include/linux/highmem-internal.h
> index 7902c7d8b55f9..4aa1031d3e4c3 100644
> --- a/include/linux/highmem-internal.h
> +++ b/include/linux/highmem-internal.h
> @@ -90,7 +90,11 @@ static inline void __kunmap_local(void *vaddr)
>
> static inline void *kmap_atomic_prot(struct page *page, pgprot_t prot)
> {
> - preempt_disable();
> + if (IS_ENABLED(CONFIG_PREEMPT_RT))
> + migrate_disable();
> + else
> + preempt_disable();
> +
> pagefault_disable();
> return __kmap_local_page_prot(page, prot);
> }
> @@ -102,7 +106,11 @@ static inline void *kmap_atomic(struct page *page)
>
> static inline void *kmap_atomic_pfn(unsigned long pfn)
> {
> - preempt_disable();
> + if (IS_ENABLED(CONFIG_PREEMPT_RT))
> + migrate_disable();
> + else
> + preempt_disable();
> +
> pagefault_disable();
> return __kmap_local_pfn_prot(pfn, kmap_prot);
> }
> @@ -111,7 +119,10 @@ static inline void __kunmap_atomic(void *addr)
> {
> kunmap_local_indexed(addr);
> pagefault_enable();
> - preempt_enable();
> + if (IS_ENABLED(CONFIG_PREEMPT_RT))
> + migrate_enable();
> + else
> + preempt_enable();
> }
>
> unsigned int __nr_free_highpages(void);
> @@ -179,7 +190,10 @@ static inline void __kunmap_local(void *addr)
>
> static inline void *kmap_atomic(struct page *page)
> {
> - preempt_disable();
> + if (IS_ENABLED(CONFIG_PREEMPT_RT))
> + migrate_disable();
> + else
> + preempt_disable();
> pagefault_disable();
> return page_address(page);
> }
> @@ -200,7 +214,10 @@ static inline void __kunmap_atomic(void *addr)
> kunmap_flush_on_unmap(addr);
> #endif
> pagefault_enable();
> - preempt_enable();
> + if (IS_ENABLED(CONFIG_PREEMPT_RT))
> + migrate_enable();
> + else
> + preempt_enable();
> }
>
> static inline unsigned int nr_free_highpages(void) { return 0; }
>
