+ highmem-dont-disable-preemption-on-rt-in-kmap_atomic.patch added to -mm tree

+ highmem-dont-disable-preemption-on-rt-in-kmap_atomic.patch added to -mm tree

[akpm]

The patch titled
     Subject: highmem: don't disable preemption on RT in kmap_atomic()
has been added to the -mm tree.  Its filename is
     highmem-dont-disable-preemption-on-rt-in-kmap_atomic.patch

This patch should soon appear at
    https://ozlabs.org/~akpm/mmots/broken-out/highmem-dont-disable-preemption-on-rt-in-kmap_atomic.patch
and later at
    https://ozlabs.org/~akpm/mmotm/broken-out/highmem-dont-disable-preemption-on-rt-in-kmap_atomic.patch

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------------------------------------------------------
From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Subject: 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>
---

 include/linux/highmem-internal.h |   27 ++++++++++++++++++++++-----
 1 file changed, 22 insertions(+), 5 deletions(-)

--- a/include/linux/highmem-internal.h~highmem-dont-disable-preemption-on-rt-in-kmap_atomic
+++ a/include/linux/highmem-internal.h
@@ -90,7 +90,11 @@ static inline void __kunmap_local(void *
 
 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 p
 
 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
 {
 	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 *
 
 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
 	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; }
_

Patches currently in -mm which might be from bigeasy@linutronix.de are

mm-slub-move-flush_cpu_slab-invocations-__free_slab-invocations-out-of-irq-context.patch
mm-slub-make-object_map_lock-a-raw_spinlock_t.patch
shmem-use-raw_spinlock_t-for-stat_lock.patch
highmem-dont-disable-preemption-on-rt-in-kmap_atomic.patch