Re: [PATCH RFC 01/13] mm/hmm: Process pud swap entry without pud_huge()

Peter Xu
2024-03-08T06:50:34Z

On Thu, Mar 07, 2024 at 02:12:33PM -0400, Jason Gunthorpe wrote:
> On Wed, Mar 06, 2024 at 06:41:35PM +0800, peterx@redhat.com wrote:
> > From: Peter Xu <peterx@redhat.com>
> >
> > Swap pud entries do not always return true for pud_huge() for all archs.
> > x86 and sparc (so far) allow it, but all the rest do not accept a swap
> > entry to be reported as pud_huge(). So it's not safe to check swap entries
> > within pud_huge(). Check swap entries before pud_huge(), so it should be
> > always safe.
> >
> > This is the only place in the kernel that (IMHO, wrongly) relies on
> > pud_huge() to return true on pud swap entries. The plan is to cleanup
> > pXd_huge() to only report non-swap mappings for all archs.
> >
> > Cc: Alistair Popple <apopple@nvidia.com>
> > Signed-off-by: Peter Xu <peterx@redhat.com>
> > ---
> > mm/hmm.c | 7 +------
> > 1 file changed, 1 insertion(+), 6 deletions(-)
>
> Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
>
> > @@ -424,7 +424,7 @@ static int hmm_vma_walk_pud(pud_t *pudp, unsigned long start, unsigned long end,
> > walk->action = ACTION_CONTINUE;
> >
> > pud = READ_ONCE(*pudp);
> > - if (pud_none(pud)) {
> > + if (pud_none(pud) || !pud_present(pud)) {
>
> Isn't this a tautology? pud_none always implies !present() ?

Hmm yes I think so, afact, it should be "all=none+swap+present". I still
remember I missed that once previously, it's not always obvious when
preparing such patches. :( I'll simplify this and also on patch 3.

Thanks,

--
Peter Xu