mm/mmap: free vm_area_struct without call_rcu in exit_mmap

call_rcu() can take a long time when callback offloading is enabled.  Its
use in the vm_area_free can cause regressions in the exit path when
multiple VMAs are being freed.

Because exit_mmap() is called only after the last mm user drops its
refcount, the page fault handlers can't be racing with it.  Any other
possible user like oom-reaper or process_mrelease are already synchronized
using mmap_lock.  Therefore exit_mmap() can free VMAs directly, without
the use of call_rcu().

Expose __vm_area_free() and use it from exit_mmap() to avoid possible
call_rcu() floods and performance regressions caused by it.

Link: https://lkml.kernel.org/r/20230227173632.3292573-33-surenb@google.com
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

include/linux/mm.h

@@ -257,6 +257,8 @@ void setup_initial_init_mm(void *start_code, void *end_code,
 struct vm_area_struct *vm_area_alloc(struct mm_struct *);
 struct vm_area_struct *vm_area_dup(struct vm_area_struct *);
 void vm_area_free(struct vm_area_struct *);
+/* Use only if VMA has no other users */
+void __vm_area_free(struct vm_area_struct *vma);
 
 #ifndef CONFIG_MMU
 extern struct rb_root nommu_region_tree;

kernel/fork.c

@@ -480,7 +480,7 @@ struct vm_area_struct *vm_area_dup(struct vm_area_struct *orig)
 	return new;
 }
 
-static void __vm_area_free(struct vm_area_struct *vma)
+void __vm_area_free(struct vm_area_struct *vma)
 {
 	free_anon_vma_name(vma);
 	kmem_cache_free(vm_area_cachep, vma);

mm/mmap.c

@@ -133,15 +133,18 @@ void unlink_file_vma(struct vm_area_struct *vma)
 /*
  * Close a vm structure and free it.
  */
-static void remove_vma(struct vm_area_struct *vma)
+static void remove_vma(struct vm_area_struct *vma, bool unreachable)
 {
 	might_sleep();
 	if (vma->vm_ops && vma->vm_ops->close)
 		vma->vm_ops->close(vma);
 	if (vma->vm_file)
 		fput(vma->vm_file);
 	mpol_put(vma_policy(vma));
-	vm_area_free(vma);
+	if (unreachable)
+		__vm_area_free(vma);
+	else
+		vm_area_free(vma);
 }
 
 static inline struct vm_area_struct *vma_prev_limit(struct vma_iterator *vmi,
@@ -2145,7 +2148,7 @@ static inline void remove_mt(struct mm_struct *mm, struct ma_state *mas)
 		if (vma->vm_flags & VM_ACCOUNT)
 			nr_accounted += nrpages;
 		vm_stat_account(mm, vma->vm_flags, -nrpages);
-		remove_vma(vma);
+		remove_vma(vma, false);
 	}
 	vm_unacct_memory(nr_accounted);
 	validate_mm(mm);
@@ -3078,7 +3081,7 @@ void exit_mmap(struct mm_struct *mm)
 	do {
 		if (vma->vm_flags & VM_ACCOUNT)
 			nr_accounted += vma_pages(vma);
-		remove_vma(vma);
+		remove_vma(vma, true);
 		count++;
 		cond_resched();
 	} while ((vma = mas_find(&mas, ULONG_MAX)) != NULL);