linux-kernel - Re: [PATCH v5 5/7] mm: Split can_change_pte_writable() into private and shared parts

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Message-ID: <b1ffcc15-6150-43ed-b49a-5d1fc8d7994f@lucifer.local>
Date: Fri, 18 Jul 2025 18:27:15 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Dev Jain <dev.jain@....com>
Cc: akpm@...ux-foundation.org, ryan.roberts@....com, david@...hat.com,
        willy@...radead.org, linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        catalin.marinas@....com, will@...nel.org, Liam.Howlett@...cle.com,
        vbabka@...e.cz, jannh@...gle.com, anshuman.khandual@....com,
        peterx@...hat.com, joey.gouly@....com, ioworker0@...il.com,
        baohua@...nel.org, kevin.brodsky@....com, quic_zhenhuah@...cinc.com,
        christophe.leroy@...roup.eu, yangyicong@...ilicon.com,
        linux-arm-kernel@...ts.infradead.org, hughd@...gle.com,
        yang@...amperecomputing.com, ziy@...dia.com
Subject: Re: [PATCH v5 5/7] mm: Split can_change_pte_writable() into private
 and shared parts

On Fri, Jul 18, 2025 at 02:32:42PM +0530, Dev Jain wrote:
> In preparation for patch 6 and modularizing the code in general, split
> can_change_pte_writable() into private and shared VMA parts. No functional
> change intended.
>
> Suggested-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> Signed-off-by: Dev Jain <dev.jain@....com>

Great thanks! This is much clearer I think (of course being the
Suggested-by here makes me somewhat biased :P) :>)

LGTM, so:

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

> ---
>  mm/mprotect.c | 50 ++++++++++++++++++++++++++++++++++++--------------
>  1 file changed, 36 insertions(+), 14 deletions(-)
>
> diff --git a/mm/mprotect.c b/mm/mprotect.c
> index 4977f198168e..a1c7d8a4648d 100644
> --- a/mm/mprotect.c
> +++ b/mm/mprotect.c
> @@ -40,11 +40,8 @@
>
>  #include "internal.h"
>
> -bool can_change_pte_writable(struct vm_area_struct *vma, unsigned long addr,
> -			     pte_t pte)
> +static bool maybe_change_pte_writable(struct vm_area_struct *vma, pte_t pte)
>  {
> -	struct page *page;
> -
>  	if (WARN_ON_ONCE(!(vma->vm_flags & VM_WRITE)))
>  		return false;
>
> @@ -60,16 +57,32 @@ bool can_change_pte_writable(struct vm_area_struct *vma, unsigned long addr,
>  	if (userfaultfd_pte_wp(vma, pte))
>  		return false;
>
> -	if (!(vma->vm_flags & VM_SHARED)) {
> -		/*
> -		 * Writable MAP_PRIVATE mapping: We can only special-case on
> -		 * exclusive anonymous pages, because we know that our
> -		 * write-fault handler similarly would map them writable without
> -		 * any additional checks while holding the PT lock.
> -		 */
> -		page = vm_normal_page(vma, addr, pte);
> -		return page && PageAnon(page) && PageAnonExclusive(page);
> -	}
> +	return true;
> +}
> +
> +static bool can_change_private_pte_writable(struct vm_area_struct *vma,
> +					    unsigned long addr, pte_t pte)
> +{
> +	struct page *page;
> +
> +	if (!maybe_change_pte_writable(vma, pte))
> +		return false;
> +
> +	/*
> +	 * Writable MAP_PRIVATE mapping: We can only special-case on
> +	 * exclusive anonymous pages, because we know that our
> +	 * write-fault handler similarly would map them writable without
> +	 * any additional checks while holding the PT lock.
> +	 */
> +	page = vm_normal_page(vma, addr, pte);
> +	return page && PageAnon(page) && PageAnonExclusive(page);
> +}
> +
> +static bool can_change_shared_pte_writable(struct vm_area_struct *vma,
> +					   pte_t pte)
> +{
> +	if (!maybe_change_pte_writable(vma, pte))
> +		return false;
>
>  	VM_WARN_ON_ONCE(is_zero_pfn(pte_pfn(pte)) && pte_dirty(pte));
>
> @@ -83,6 +96,15 @@ bool can_change_pte_writable(struct vm_area_struct *vma, unsigned long addr,
>  	return pte_dirty(pte);
>  }
>
> +bool can_change_pte_writable(struct vm_area_struct *vma, unsigned long addr,
> +			     pte_t pte)
> +{
> +	if (!(vma->vm_flags & VM_SHARED))
> +		return can_change_private_pte_writable(vma, addr, pte);
> +
> +	return can_change_shared_pte_writable(vma, pte);
> +}
> +
>  static int mprotect_folio_pte_batch(struct folio *folio, pte_t *ptep,
>  				    pte_t pte, int max_nr_ptes)
>  {
> --
> 2.30.2
>