Re: [RFC PATCH v2 06/47] hugetlb: extend vma lock for shared vmas

[James Houghton]

On Wed, Nov 30, 2022 at 4:01 PM Peter Xu <peterx@xxxxxxxxxx> wrote:
>
> On Fri, Oct 21, 2022 at 04:36:22PM +0000, James Houghton wrote:
> > This allows us to add more data into the shared structure, which we will
> > use to store whether or not HGM is enabled for this VMA or not, as HGM
> > is only available for shared mappings.
> >
> > It may be better to include HGM as a VMA flag instead of extending the
> > VMA lock structure.
> >
> > Signed-off-by: James Houghton <jthoughton@xxxxxxxxxx>
> > ---
> >  include/linux/hugetlb.h |  4 +++
> >  mm/hugetlb.c            | 65 +++++++++++++++++++++--------------------
> >  2 files changed, 37 insertions(+), 32 deletions(-)
> >
> > diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
> > index a899bc76d677..534958499ac4 100644
> > --- a/include/linux/hugetlb.h
> > +++ b/include/linux/hugetlb.h
> > @@ -121,6 +121,10 @@ struct hugetlb_vma_lock {
> >       struct vm_area_struct *vma;
> >  };
> >
> > +struct hugetlb_shared_vma_data {
> > +     struct hugetlb_vma_lock vma_lock;
> > +};
>
> How about add a comment above hugetlb_vma_lock showing how it should be
> used correctly?  We lacked documents on the lock for pmd sharing
> protections, now if to reuse the same lock for HGM pgtables I think some
> doc will definitely help.
>
> To summarize, I think so far it means:
>
>   - Read lock needed when one wants to stablize VM_SHARED pgtables (covers
>     both pmd shared pgtables or hgm low-level pgtables)
>
>   - Write lock needed when one wants to release VM_SHARED pgtable pages
>     (covers both pmd unshare or releasing hgm low-level pgtables)
>
> Or something like that.

Will do. I'll make this change together with the rmap comment update
("rmap: update hugetlb lock comment for HGM").

- James

>
> --
> Peter Xu
>