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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>
This commit is contained in:
Sebastian Andrzej Siewior 2021-09-07 19:56:09 -07:00 committed by Linus Torvalds
parent 395519b4b6
commit 513861202d
1 changed files with 22 additions and 5 deletions
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27
include/linux/highmem-internal.h
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@ -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; }