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

[Suren Baghdasaryan]

On Wed, Jan 15, 2025 at 5:37 PM Wei Yang <richard.weiyang@xxxxxxxxx> wrote:
>
> On Wed, Jan 15, 2025 at 07:01:56AM -0800, Suren Baghdasaryan wrote:
> >On Wed, Jan 15, 2025 at 4:05 AM Wei Yang <richard.weiyang@xxxxxxxxx> 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@xxxxxxxxx> 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.
> >
>
> Yes, this makes me confused about what VMA_LOCK_SUCCESS represents.

I'll need to look into the history of why we account it this way but
this is out of scope for this patchset.

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