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

Wei Yang
2025-01-15T12:05:35Z

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.

>>
>> > /*
>> > * 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