Re: [PATCH 05/19] mm/gup: Detect huge pfnmap entries in gup-fast

Peter Xu
2024-08-09T16:59:47Z

On Fri, Aug 09, 2024 at 06:23:53PM +0200, David Hildenbrand wrote:
> On 09.08.24 18:08, Peter Xu wrote:
> > Since gup-fast doesn't have the vma reference, teach it to detect such huge
> > pfnmaps by checking the special bit for pmd/pud too, just like ptes.
> >
> > Signed-off-by: Peter Xu <peterx@redhat.com>
> > ---
> > mm/gup.c | 6 ++++++
> > 1 file changed, 6 insertions(+)
> >
> > diff --git a/mm/gup.c b/mm/gup.c
> > index d19884e097fd..a49f67a512ee 100644
> > --- a/mm/gup.c
> > +++ b/mm/gup.c
> > @@ -3038,6 +3038,9 @@ static int gup_fast_pmd_leaf(pmd_t orig, pmd_t *pmdp, unsigned long addr,
> > if (!pmd_access_permitted(orig, flags & FOLL_WRITE))
> > return 0;
> > + if (pmd_special(orig))
> > + return 0;
> > +
> > if (pmd_devmap(orig)) {
> > if (unlikely(flags & FOLL_LONGTERM))
> > return 0;
> > @@ -3082,6 +3085,9 @@ static int gup_fast_pud_leaf(pud_t orig, pud_t *pudp, unsigned long addr,
> > if (!pud_access_permitted(orig, flags & FOLL_WRITE))
> > return 0;
> > + if (pud_special(orig))
> > + return 0;
> > +
> > if (pud_devmap(orig)) {
> > if (unlikely(flags & FOLL_LONGTERM))
> > return 0;
>
> In gup_fast_pte_range() we check after checking pte_devmap(). Do we want to
> do it in a similar fashion here, or is there a reason to do it differently?

IIUC they should behave the same, as the two should be mutual exclusive so
far. E.g. see insert_pfn():

if (pfn_t_devmap(pfn))
entry = pte_mkdevmap(pfn_t_pte(pfn, prot));
else
entry = pte_mkspecial(pfn_t_pte(pfn, prot));

It might change for sure if Alistair move on with the devmap work, though..
these two always are processed together now, so I hope that won't add much
burden which series will land first, then we may need some care on merging
them. I don't expect anything too tricky in merge if that was about
removal of the devmap bits.

>
> Acked-by: David Hildenbrand <david@redhat.com>

Thanks, I'll take this one first.

--
Peter Xu