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

[Suren Baghdasaryan <surenb@google.com>]

On Wed, Jan 15, 2025 at 4:05 AM Wei Yang <richard.weiyang@gmail.com> wrote:
>
> On Tue, Jan 14, 2025 at 07:12:20PM -0800, Suren Baghdasaryan wrote:
> >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.
> >
>
> Looks good to me.
>
> >>
> >> 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
>
> Something like this. I thought we would increase VMA_LOCK_SUCCESS on success.
>
> >page fault (in which we increment VMA_LOCK_RETRY instead)?
> >
>
> I don't follow this.

Sorry, I meant to say "in which case we increment VMA_LOCK_RETRY
instead". IOW, when we successfully lock the vma but have to retry the
pagefault, we increment VMA_LOCK_RETRY without incrementing
VMA_LOCK_SUCCESS.

>
> >>
> >> >       /*
> >> >        * 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
>
> --
> Wei Yang
> Help you, Help me