| Message ID | 20210810091116.pocdmaatdcogvdso@linutronix.de (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | highmem: Don't disable preemption on RT in kmap_atomic() | expand |
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@linutronix.de> Acked-by: Vlastimil Babka <vbabka@suse.cz> 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; } >
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; }
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@linutronix.de> --- include/linux/highmem-internal.h | 27 ++++++++++++++++++++++----- 1 file changed, 22 insertions(+), 5 deletions(-)