linux-kernel - Re: [PATCH v2 5/5] mm: Add update_mmu_tlb

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Date: Fri, 10 May 2024 10:05:33 +0100
From: Ryan Roberts <ryan.roberts@....com>
To: Bang Li <libang.li@...group.com>, akpm@...ux-foundation.org,
 chenhuacai@...nel.org, tsbogend@...ha.franken.de, paul.walmsley@...ive.com,
 palmer@...belt.com, chris@...kel.net, jcmvbkbc@...il.com
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
 loongarch@...ts.linux.dev, linux-riscv@...ts.infradead.org,
 david@...hat.com, ioworker0@...il.com, libang.linux@...il.com
Subject: Re: [PATCH v2 5/5] mm: Add update_mmu_tlb_range()

On 06/05/2024 16:51, Bang Li wrote:
> After the commit 19eaf44954df ("mm: thp: support allocation of anonymous
> multi-size THP"), it may need to batch update tlb of an address range
> through the update_mmu_tlb function. We can simplify this operation by
> adding the update_mmu_tlb_range function, which may also reduce the
> execution of some unnecessary code in some architectures.
> 
> Signed-off-by: Bang Li <libang.li@...group.com>
> ---
>  include/linux/pgtable.h | 8 ++++++++
>  mm/memory.c             | 4 +---
>  2 files changed, 9 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
> index 18019f037bae..869bfe6054f1 100644
> --- a/include/linux/pgtable.h
> +++ b/include/linux/pgtable.h
> @@ -737,6 +737,14 @@ static inline void update_mmu_tlb(struct vm_area_struct *vma,
>  #define __HAVE_ARCH_UPDATE_MMU_TLB
>  #endif

Given you are implementing update_mmu_tlb_range() in all the arches that
currently override update_mmu_tlb() I wonder if it would be cleaner to remove
update_mmu_tlb() from all those arches, and define generically, removing the
ability for arches to override it:

static inline void update_mmu_tlb(struct vm_area_struct *vma,
				unsigned long address, pte_t *ptep)
{
	update_mmu_tlb_range(vma, address, ptep, 1);
}

>  
> +#ifndef __HAVE_ARCH_UPDATE_MMU_TLB_RANGE
> +static inline void update_mmu_tlb_range(struct vm_area_struct *vma,
> +				unsigned long address, pte_t *ptep, unsigned int nr)
> +{
> +}
> +#define __HAVE_ARCH_UPDATE_MMU_TLB_RANGE
> +#endif

Then you could use the modern override scheme as Lance suggested and you won't
have any confusion with __HAVE_ARCH_UPDATE_MMU_TLB because it won't exist anymore.

> +
>  /*
>   * Some architectures may be able to avoid expensive synchronization
>   * primitives when modifications are made to PTE's which are already
> diff --git a/mm/memory.c b/mm/memory.c
> index eea6e4984eae..2d53e29cf76e 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -4421,7 +4421,6 @@ static vm_fault_t do_anonymous_page(struct vm_fault *vmf)
>  	vm_fault_t ret = 0;
>  	int nr_pages = 1;
>  	pte_t entry;
> -	int i;
>  
>  	/* File mapping without ->vm_ops ? */
>  	if (vma->vm_flags & VM_SHARED)
> @@ -4491,8 +4490,7 @@ static vm_fault_t do_anonymous_page(struct vm_fault *vmf)
>  		update_mmu_tlb(vma, addr, vmf->pte);
>  		goto release;
>  	} else if (nr_pages > 1 && !pte_range_none(vmf->pte, nr_pages)) {
> -		for (i = 0; i < nr_pages; i++)
> -			update_mmu_tlb(vma, addr + PAGE_SIZE * i, vmf->pte + i);
> +		update_mmu_tlb_range(vma, addr, vmf->pte, nr_pages);

I certainly agree that this will be a useful helper to have. I expect there will
be more users in future.

>  		goto release;
>  	}
>