linux-kernel - Re: [PATCH 13/41] mm: introduce vma->vm

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Message-ID: <CAJuCfpFtMjuJS6-4FYR8gWwKJjRuX5Dd2DQLHLiGLUuij_YjMQ@mail.gmail.com>
Date:   Tue, 17 Jan 2023 18:07:49 -0800
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     Michal Hocko <mhocko@...e.com>
Cc:     akpm@...ux-foundation.org, michel@...pinasse.org,
        jglisse@...gle.com, vbabka@...e.cz, hannes@...xchg.org,
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Subject: Re: [PATCH 13/41] mm: introduce vma->vm_flags modifier functions

On Tue, Jan 17, 2023 at 7:15 AM 'Michal Hocko' via kernel-team
<kernel-team@...roid.com> wrote:
>
> On Tue 17-01-23 16:09:03, Michal Hocko wrote:
> > On Mon 09-01-23 12:53:08, Suren Baghdasaryan wrote:
> > > To keep vma locking correctness when vm_flags are modified, add modifier
> > > functions to be used whenever flags are updated.
> > >
> > > Signed-off-by: Suren Baghdasaryan <surenb@...gle.com>
> > > ---
> > >  include/linux/mm.h       | 38 ++++++++++++++++++++++++++++++++++++++
> > >  include/linux/mm_types.h |  8 +++++++-
> > >  2 files changed, 45 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > > index ec2c4c227d51..35cf0a6cbcc2 100644
> > > --- a/include/linux/mm.h
> > > +++ b/include/linux/mm.h
> > > @@ -702,6 +702,44 @@ static inline void vma_init(struct vm_area_struct *vma, struct mm_struct *mm)
> > >     vma_init_lock(vma);
> > >  }
> > >
> > > +/* Use when VMA is not part of the VMA tree and needs no locking */
> > > +static inline
> > > +void init_vm_flags(struct vm_area_struct *vma, unsigned long flags)
> > > +{
> > > +   WRITE_ONCE(vma->vm_flags, flags);
> > > +}
> >
> > Why do we need WRITE_ONCE here? Isn't vma invisible during its
> > initialization?

Ack. Will change to a simple assignment.

> >
> > > +
> > > +/* Use when VMA is part of the VMA tree and needs appropriate locking */
> > > +static inline
> > > +void reset_vm_flags(struct vm_area_struct *vma, unsigned long flags)
> > > +{
> > > +   vma_write_lock(vma);
> > > +   init_vm_flags(vma, flags);
> > > +}
> > > +
> > > +static inline
> > > +void set_vm_flags(struct vm_area_struct *vma, unsigned long flags)
> > > +{
> > > +   vma_write_lock(vma);
> > > +   vma->vm_flags |= flags;
> > > +}
> > > +
> > > +static inline
> > > +void clear_vm_flags(struct vm_area_struct *vma, unsigned long flags)
> > > +{
> > > +   vma_write_lock(vma);
> > > +   vma->vm_flags &= ~flags;
> > > +}
> > > +
> > > +static inline
> > > +void mod_vm_flags(struct vm_area_struct *vma,
> > > +             unsigned long set, unsigned long clear)
> > > +{
> > > +   vma_write_lock(vma);
> > > +   vma->vm_flags |= set;
> > > +   vma->vm_flags &= ~clear;
> > > +}
> > > +
> >
> > This is rather unusual pattern. There is no note about locking involved
> > in the naming and also why is the locking part of this interface in the
> > first place? I can see reason for access functions to actually check for
> > lock asserts.
>
> OK, it took me a while but it is clear to me now. The confusion comes
> from the naming vma_write_lock is no a lock in its usual terms. It is
> more of a vma_mark_modified with side effects to read locking which is a
> real lock. With that it makes more sense to have this done in these
> helpers rather than requiring all users to keep this subtletly in mind.

If renaming vma-locking primitives the way Matthew suggested in
https://lore.kernel.org/all/Y8cZMt01Z1FvVFXh@casper.infradead.org/
makes it easier to read/understand, I'm all for it. Let's discuss the
naming in that email thread because that's where these functions are
introduced.

>
> --
> Michal Hocko
> SUSE Labs
>
> --
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>