[PATCH v2 hmm 03/11] mm/hmm: Hold a mmgrab from hmm to mm

[Jason Gunthorpe]

On Thu, Jun 06, 2019 at 07:44:58PM -0700, John Hubbard wrote:
> On 6/6/19 11:44 AM, Jason Gunthorpe wrote:
> > From: Jason Gunthorpe <jgg at mellanox.com>
> > 
> > So long a a struct hmm pointer exists, so should the struct mm it is
> > linked too. Hold the mmgrab() as soon as a hmm is created, and mmdrop() it
> > once the hmm refcount goes to zero.
> > 
> > Since mmdrop() (ie a 0 kref on struct mm) is now impossible with a !NULL
> > mm->hmm delete the hmm_hmm_destroy().
> > 
> > Signed-off-by: Jason Gunthorpe <jgg at mellanox.com>
> > Reviewed-by: Jérôme Glisse <jglisse at redhat.com>
> > v2:
> >  - Fix error unwind paths in hmm_get_or_create (Jerome/Jason)
> >  include/linux/hmm.h |  3 ---
> >  kernel/fork.c       |  1 -
> >  mm/hmm.c            | 22 ++++------------------
> >  3 files changed, 4 insertions(+), 22 deletions(-)
> > 
> > diff --git a/include/linux/hmm.h b/include/linux/hmm.h
> > index 2d519797cb134a..4ee3acabe5ed22 100644
> > +++ b/include/linux/hmm.h
> > @@ -586,14 +586,11 @@ static inline int hmm_vma_fault(struct hmm_mirror *mirror,
> >  }
> >  
> >  /* Below are for HMM internal use only! Not to be used by device driver! */
> > -void hmm_mm_destroy(struct mm_struct *mm);
> > -
> >  static inline void hmm_mm_init(struct mm_struct *mm)
> >  {
> >  	mm->hmm = NULL;
> >  }
> >  #else /* IS_ENABLED(CONFIG_HMM_MIRROR) */
> > -static inline void hmm_mm_destroy(struct mm_struct *mm) {}
> >  static inline void hmm_mm_init(struct mm_struct *mm) {}
> >  #endif /* IS_ENABLED(CONFIG_HMM_MIRROR) */
> >  
> > diff --git a/kernel/fork.c b/kernel/fork.c
> > index b2b87d450b80b5..588c768ae72451 100644
> > +++ b/kernel/fork.c
> > @@ -673,7 +673,6 @@ void __mmdrop(struct mm_struct *mm)
> >  	WARN_ON_ONCE(mm == current->active_mm);
> >  	mm_free_pgd(mm);
> >  	destroy_context(mm);
> > -	hmm_mm_destroy(mm);
> 
> 
> This is particularly welcome, not to have an "HMM is special" case
> in such a core part of process/mm code. 

I would very much like to propose something like 'per-net' for struct
mm, as rdma also need to add some data to each mm to make it's use of
mmu notifiers work (for basically this same reason as HMM)

Thanks,
Jason