Re: [PATCH v9 11/17] mm: replace vm_lock and detached flag with a reference count

[Suren Baghdasaryan <surenb@google.com>]

On Tue, Jan 14, 2025 at 6:58 PM Wei Yang <richard.weiyang@gmail.com> wrote:
>
> On Fri, Jan 10, 2025 at 08:25:58PM -0800, Suren Baghdasaryan wrote:
> >@@ -6354,7 +6422,6 @@ struct vm_area_struct *lock_vma_under_rcu(struct mm_struct *mm,
> >       struct vm_area_struct *vma;
> >
> >       rcu_read_lock();
> >-retry:
> >       vma = mas_walk(&mas);
> >       if (!vma)
> >               goto inval;
> >@@ -6362,13 +6429,6 @@ struct vm_area_struct *lock_vma_under_rcu(struct mm_struct *mm,
> >       if (!vma_start_read(vma))
> >               goto inval;
> >
> >-      /* Check if the VMA got isolated after we found it */
> >-      if (is_vma_detached(vma)) {
> >-              vma_end_read(vma);
> >-              count_vm_vma_lock_event(VMA_LOCK_MISS);
> >-              /* The area was replaced with another one */
> >-              goto retry;
> >-      }
>
> We have a little behavior change here.
>
> Originally, if we found an detached vma, we may retry. But now, we would go to
> the slow path directly.

Hmm. Good point. I think the easiest way to keep the same
functionality is to make vma_start_read() return vm_area_struct* on
success, NULL on locking failure and EAGAIN if vma was detached
(vm_refcnt==0). Then the same retry with VMA_LOCK_MISS can be done in
the case of EAGAIN.

>
> Maybe we can compare the event VMA_LOCK_MISS and VMA_LOCK_ABORT
> to see the percentage of this case. If it shows this is a too rare
> case to impact performance, we can ignore it.
>
> Also the event VMA_LOCK_MISS recording is removed, but the definition is
> there. We may record it in the vma_start_read() when oldcnt is 0.
>
> BTW, the name of VMA_LOCK_SUCCESS confuse me a little. I thought it indicates
> lock_vma_under_rcu() successfully get a valid vma. But seems not. Sounds we
> don't have an overall success/failure statistic in vmstat.

Are you referring to the fact that we do not increment
VMA_LOCK_SUCCESS if we successfully locked a vma but have to retry the
page fault (in which we increment VMA_LOCK_RETRY instead)?

>
> >       /*
> >        * At this point, we have a stable reference to a VMA: The VMA is
> >        * locked and we know it hasn't already been isolated.
>
> --
> Wei Yang
> Help you, Help me