[PATCH v5 07/12] mm: bail out of lazy_mmu_mode_* in interrupt context

[Kevin Brodsky <kevin.brodsky@xxxxxxx>]

The lazy MMU mode cannot be used in interrupt context. This is
documented in <linux/pgtable.h>, but isn't consistently handled
across architectures.

arm64 ensures that calls to lazy_mmu_mode_* have no effect in
interrupt context, because such calls do occur in certain
configurations - see commit b81c688426a9 ("arm64/mm: Disable barrier
batching in interrupt contexts"). Other architectures do not check
this situation, most likely because it hasn't occurred so far.

Let's handle this in the new generic lazy_mmu layer, in the same
fashion as arm64: bail out of lazy_mmu_mode_* if in_interrupt().
Also remove the arm64 handling that is now redundant.

Both arm64 and x86/Xen also ensure that any lazy MMU optimisation is
disabled while in interrupt (see queue_pte_barriers() and
xen_get_lazy_mode() respectively). This will be handled in the
generic layer in a subsequent patch.

Signed-off-by: Kevin Brodsky <kevin.brodsky@xxxxxxx>
---
 arch/arm64/include/asm/pgtable.h |  9 ---------
 include/linux/pgtable.h          | 17 +++++++++++++++--
 2 files changed, 15 insertions(+), 11 deletions(-)

diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
index 54f8d6bb6f22..e596899f4029 100644
--- a/arch/arm64/include/asm/pgtable.h
+++ b/arch/arm64/include/asm/pgtable.h
@@ -94,26 +94,17 @@ static inline void arch_enter_lazy_mmu_mode(void)
         * keeps tracking simple.
         */
 
-       if (in_interrupt())
-               return;
-
        set_thread_flag(TIF_LAZY_MMU);
 }
 
 static inline void arch_flush_lazy_mmu_mode(void)
 {
-       if (in_interrupt())
-               return;
-
        if (test_and_clear_thread_flag(TIF_LAZY_MMU_PENDING))
                emit_pte_barriers();
 }
 
 static inline void arch_leave_lazy_mmu_mode(void)
 {
-       if (in_interrupt())
-               return;
-
        arch_flush_lazy_mmu_mode();
        clear_thread_flag(TIF_LAZY_MMU);
 }
diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
index c121358dba15..8ff6fdb4b13d 100644
--- a/include/linux/pgtable.h
+++ b/include/linux/pgtable.h
@@ -228,27 +228,40 @@ static inline int pmd_dirty(pmd_t pmd)
  * of the lazy mode. So the implementation must assume preemption may be 
enabled
  * and cpu migration is possible; it must take steps to be robust against this.
  * (In practice, for user PTE updates, the appropriate page table lock(s) are
- * held, but for kernel PTE updates, no lock is held). Nesting is not permitted
- * and the mode cannot be used in interrupt context.
+ * held, but for kernel PTE updates, no lock is held). The mode is disabled in
+ * interrupt context and calls to the lazy_mmu API have no effect.
+ * Nesting is not permitted.
  */
 #ifdef CONFIG_ARCH_HAS_LAZY_MMU_MODE
 static inline void lazy_mmu_mode_enable(void)
 {
+       if (in_interrupt())
+               return;
+
        arch_enter_lazy_mmu_mode();
 }
 
 static inline void lazy_mmu_mode_disable(void)
 {
+       if (in_interrupt())
+               return;
+
        arch_leave_lazy_mmu_mode();
 }
 
 static inline void lazy_mmu_mode_pause(void)
 {
+       if (in_interrupt())
+               return;
+
        arch_leave_lazy_mmu_mode();
 }
 
 static inline void lazy_mmu_mode_resume(void)
 {
+       if (in_interrupt())
+               return;
+
        arch_enter_lazy_mmu_mode();
 }
 #else
-- 
2.51.2