[PATCH v5] um: Enable preemption in UML

[Johannes Berg]

On Fri, 2023-09-22 at 10:19 +0100, Anton Ivanov wrote:
> > 
> > So maybe that works - perhaps with a big comment?
> 
> Ack. Will add this to the patch and run it through its paces.
> 

We can also just get rid of it entirely:

diff --git a/arch/um/include/asm/mmu_context.h b/arch/um/include/asm/mmu_context.h
index 68e2eb9cfb47..23dcc914d44e 100644
--- a/arch/um/include/asm/mmu_context.h
+++ b/arch/um/include/asm/mmu_context.h
@@ -13,8 +13,6 @@
 #include <asm/mm_hooks.h>
 #include <asm/mmu.h>
 
-extern void force_flush_all(void);
-
 #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..a024acd6d85c 100644
--- a/arch/um/kernel/process.c
+++ b/arch/um/kernel/process.c
@@ -25,7 +25,6 @@
 #include <linux/threads.h>
 #include <linux/resume_user_mode.h>
 #include <asm/current.h>
-#include <asm/mmu_context.h>
 #include <linux/uaccess.h>
 #include <as-layout.h>
 #include <kern_util.h>
@@ -139,8 +138,6 @@ 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 656fe16c9b63..f3766dbbc4ee 100644
--- a/arch/um/kernel/skas/mmu.c
+++ b/arch/um/kernel/skas/mmu.c
@@ -30,10 +30,7 @@ int init_new_context(struct task_struct *task, struct mm_struct *mm)
 		from_mm = &current->mm->context;
 
 	block_signals_trace();
-	if (from_mm)
-		to_mm->id.u.pid = copy_context_skas0(stack,
-						     from_mm->id.u.pid);
-	else to_mm->id.u.pid = start_userspace(stack);
+	to_mm->id.u.pid = start_userspace(stack);
 	unblock_signals_trace();
 
 	if (to_mm->id.u.pid < 0) {
diff --git a/arch/um/kernel/tlb.c b/arch/um/kernel/tlb.c
index 34ec8e677fb9..ce7fb5a34f0f 100644
--- a/arch/um/kernel/tlb.c
+++ b/arch/um/kernel/tlb.c
@@ -599,15 +599,3 @@ void flush_tlb_mm(struct mm_struct *mm)
 	for_each_vma(vmi, vma)
 		fix_range(mm, vma->vm_start, vma->vm_end, 0);
 }
-
-void force_flush_all(void)
-{
-	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);
-}




I think that _might_ be slower for fork() happy workloads, since we
don't init from skas0, but I'm not even sure what's _in_ skas0 at that
point? Does it matter?

johannes