Re: [PATCH v2] mm: thp: fix flags for pmd migration when split

[Konstantin Khlebnikov]

On Wed, Dec 12, 2018 at 8:15 AM Peter Xu <peterx@xxxxxxxxxx> wrote:
>
> On Tue, Dec 11, 2018 at 11:21:44AM +0300, Konstantin Khlebnikov wrote:
> > On 11.12.2018 8:12, Peter Xu wrote:
> > > When splitting a huge migrating PMD, we'll transfer all the existing
> > > PMD bits and apply them again onto the small PTEs.  However we are
> > > fetching the bits unconditionally via pmd_soft_dirty(), pmd_write()
> > > or pmd_yound() while actually they don't make sense at all when it's
> > > a migration entry.  Fix them up by make it conditional.
> > >
> > > Note that if my understanding is correct about the problem then if
> > > without the patch there is chance to lose some of the dirty bits in
> > > the migrating pmd pages (on x86_64 we're fetching bit 11 which is part
> > > of swap offset instead of bit 2) and it could potentially corrupt the
> > > memory of an userspace program which depends on the dirty bit.
> > >
> > > CC: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> > > CC: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> > > CC: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
> > > CC: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> > > CC: Michal Hocko <mhocko@xxxxxxxx>
> > > CC: Dave Jiang <dave.jiang@xxxxxxxxx>
> > > CC: "Aneesh Kumar K.V" <aneesh.kumar@xxxxxxxxxxxxxxxxxx>
> > > CC: Souptick Joarder <jrdr.linux@xxxxxxxxx>
> > > CC: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx>
> > > CC: linux-mm@xxxxxxxxx
> > > CC: linux-kernel@xxxxxxxxxxxxxxx
> > > Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
> > > ---
> > > v2:
> > > - fix it up for young/write/dirty bits too [Konstantin]
> > > ---
> > >   mm/huge_memory.c | 15 ++++++++++-----
> > >   1 file changed, 10 insertions(+), 5 deletions(-)
> > >
> > > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > > index f2d19e4fe854..b00941b3d342 100644
> > > --- a/mm/huge_memory.c
> > > +++ b/mm/huge_memory.c
> > > @@ -2157,11 +2157,16 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
> > >             page = pmd_page(old_pmd);
> > >     VM_BUG_ON_PAGE(!page_count(page), page);
> > >     page_ref_add(page, HPAGE_PMD_NR - 1);
> > > -   if (pmd_dirty(old_pmd))
> > > -           SetPageDirty(page);
> > > -   write = pmd_write(old_pmd);
> > > -   young = pmd_young(old_pmd);
> > > -   soft_dirty = pmd_soft_dirty(old_pmd);
> > > +   if (unlikely(pmd_migration)) {
> > > +           soft_dirty = pmd_swp_soft_dirty(old_pmd);
> > > +           young = write = false;
> > > +   } else {
> > > +           if (pmd_dirty(old_pmd))
> > > +                   SetPageDirty(page);
> > > +           write = pmd_write(old_pmd);
> > > +           young = pmd_young(old_pmd);
> > > +           soft_dirty = pmd_soft_dirty(old_pmd);
> > > +   }
> >
> > Write/read-only is encoded into migration entry.
> > I suppose there should be something like this:
> >
> > --- a/mm/huge_memory.c
> > +++ b/mm/huge_memory.c
> > @@ -2151,16 +2151,21 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
> >
> >                 entry = pmd_to_swp_entry(old_pmd);
> >                 page = pfn_to_page(swp_offset(entry));
> > +               write = is_write_migration_entry(entry);
> > +               young = false;
> > +               soft_dirty = pmd_swp_soft_dirty(old_pmd);
> >         } else
> >  #endif
> > +       {
> >                 page = pmd_page(old_pmd);
> > +               if (pmd_dirty(old_pmd))
> > +                       SetPageDirty(page);
> > +               write = pmd_write(old_pmd);
> > +               young = pmd_young(old_pmd);
> > +               soft_dirty = pmd_soft_dirty(old_pmd);
> > +       }
> >         VM_BUG_ON_PAGE(!page_count(page), page);
> >         page_ref_add(page, HPAGE_PMD_NR - 1);
> > -       if (pmd_dirty(old_pmd))
> > -               SetPageDirty(page);
> > -       write = pmd_write(old_pmd);
> > -       young = pmd_young(old_pmd);
> > -       soft_dirty = pmd_soft_dirty(old_pmd);
> >
> >         /*
> >          * Withdraw the table only after we mark the pmd entry invalid.
> >
>
> Oops yes, I missed the write bit.  Thanks for pointing it out.
>
> Should I repost with your authorship and your sign-off?

Feel free to use this piece for your own patch.

> Or even I'll
> consider to directly drop the CONFIG_ARCH_ENABLE_THP_MIGRATION if with
> that since I don't see much gain to keep it:

Yep, this ifdef could be removed.
Without CONFIG_ARCH_ENABLE_THP_MIGRATION
is_pmd_migration_entry() is constant 0 so compiler should eliminate "if" branch.

>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index f2d19e4fe854..aebade83cec9 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -2145,23 +2145,25 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
>          */
>         old_pmd = pmdp_invalidate(vma, haddr, pmd);
>
> -#ifdef CONFIG_ARCH_ENABLE_THP_MIGRATION
>         pmd_migration = is_pmd_migration_entry(old_pmd);
> -       if (pmd_migration) {
> +       if (unlikely(pmd_migration)) {
>                 swp_entry_t entry;
>
>                 entry = pmd_to_swp_entry(old_pmd);
>                 page = pfn_to_page(swp_offset(entry));
> -       } else
> -#endif
> +               write = is_write_migration_entry(entry);
> +               young = false;
> +               soft_dirty = pmd_swp_soft_dirty(old_pmd);
> +       } else {
>                 page = pmd_page(old_pmd);
> +               if (pmd_dirty(old_pmd))
> +                       SetPageDirty(page);
> +               write = pmd_write(old_pmd);
> +               young = pmd_young(old_pmd);
> +               soft_dirty = pmd_soft_dirty(old_pmd);
> +       }
>         VM_BUG_ON_PAGE(!page_count(page), page);
>         page_ref_add(page, HPAGE_PMD_NR - 1);
> -       if (pmd_dirty(old_pmd))
> -               SetPageDirty(page);
> -       write = pmd_write(old_pmd);
> -       young = pmd_young(old_pmd);
> -       soft_dirty = pmd_soft_dirty(old_pmd);
>
>         /*
>          * Withdraw the table only after we mark the pmd entry invalid.
>
> Thanks,
>
> --
> Peter Xu
>