[PATCH v2 32/33] mm/mmap: free vm_area_struct without call_rcu in exit_mmap

[Suren Baghdasaryan]

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.

Signed-off-by: Suren Baghdasaryan <sur...@google.com>
---
 include/linux/mm.h |  2 ++
 kernel/fork.c      |  2 +-
 mm/mmap.c          | 11 +++++++----
 3 files changed, 10 insertions(+), 5 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index a6de58bb40c7..1c4ddcd6fd84 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -250,6 +250,8 @@ 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;
diff --git a/kernel/fork.c b/kernel/fork.c
index a08cc0e2bfde..d0999de82f94 100644
--- a/kernel/fork.c
+++ b/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);
diff --git a/mm/mmap.c b/mm/mmap.c
index 3d0cfbc92745..1028fe131bb7 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -133,7 +133,7 @@ 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)
@@ -141,7 +141,10 @@ static void remove_vma(struct vm_area_struct *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,
@@ -2134,7 +2137,7 @@ 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);
@@ -3083,7 +3086,7 @@ 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);
-- 
2.39.1