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

Jason Gunthorpe
2024-03-07T18:12:37Z

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() ?

Jason