Re: [RFC PATCH RESEND 28/28] kernel/fork: throttle call_rcu() calls in vm_area_free

[Suren Baghdasaryan]

On Fri, Sep 9, 2022 at 8:19 AM Laurent Dufour <ldufour@xxxxxxxxxxxxx> wrote:
>
> Le 01/09/2022 à 19:35, Suren Baghdasaryan a écrit :
> > 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. To minimize that impact, place VMAs into
> > a list and free them in groups using one call_rcu() call per group.
> >
> > Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>
> > ---
> >  include/linux/mm.h       |  1 +
> >  include/linux/mm_types.h | 11 ++++++-
> >  kernel/fork.c            | 68 +++++++++++++++++++++++++++++++++++-----
> >  mm/init-mm.c             |  3 ++
> >  mm/mmap.c                |  1 +
> >  5 files changed, 75 insertions(+), 9 deletions(-)
> >
> > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > index a3cbaa7b9119..81dff694ac14 100644
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h
> > @@ -249,6 +249,7 @@ 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 *);
> > +void drain_free_vmas(struct mm_struct *mm);
> >
> >  #ifndef CONFIG_MMU
> >  extern struct rb_root nommu_region_tree;
> > diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> > index 36562e702baf..6f3effc493b1 100644
> > --- a/include/linux/mm_types.h
> > +++ b/include/linux/mm_types.h
> > @@ -412,7 +412,11 @@ struct vm_area_struct {
> >                       struct vm_area_struct *vm_next, *vm_prev;
> >               };
> >  #ifdef CONFIG_PER_VMA_LOCK
> > -             struct rcu_head vm_rcu; /* Used for deferred freeing. */
> > +             struct {
> > +                     struct list_head vm_free_list;
> > +                     /* Used for deferred freeing. */
> > +                     struct rcu_head vm_rcu;
> > +             };
> >  #endif
> >       };
> >
> > @@ -573,6 +577,11 @@ struct mm_struct {
> >                                         */
> >  #ifdef CONFIG_PER_VMA_LOCK
> >               int mm_lock_seq;
> > +             struct {
> > +                     struct list_head head;
> > +                     spinlock_t lock;
> > +                     int size;
> > +             } vma_free_list;
> >  #endif
> >
> >
> > diff --git a/kernel/fork.c b/kernel/fork.c
> > index b443ba3a247a..7c88710aed72 100644
> > --- a/kernel/fork.c
> > +++ b/kernel/fork.c
> > @@ -483,26 +483,75 @@ struct vm_area_struct *vm_area_dup(struct vm_area_struct *orig)
> >  }
> >
> >  #ifdef CONFIG_PER_VMA_LOCK
> > -static void __vm_area_free(struct rcu_head *head)
> > +static inline void __vm_area_free(struct vm_area_struct *vma)
> >  {
> > -     struct vm_area_struct *vma = container_of(head, struct vm_area_struct,
> > -                                               vm_rcu);
> >       /* The vma should either have no lock holders or be write-locked. */
> >       vma_assert_no_reader(vma);
> >       kmem_cache_free(vm_area_cachep, vma);
> >  }
> > -#endif
> > +
> > +static void vma_free_rcu_callback(struct rcu_head *head)
> > +{
> > +     struct vm_area_struct *first_vma;
> > +     struct vm_area_struct *vma, *vma2;
> > +
> > +     first_vma = container_of(head, struct vm_area_struct, vm_rcu);
> > +     list_for_each_entry_safe(vma, vma2, &first_vma->vm_free_list, vm_free_list)
>
> Is that safe to walk the list against concurrent calls to
> list_splice_init(), or list_add()?

I think it is. drain_free_vmas() moves the to-be-destroyed and already
isolated VMAs from mm->vma_free_list into to_destroy list and then
passes that list to vma_free_rcu_callback(). At this point the list of
VMAs passed to vma_free_rcu_callback() is not accessible either from
mm (VMAs were isolated before vm_area_free() was called) or from
drain_free_vmas() since they were already removed from
mm->vma_free_list. Does that make sense?

>
> > +             __vm_area_free(vma);
> > +     __vm_area_free(first_vma);
> > +}
> > +
> > +void drain_free_vmas(struct mm_struct *mm)
> > +{
> > +     struct vm_area_struct *first_vma;
> > +     LIST_HEAD(to_destroy);
> > +
> > +     spin_lock(&mm->vma_free_list.lock);
> > +     list_splice_init(&mm->vma_free_list.head, &to_destroy);
> > +     mm->vma_free_list.size = 0;
> > +     spin_unlock(&mm->vma_free_list.lock);
> > +
> > +     if (list_empty(&to_destroy))
> > +             return;
> > +
> > +     first_vma = list_first_entry(&to_destroy, struct vm_area_struct, vm_free_list);
> > +     /* Remove the head which is allocated on the stack */
> > +     list_del(&to_destroy);
> > +
> > +     call_rcu(&first_vma->vm_rcu, vma_free_rcu_callback);
> > +}
> > +
> > +#define VM_AREA_FREE_LIST_MAX        32
> > +
> > +void vm_area_free(struct vm_area_struct *vma)
> > +{
> > +     struct mm_struct *mm = vma->vm_mm;
> > +     bool drain;
> > +
> > +     free_anon_vma_name(vma);
> > +
> > +     spin_lock(&mm->vma_free_list.lock);
> > +     list_add(&vma->vm_free_list, &mm->vma_free_list.head);
> > +     mm->vma_free_list.size++;
> > +     drain = mm->vma_free_list.size > VM_AREA_FREE_LIST_MAX;
> > +     spin_unlock(&mm->vma_free_list.lock);
> > +
> > +     if (drain)
> > +             drain_free_vmas(mm);
> > +}
> > +
> > +#else /* CONFIG_PER_VMA_LOCK */
> > +
> > +void drain_free_vmas(struct mm_struct *mm) {}
> >
> >  void vm_area_free(struct vm_area_struct *vma)
> >  {
> >       free_anon_vma_name(vma);
> > -#ifdef CONFIG_PER_VMA_LOCK
> > -     call_rcu(&vma->vm_rcu, __vm_area_free);
> > -#else
> >       kmem_cache_free(vm_area_cachep, vma);
> > -#endif
> >  }
> >
> > +#endif /* CONFIG_PER_VMA_LOCK */
> > +
> >  static void account_kernel_stack(struct task_struct *tsk, int account)
> >  {
> >       if (IS_ENABLED(CONFIG_VMAP_STACK)) {
> > @@ -1137,6 +1186,9 @@ static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p,
> >       INIT_LIST_HEAD(&mm->mmlist);
> >  #ifdef CONFIG_PER_VMA_LOCK
> >       WRITE_ONCE(mm->mm_lock_seq, 0);
> > +     INIT_LIST_HEAD(&mm->vma_free_list.head);
> > +     spin_lock_init(&mm->vma_free_list.lock);
> > +     mm->vma_free_list.size = 0;
> >  #endif
> >       mm_pgtables_bytes_init(mm);
> >       mm->map_count = 0;
> > diff --git a/mm/init-mm.c b/mm/init-mm.c
> > index 8399f90d631c..7b6d2460545f 100644
> > --- a/mm/init-mm.c
> > +++ b/mm/init-mm.c
> > @@ -39,6 +39,9 @@ struct mm_struct init_mm = {
> >       .mmlist         = LIST_HEAD_INIT(init_mm.mmlist),
> >  #ifdef CONFIG_PER_VMA_LOCK
> >       .mm_lock_seq    = 0,
> > +     .vma_free_list.head = LIST_HEAD_INIT(init_mm.vma_free_list.head),
> > +     .vma_free_list.lock =  __SPIN_LOCK_UNLOCKED(init_mm.vma_free_list.lock),
> > +     .vma_free_list.size = 0,
> >  #endif
> >       .user_ns        = &init_user_ns,
> >       .cpu_bitmap     = CPU_BITS_NONE,
> > diff --git a/mm/mmap.c b/mm/mmap.c
> > index 1edfcd384f5e..d61b7ef84ba6 100644
> > --- a/mm/mmap.c
> > +++ b/mm/mmap.c
> > @@ -3149,6 +3149,7 @@ void exit_mmap(struct mm_struct *mm)
> >       }
> >       mm->mmap = NULL;
> >       mmap_write_unlock(mm);
> > +     drain_free_vmas(mm);
> >       vm_unacct_memory(nr_accounted);
> >  }
> >
>