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Message-ID: <CAJuCfpEvbCJeQDnMqJK7F9dCLX+4_kx3THuRq5yuf5U9oYoEKQ@mail.gmail.com>
Date: Tue, 14 Jan 2025 19:12:20 -0800
From: Suren Baghdasaryan <surenb@...gle.com>
To: Wei Yang <richard.weiyang@...il.com>
Cc: 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 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.

>
> 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
page fault (in which we increment VMA_LOCK_RETRY instead)?

>
> >       /*
> >        * 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

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