linux-kernel - Re: [PATCH mm-new 2/3] mm: clean up and expose is_guard_pte

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Message-ID: <a0133269-82dc-4249-bb78-202b44a4a25e@lucifer.local>
Date: Mon, 15 Sep 2025 14:54:25 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Lance Yang <lance.yang@...ux.dev>
Cc: akpm@...ux-foundation.org, david@...hat.com, ziy@...dia.com,
        baolin.wang@...ux.alibaba.com, Liam.Howlett@...cle.com,
        npache@...hat.com, ryan.roberts@....com, dev.jain@....com,
        baohua@...nel.org, ioworker0@...il.com, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, Kairui Song <kasong@...cent.com>
Subject: Re: [PATCH mm-new 2/3] mm: clean up and expose is_guard_pte_marker()

On Sun, Sep 14, 2025 at 10:35:46PM +0800, Lance Yang wrote:
> From: Lance Yang <lance.yang@...ux.dev>
>
> is_guard_pte_marker() performs a redundant check because it calls both
> is_pte_marker() and is_guard_swp_entry(), both of which internally check
> for a PTE marker.
>
> is_guard_pte_marker()
>  |- is_pte_marker()
>  |   `- is_pte_marker_entry()  // First check
>  `- is_guard_swp_entry()
>      `- is_pte_marker_entry()  // Second, redundant check
>

I mean, it expands to:

is_swap_pte(pte) && is_pte_marker_entry(pte_to_swp_entry(pte)) &&
is_pte_marker_entry(pte_to_swp_entry(pte))

So I don't think it's really unreasonable to expect compiler magic here...

But you're right that I should have just used is_swap_pte() really, it's a bit
silly not to, so this is fine :)

> While a modern compiler could likely optimize this away, let's have clean
> code and not rely on it ;)

Please don't put smileys in commit messages :) as cute as they are, this is
going on the permanent kernel record and while we all love them, it's
probably not the best place to put them :P

>
> Also, make it available for hugepage collapsing code.

Nit but put a newline after this.

I think probably if I'm really really nitty I'd say that you should put
this bit first, as it's the primary motivation for the change, and put the
refactoring stuff after.

> Cc: Kairui Song <kasong@...cent.com>
> Signed-off-by: Lance Yang <lance.yang@...ux.dev>

This seems fine to me, so:

Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>

> ---
>  include/linux/swapops.h | 6 ++++++
>  mm/madvise.c            | 6 ------
>  2 files changed, 6 insertions(+), 6 deletions(-)
>
> diff --git a/include/linux/swapops.h b/include/linux/swapops.h
> index 59c5889a4d54..7f5684fa043b 100644
> --- a/include/linux/swapops.h
> +++ b/include/linux/swapops.h
> @@ -469,6 +469,12 @@ static inline int is_guard_swp_entry(swp_entry_t entry)
>  		(pte_marker_get(entry) & PTE_MARKER_GUARD);
>  }
>
> +static inline bool is_guard_pte_marker(pte_t ptent)
> +{
> +	return is_swap_pte(ptent) &&
> +	       is_guard_swp_entry(pte_to_swp_entry(ptent));
> +}
> +
>  /*
>   * This is a special version to check pte_none() just to cover the case when
>   * the pte is a pte marker.  It existed because in many cases the pte marker
> diff --git a/mm/madvise.c b/mm/madvise.c
> index 35ed4ab0d7c5..bd46e6788fac 100644
> --- a/mm/madvise.c
> +++ b/mm/madvise.c
> @@ -1069,12 +1069,6 @@ static bool is_valid_guard_vma(struct vm_area_struct *vma, bool allow_locked)
>  	return !(vma->vm_flags & disallowed);
>  }
>
> -static bool is_guard_pte_marker(pte_t ptent)
> -{
> -	return is_pte_marker(ptent) &&
> -		is_guard_swp_entry(pte_to_swp_entry(ptent));
> -}
> -
>  static int guard_install_pud_entry(pud_t *pud, unsigned long addr,
>  				   unsigned long next, struct mm_walk *walk)
>  {
> --
> 2.49.0
>