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

From: Jason Gunthorpe <[email-masked]>
Date: Thu, 7 Mar 2024 14:12:33 -0400

On Wed, Mar 06, 2024 at 06:41:35PM +0800, [email-masked] wrote:
> From: Peter Xu <[email-masked]>
> 
> 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 <[email-masked]>
> Signed-off-by: Peter Xu <[email-masked]>
> ---
>  mm/hmm.c | 7 +------
>  1 file changed, 1 insertion(+), 6 deletions(-)

Reviewed-by: Jason Gunthorpe <[email-masked]>

> @@ -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