Subject: Re: [PATCH v2 03/13] mm: Provide generic pmd_thp_or_huge()

From: Jason Gunthorpe <[email-masked]>
Date: Mon, 15 Jan 2024 13:55:51 -0400

On Wed, Jan 03, 2024 at 05:14:13PM +0800, [email-masked] wrote:
> From: Peter Xu <[email-masked]>
> 
> ARM defines pmd_thp_or_huge(), detecting either a THP or a huge PMD.  It
> can be a helpful helper if we want to merge more THP and hugetlb code
> paths.  Make it a generic default implementation, only exist when
> CONFIG_MMU.  Arch can overwrite it by defining its own version.
> 
> For example, ARM's pgtable-2level.h defines it to always return false.
> 
> Keep the macro declared with all config, it should be optimized to a false
> anyway if !THP && !HUGETLB.
> 
> Signed-off-by: Peter Xu <[email-masked]>
> ---
>  include/linux/pgtable.h | 4 ++++
>  mm/gup.c                | 3 +--
>  2 files changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
> index 466cf477551a..2b42e95a4e3a 100644
> --- a/include/linux/pgtable.h
> +++ b/include/linux/pgtable.h
> @@ -1362,6 +1362,10 @@ static inline int pmd_write(pmd_t pmd)
>  #endif /* pmd_write */
>  #endif /* CONFIG_TRANSPARENT_HUGEPAGE */
>  
> +#ifndef pmd_thp_or_huge
> +#define pmd_thp_or_huge(pmd)	(pmd_huge(pmd) || pmd_trans_huge(pmd))
> +#endif

Why not just use pmd_leaf() ?

This GUP case seems to me exactly like what pmd_leaf() should really
do and be used for..

eg x86 does:

#define pmd_leaf	pmd_large
static inline int pmd_large(pmd_t pte)
	return pmd_flags(pte) & _PAGE_PSE;

static inline int pmd_trans_huge(pmd_t pmd)
	return (pmd_val(pmd) & (_PAGE_PSE|_PAGE_DEVMAP)) == _PAGE_PSE;

int pmd_huge(pmd_t pmd)
        return !pmd_none(pmd) &&
                (pmd_val(pmd) & (_PAGE_PRESENT|_PAGE_PSE)) != _PAGE_PRESENT;

I spot checked a couple arches and it looks like it holds up.

Further, it looks to me like this site in GUP is the only core code
caller..

So, I'd suggest a small series to go arch by arch and convert the arch
to use pmd_huge() == pmd_leaf(). Then retire pmd_huge() as a public
API.

> diff --git a/mm/gup.c b/mm/gup.c
> index df83182ec72d..eebae70d2465 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -3004,8 +3004,7 @@ static int gup_pmd_range(pud_t *pudp, pud_t pud, unsigned long addr, unsigned lo
>  		if (!pmd_present(pmd))
>  			return 0;
>  
> -		if (unlikely(pmd_trans_huge(pmd) || pmd_huge(pmd) ||
> -			     pmd_devmap(pmd))) {
> +		if (unlikely(pmd_thp_or_huge(pmd) || pmd_devmap(pmd))) {
>  			/* See gup_pte_range() */
>  			if (pmd_protnone(pmd))
>  				return 0;

And the devmap thing here doesn't make any sense either. The arch
should ensure that pmd_devmap() implies pmd_leaf(). Since devmap is a
purely SW construct it almost certainly does already anyhow.

Jason