Re: [PATCH v3 4/5] mm: hugetlb: fix missing cache flush in hugetlb_mcopy_atomic_pte()

[Muchun Song]

On Wed, Feb 2, 2022 at 6:24 AM Mike Kravetz <mike.kravetz@xxxxxxxxxx> wrote:
>
> Cc: Matthew
>
> On 1/31/22 08:02, Muchun Song wrote:
> > folio_copy() will copy the data from one page to the target page, then
> > the target page will be mapped to the user space address, which might
> > have an alias issue with the kernel address used to copy the data from
> > the page to.  Fix this issue by flushing dcache but not use
> > flush_dcache_folio() since it is not backportable.
> >
> > Fixes: 8cc5fcbb5be8 ("mm, hugetlb: fix racy resv_huge_pages underflow on UFFDIO_COPY")
> > Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
> > ---
> >  mm/hugetlb.c | 5 +++++
> >  1 file changed, 5 insertions(+)
> >
> > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > index a1baa198519a..f1f1ab31dc8a 100644
> > --- a/mm/hugetlb.c
> > +++ b/mm/hugetlb.c
> > @@ -5804,6 +5804,8 @@ int hugetlb_mcopy_atomic_pte(struct mm_struct *dst_mm,
> >                       goto out;
> >               }
> >       } else {
> > +             int i, nr;
> > +
> >               if (vm_shared &&
> >                   hugetlbfs_pagecache_present(h, dst_vma, dst_addr)) {
> >                       put_page(*pagep);
> > @@ -5819,6 +5821,9 @@ int hugetlb_mcopy_atomic_pte(struct mm_struct *dst_mm,
> >                       goto out;
> >               }
> >               folio_copy(page_folio(page), page_folio(*pagep));
>
> What if we changed that folio_copy() to?
>
>                 copy_user_huge_page(page, *pagep, dst_addr, dst_vma,
>                         pages_per_huge_page(h));
>
> Seems like that would take care of the flush_dcache_page and it would be
> backportable.

Agree. I also can replace folio_copy() with copy_user_huge_page().

>
> Of course, Matthew may hate the idea.  Not sure if there are any plans to
> convert copy_user_huge_page to use folio type as it has some special hinting
> logic.