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

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Message-ID: <4ee7a430-f92d-409b-89a8-f084605eda98@linux.dev>
Date: Mon, 15 Sep 2025 22:26:51 +0800
From: Lance Yang <lance.yang@...ux.dev>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
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()

Hi Lorenzo,

Thanks for taking time to review!

On 2025/9/15 21:54, Lorenzo Stoakes wrote:
> 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))

Yes, that's a much clearer way to lay it out ;)

> 
> 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 :)

Exactly. Glad we're on the same page!

> 
>> 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.

Got it. Will fix up all nits in v2.

> 
> 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.

Ah, right. The motivation for exposing the helper should come first. I'll
reorder this changelog in v2.

> 
>> 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>

Thanks,
Lance

> 
>> ---
>>   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
>>