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Date:   Tue, 17 Jan 2023 16:15:04 +0100
From:   Michal Hocko <mhocko@suse.com>
To:     Suren Baghdasaryan <surenb@google.com>
Cc:     akpm@linux-foundation.org, michel@lespinasse.org,
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Subject: Re: [PATCH 13/41] mm: introduce vma->vm_flags modifier functions
Message-ID: <Y8a7eJQ6upFyI6pf@dhcp22.suse.cz>
References: <20230109205336.3665937-1-surenb@google.com>
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 <Y8a6DhA6o3jcZaM3@dhcp22.suse.cz>
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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@google.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?
> 
> > +
> > +/* 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.

-- 
Michal Hocko
SUSE Labs