Commit 51386120 authored Sep 07, 2021 by Sebastian Andrzej Siewior Committed by Linus Torvalds Sep 08, 2021

highmem: don't disable preemption on RT in kmap_atomic()

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.

Link: https://lkml.kernel.org/r/20210810091116.pocdmaatdcogvdso@linutronix.de


Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent 395519b4

include/linux/highmem-internal.h

+22 −5

Original line number	Diff line number	Diff line
		@@ -90,7 +90,11 @@ static inline void __kunmap_local(void *vaddr)

		static inline void kmap_atomic_prot(struct page page, pgprot_t prot)
		{
		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)
		{
		if (IS_ENABLED(CONFIG_PREEMPT_RT))
		migrate_disable();
		else
		preempt_disable();

		pagefault_disable();
		return __kmap_local_pfn_prot(pfn, kmap_prot);
		}
		@@ -111,6 +119,9 @@ static inline void __kunmap_atomic(void *addr)
		{
		kunmap_local_indexed(addr);
		pagefault_enable();
		if (IS_ENABLED(CONFIG_PREEMPT_RT))
		migrate_enable();
		else
		preempt_enable();
		}

		@@ -179,6 +190,9 @@ static inline void __kunmap_local(void *addr)

		static inline void kmap_atomic(struct page page)
		{
		if (IS_ENABLED(CONFIG_PREEMPT_RT))
		migrate_disable();
		else
		preempt_disable();
		pagefault_disable();
		return page_address(page);
		@@ -200,6 +214,9 @@ static inline void __kunmap_atomic(void *addr)
		kunmap_flush_on_unmap(addr);
		#endif
		pagefault_enable();
		if (IS_ENABLED(CONFIG_PREEMPT_RT))
		migrate_enable();
		else
		preempt_enable();
		}