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@xxxxxxxxxx>
> > ---
> > 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
