Re: [PATCH v3 5/7] mm: replace vma->vm_flags indirect modification in ksm_madvise

[Suren Baghdasaryan <surenb@google.com>]

On Thu, Jan 26, 2023 at 7:19 AM Mel Gorman <mgorman@techsingularity.net> wrote:
>
> On Wed, Jan 25, 2023 at 03:35:52PM -0800, Suren Baghdasaryan wrote:
> > Replace indirect modifications to vma->vm_flags with calls to modifier
> > functions to be able to track flag changes and to keep vma locking
> > correctness.
> >
> > Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> > Acked-by: Michal Hocko <mhocko@suse.com>
> > ---
> >  arch/powerpc/kvm/book3s_hv_uvmem.c | 5 ++++-
> >  arch/s390/mm/gmap.c                | 5 ++++-
> >  2 files changed, 8 insertions(+), 2 deletions(-)
> >
> > diff --git a/arch/powerpc/kvm/book3s_hv_uvmem.c b/arch/powerpc/kvm/book3s_hv_uvmem.c
> > index 1d67baa5557a..325a7a47d348 100644
> > --- a/arch/powerpc/kvm/book3s_hv_uvmem.c
> > +++ b/arch/powerpc/kvm/book3s_hv_uvmem.c
> > @@ -393,6 +393,7 @@ static int kvmppc_memslot_page_merge(struct kvm *kvm,
> >  {
> >       unsigned long gfn = memslot->base_gfn;
> >       unsigned long end, start = gfn_to_hva(kvm, gfn);
> > +     unsigned long vm_flags;
> >       int ret = 0;
> >       struct vm_area_struct *vma;
> >       int merge_flag = (merge) ? MADV_MERGEABLE : MADV_UNMERGEABLE;
> > @@ -409,12 +410,14 @@ static int kvmppc_memslot_page_merge(struct kvm *kvm,
> >                       ret = H_STATE;
> >                       break;
> >               }
> > +             vm_flags = vma->vm_flags;
> >               ret = ksm_madvise(vma, vma->vm_start, vma->vm_end,
> > -                       merge_flag, &vma->vm_flags);
> > +                       merge_flag, &vm_flags);
> >               if (ret) {
> >                       ret = H_STATE;
> >                       break;
> >               }
> > +             reset_vm_flags(vma, vm_flags);
> >               start = vma->vm_end;
> >       } while (end > vma->vm_end);
>
> Add a comment on why the vm_flags are copied in case someone "optimises"
> this in the future? Something like
>
> /* Copy vm_flags to avoid any partial modifications in ksm_madvise. */

Ack.

>
> >
> > diff --git a/arch/s390/mm/gmap.c b/arch/s390/mm/gmap.c
> > index 3a695b8a1e3c..d5eb47dcdacb 100644
> > --- a/arch/s390/mm/gmap.c
> > +++ b/arch/s390/mm/gmap.c
> > @@ -2587,14 +2587,17 @@ int gmap_mark_unmergeable(void)
> >  {
> >       struct mm_struct *mm = current->mm;
> >       struct vm_area_struct *vma;
> > +     unsigned long vm_flags;
> >       int ret;
> >       VMA_ITERATOR(vmi, mm, 0);
> >
> >       for_each_vma(vmi, vma) {
> > +             vm_flags = vma->vm_flags;
> >               ret = ksm_madvise(vma, vma->vm_start, vma->vm_end,
> > -                               MADV_UNMERGEABLE, &vma->vm_flags);
> > +                               MADV_UNMERGEABLE, &vm_flags);
> >               if (ret)
> >                       return ret;
> > +             reset_vm_flags(vma, vm_flags);
>
> Same.
>
> Not necessary as such as there are few users of ksm_madvise and I doubt
> it'll introduce new surprises.
>
> With or without the comment;
>
> Acked-by: Mel Gorman <mgorman@techsingularity.net>

Thanks!

>
> --
> Mel Gorman
> SUSE Labs