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

Wei Yang
2025-01-16T01:37:52Z

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@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.
>

Yes, this makes me confused about what VMA_LOCK_SUCCESS represents.

>>
>> >>
>> >> > /*
>> >> > * 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