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

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Date:   Thu, 26 Jan 2023 08:11:28 -0800
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     Mel Gorman <mgorman@...hsingularity.net>
Cc:     akpm@...ux-foundation.org, michel@...pinasse.org,
        jglisse@...gle.com, mhocko@...e.com, vbabka@...e.cz,
        hannes@...xchg.org, dave@...olabs.net, willy@...radead.org,
        liam.howlett@...cle.com, peterz@...radead.org,
        ldufour@...ux.ibm.com, paulmck@...nel.org, mingo@...hat.com,
        will@...nel.org, luto@...nel.org, songliubraving@...com,
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        hughd@...gle.com, bigeasy@...utronix.de, kent.overstreet@...ux.dev,
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        leewalsh@...gle.com, posk@...gle.com, linux-mm@...ck.org,
        linux-arm-kernel@...ts.infradead.org,
        linuxppc-dev@...ts.ozlabs.org, x86@...nel.org,
        linux-kernel@...r.kernel.org, kernel-team@...roid.com
Subject: Re: [PATCH v3 5/7] mm: replace vma->vm_flags indirect modification in ksm_madvise

On Thu, Jan 26, 2023 at 7:19 AM Mel Gorman <mgorman@...hsingularity.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@...gle.com>
> > Acked-by: Michal Hocko <mhocko@...e.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@...hsingularity.net>

Thanks!

>
> --
> Mel Gorman
> SUSE Labs