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

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Message-ID: <20250116013747.akajp2kdwhmbgq5r@master>
Date: Thu, 16 Jan 2025 01:37:47 +0000
From: Wei Yang <richard.weiyang@...il.com>
To: Suren Baghdasaryan <surenb@...gle.com>
Cc: Wei Yang <richard.weiyang@...il.com>, akpm@...ux-foundation.org,
	peterz@...radead.org, willy@...radead.org, liam.howlett@...cle.com,
	lorenzo.stoakes@...cle.com, david.laight.linux@...il.com,
	mhocko@...e.com, vbabka@...e.cz, hannes@...xchg.org,
	mjguzik@...il.com, oliver.sang@...el.com,
	mgorman@...hsingularity.net, david@...hat.com, peterx@...hat.com,
	oleg@...hat.com, dave@...olabs.net, paulmck@...nel.org,
	brauner@...nel.org, dhowells@...hat.com, hdanton@...a.com,
	hughd@...gle.com, lokeshgidra@...gle.com, minchan@...gle.com,
	jannh@...gle.com, shakeel.butt@...ux.dev, souravpanda@...gle.com,
	pasha.tatashin@...een.com, klarasmodin@...il.com, corbet@....net,
	linux-doc@...r.kernel.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, kernel-team@...roid.com
Subject: Re: [PATCH v9 11/17] mm: replace vm_lock and detached flag with a
 reference count

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@...il.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@...il.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