linux-kernel - Re: [PATCH v6 10/16] mm: replace vm_lock and detached flag with a reference count

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Message-ID: <CAJuCfpF5eLoWd9FCnM+aXKjnMQ_VbFmuELbp=pyQ2exYM1W6sg@mail.gmail.com>
Date: Thu, 26 Dec 2024 09:12:17 -0800
From: Suren Baghdasaryan <surenb@...gle.com>
To: "Liam R. Howlett" <Liam.Howlett@...cle.com>, Peter Zijlstra <peterz@...radead.org>, 
	Suren Baghdasaryan <surenb@...gle.com>, akpm@...ux-foundation.org, willy@...radead.org, 
	lorenzo.stoakes@...cle.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 v6 10/16] mm: replace vm_lock and detached flag with a
 reference count

On Sun, Dec 22, 2024 at 7:03 PM Suren Baghdasaryan <surenb@...gle.com> wrote:
>
> On Thu, Dec 19, 2024 at 10:55 AM Liam R. Howlett
> <Liam.Howlett@...cle.com> wrote:
> >
> > * Peter Zijlstra <peterz@...radead.org> [241219 13:47]:
> > > On Thu, Dec 19, 2024 at 01:18:23PM -0500, Liam R. Howlett wrote:
> > >
> > > > > For RCU lookups only the mas tree matters -- and its left present there.
> > > > >
> > > > > If you really want to block RCU readers, I would suggest punching a hole
> > > > > in the mm_mt. All the traditional code won't notice anyway, this is all
> > > > > with mmap_lock held for writing.
> > > >
> > > > We don't want to block all rcu readers, we want to block the rcu readers
> > > > that would see the problem - that is, anyone trying to read a particular
> > > > area.
> > > >
> > > > Right now we can page fault in unpopulated vmas while writing other vmas
> > > > to the tree.  We are also moving more users to rcu reading to use the
> > > > vmas they need without waiting on writes to finish.
> > > >
> > > > Maybe I don't understand your suggestion, but I would think punching a
> > > > hole would lose this advantage?
> > >
> > > My suggestion was to remove the range stuck in mas_detach from mm_mt.
> > > That is exactly the affected range, no?
> >
> > Yes.
> >
> > But then looping over the vmas will show a gap where there should not be
> > a gap.
> >
> > If we stop rcu readers entirely we lose the advantage.
> >
> > This is exactly the issue that the locking dance was working around :)
>
> IOW we write-lock the entire range before removing any part of it for
> the whole transaction to be atomic, correct?
>
>
> Peter, you suggested the following pattern for ensuring vma is
> detached with no possible readers:
>
> vma_iter_store()
> vma_start_write()
> vma_mark_detached()
>
> What do you think about this alternative?
>
> vma_start_write()
> ...
> vma_iter_store()
> vma_mark_detached()
>         vma_assert_write_locked(vma)
>         if (unlikely(!refcount_dec_and_test(&vma->vm_refcnt)))
>                 vma_start_write()
>
> The second vma_start_write() is unlikely to be executed because the
> vma is locked, vm_refcnt might be increased only temporarily by
> readers before they realize the vma is locked and that's a very narrow
> window. I think performance should not visibly suffer?
> OTOH this would let us keep current locking patterns and would
> guarantee that vma_mark_detached() always exits with a detached and
> unused vma (less possibilities for someone not following an exact
> pattern and ending up with a detached but still used vma).

I posted v7 of this patchset at
https://lore.kernel.org/all/20241226170710.1159679-1-surenb@google.com/
>From the things we discussed, I didn't include the following:
- Changing vma locking patterns
- Changing do_vmi_align_munmap() to avoid reattach_vmas()
It seems we need more discussion for the first one and the second one
can be done completely independent from this patchset. I feel this
patchset is already quite large, so trying to keep its size
manageable.
Thanks,
Suren.

>
> >