linux-kernel - Re: [PATCH v1 2/4] mm: Cleanup apply_to_pte

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Message-Id: <D93LW58FLXOS.2U8X0CO2H9H5S@gmail.com>
Date: Fri, 11 Apr 2025 16:46:58 +1000
From: "Nicholas Piggin" <npiggin@...il.com>
To: "Alexander Gordeev" <agordeev@...ux.ibm.com>, "Andrew Morton"
 <akpm@...ux-foundation.org>, "Andrey Ryabinin" <ryabinin.a.a@...il.com>
Cc: "Hugh Dickins" <hughd@...gle.com>, "Guenter Roeck" <linux@...ck-us.net>,
 "Juergen Gross" <jgross@...e.com>, "Jeremy Fitzhardinge" <jeremy@...p.org>,
 <linux-kernel@...r.kernel.org>, <linux-mm@...ck.org>,
 <kasan-dev@...glegroups.com>, <sparclinux@...r.kernel.org>,
 <xen-devel@...ts.xenproject.org>, <linuxppc-dev@...ts.ozlabs.org>,
 <linux-s390@...r.kernel.org>
Subject: Re: [PATCH v1 2/4] mm: Cleanup apply_to_pte_range() routine

On Tue Apr 8, 2025 at 1:11 AM AEST, Alexander Gordeev wrote:
> Reverse 'create' vs 'mm == &init_mm' conditions and move
> page table mask modification out of the atomic context.
>
> Signed-off-by: Alexander Gordeev <agordeev@...ux.ibm.com>
> ---
>  mm/memory.c | 28 +++++++++++++++++-----------
>  1 file changed, 17 insertions(+), 11 deletions(-)
>
> diff --git a/mm/memory.c b/mm/memory.c
> index 2d8c265fc7d6..f0201c8ec1ce 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -2915,24 +2915,28 @@ static int apply_to_pte_range(struct mm_struct *mm, pmd_t *pmd,
>  				     pte_fn_t fn, void *data, bool create,
>  				     pgtbl_mod_mask *mask)
>  {
> +	int err = create ? -ENOMEM : -EINVAL;

Could you make this a new variable instead of reusing
existing err? 'const int pte_err' or something?

>  	pte_t *pte, *mapped_pte;
> -	int err = 0;
>  	spinlock_t *ptl;
>  
> -	if (create) {
> -		mapped_pte = pte = (mm == &init_mm) ?
> -			pte_alloc_kernel_track(pmd, addr, mask) :
> -			pte_alloc_map_lock(mm, pmd, addr, &ptl);
> +	if (mm == &init_mm) {
> +		if (create)
> +			pte = pte_alloc_kernel_track(pmd, addr, mask);
> +		else
> +			pte = pte_offset_kernel(pmd, addr);
>  		if (!pte)
> -			return -ENOMEM;
> +			return err;
>  	} else {
> -		mapped_pte = pte = (mm == &init_mm) ?
> -			pte_offset_kernel(pmd, addr) :
> -			pte_offset_map_lock(mm, pmd, addr, &ptl);
> +		if (create)
> +			pte = pte_alloc_map_lock(mm, pmd, addr, &ptl);
> +		else
> +			pte = pte_offset_map_lock(mm, pmd, addr, &ptl);
>  		if (!pte)
> -			return -EINVAL;
> +			return err;
> +		mapped_pte = pte;
>  	}
>  
> +	err = 0;
>  	arch_enter_lazy_mmu_mode();
>  
>  	if (fn) {
> @@ -2944,12 +2948,14 @@ static int apply_to_pte_range(struct mm_struct *mm, pmd_t *pmd,
>  			}
>  		} while (addr += PAGE_SIZE, addr != end);
>  	}
> -	*mask |= PGTBL_PTE_MODIFIED;
>  
>  	arch_leave_lazy_mmu_mode();
>  
>  	if (mm != &init_mm)
>  		pte_unmap_unlock(mapped_pte, ptl);
> +
> +	*mask |= PGTBL_PTE_MODIFIED;

This is done just because we might as well? Less work in critical
section?

Reviewed-by: Nicholas Piggin <npiggin@...il.com>

> +
>  	return err;
>  }
>