[RFC PATCH 3/3] um: don't force-flush in mm/userspace process start

[Johannes Berg]

From: Johannes Berg <johannes.berg at intel.com>

The reason to flush here is to resynchronize the mm after
a copy. For exec we don't need to since flush_thread()
will anyway clear everything.

Thus, we can just do this in arch_dup_mmap(), since when
that's invoked, the new mm has been setup sufficiently.
This isn't true if we just flush in init_new_context().

Signed-off-by: Johannes Berg <johannes.berg at intel.com>
---
 arch/um/include/asm/Kbuild        |  1 -
 arch/um/include/asm/mm_hooks.h    | 22 ++++++++++++++++++++++
 arch/um/include/asm/mmu_context.h |  2 +-
 arch/um/kernel/process.c          |  2 --
 arch/um/kernel/skas/mmu.c         |  7 +++++++
 arch/um/kernel/tlb.c              |  5 +----
 6 files changed, 31 insertions(+), 8 deletions(-)
 create mode 100644 arch/um/include/asm/mm_hooks.h

diff --git a/arch/um/include/asm/Kbuild b/arch/um/include/asm/Kbuild
index b2d834a29f3a..de8d82a6fd7b 100644
--- a/arch/um/include/asm/Kbuild
+++ b/arch/um/include/asm/Kbuild
@@ -26,5 +26,4 @@ generic-y += switch_to.h
 generic-y += topology.h
 generic-y += trace_clock.h
 generic-y += kprobes.h
-generic-y += mm_hooks.h
 generic-y += vga.h
diff --git a/arch/um/include/asm/mm_hooks.h b/arch/um/include/asm/mm_hooks.h
new file mode 100644
index 000000000000..b1016520c5b8
--- /dev/null
+++ b/arch/um/include/asm/mm_hooks.h
@@ -0,0 +1,22 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _ASM_UM_MM_HOOKS_H
+#define _ASM_UM_MM_HOOKS_H
+
+int arch_dup_mmap(struct mm_struct *oldmm, struct mm_struct *mm);
+
+static inline void arch_exit_mmap(struct mm_struct *mm)
+{
+}
+
+static inline void arch_unmap(struct mm_struct *mm,
+			unsigned long start, unsigned long end)
+{
+}
+
+static inline bool arch_vma_access_permitted(struct vm_area_struct *vma,
+		bool write, bool execute, bool foreign)
+{
+	/* by default, allow everything */
+	return true;
+}
+#endif	/* _ASM_UM_MM_HOOKS_H */
diff --git a/arch/um/include/asm/mmu_context.h b/arch/um/include/asm/mmu_context.h
index 68e2eb9cfb47..8668861d4a85 100644
--- a/arch/um/include/asm/mmu_context.h
+++ b/arch/um/include/asm/mmu_context.h
@@ -13,7 +13,7 @@
 #include <asm/mm_hooks.h>
 #include <asm/mmu.h>
 
-extern void force_flush_all(void);
+void force_flush_all(struct mm_struct *mm);
 
 #define activate_mm activate_mm
 static inline void activate_mm(struct mm_struct *old, struct mm_struct *new)
diff --git a/arch/um/kernel/process.c b/arch/um/kernel/process.c
index 6daffb9d8a8d..d7545ba6522a 100644
--- a/arch/um/kernel/process.c
+++ b/arch/um/kernel/process.c
@@ -139,8 +139,6 @@ void new_thread_handler(void)
 /* Called magically, see new_thread_handler above */
 void fork_handler(void)
 {
-	force_flush_all();
-
 	schedule_tail(current->thread.prev_sched);
 
 	/*
diff --git a/arch/um/kernel/skas/mmu.c b/arch/um/kernel/skas/mmu.c
index ea0c37b92581..97e731180f70 100644
--- a/arch/um/kernel/skas/mmu.c
+++ b/arch/um/kernel/skas/mmu.c
@@ -10,6 +10,7 @@
 
 #include <asm/pgalloc.h>
 #include <asm/sections.h>
+#include <asm/mmu_context.h>
 #include <as-layout.h>
 #include <os.h>
 #include <skas.h>
@@ -69,6 +70,12 @@ int init_new_context(struct task_struct *task, struct mm_struct *mm)
 	return ret;
 }
 
+int arch_dup_mmap(struct mm_struct *oldmm, struct mm_struct *mm)
+{
+	force_flush_all(mm);
+	return 0;
+}
+
 void destroy_context(struct mm_struct *mm)
 {
 	struct mm_context *mmu = &mm->context;
diff --git a/arch/um/kernel/tlb.c b/arch/um/kernel/tlb.c
index 34ec8e677fb9..7c0161321fd9 100644
--- a/arch/um/kernel/tlb.c
+++ b/arch/um/kernel/tlb.c
@@ -600,14 +600,11 @@ void flush_tlb_mm(struct mm_struct *mm)
 		fix_range(mm, vma->vm_start, vma->vm_end, 0);
 }
 
-void force_flush_all(void)
+void force_flush_all(struct mm_struct *mm)
 {
-	struct mm_struct *mm = current->mm;
 	struct vm_area_struct *vma;
 	VMA_ITERATOR(vmi, mm, 0);
 
-	mmap_read_lock(mm);
 	for_each_vma(vmi, vma)
 		fix_range(mm, vma->vm_start, vma->vm_end, 1);
-	mmap_read_unlock(mm);
 }
-- 
2.41.0