linux-kernel - Re: [PATCH 2/2] arm64: mm: relax VM_ALLOW_HUGE_VMAP if BBML2

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Message-ID: <eaa67b0d-e256-4db6-82e7-c4d56df70c09@arm.com>
Date: Wed, 15 Oct 2025 12:20:51 +0530
From: Dev Jain <dev.jain@....com>
To: Yang Shi <yang@...amperecomputing.com>, ryan.roberts@....com,
 cl@...two.org, catalin.marinas@....com, will@...nel.org
Cc: linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] arm64: mm: relax VM_ALLOW_HUGE_VMAP if BBML2_NOABORT
 is supported


On 14/10/25 4:57 am, Yang Shi wrote:
> When changing permissions for vmalloc area, VM_ALLOW_HUGE_VMAP area is
> exclueded because kernel can't split the va mapping if it is called on
> partial range.
> It is no longer true if the machines support BBML2_NOABORT after commit
> a166563e7ec3 ("arm64: mm: support large block mapping when rodata=full").
> So we can relax this restriction and update the comments accordingly.
>
> Fixes: a166563e7ec3 ("arm64: mm: support large block mapping when rodata=full")
> Signed-off-by: Yang Shi <yang@...amperecomputing.com>
> ---
>   arch/arm64/mm/pageattr.c | 13 +++++++------
>   1 file changed, 7 insertions(+), 6 deletions(-)
>
> diff --git a/arch/arm64/mm/pageattr.c b/arch/arm64/mm/pageattr.c
> index c21a2c319028..b4dcae6273a8 100644
> --- a/arch/arm64/mm/pageattr.c
> +++ b/arch/arm64/mm/pageattr.c
> @@ -157,13 +157,13 @@ static int change_memory_common(unsigned long addr, int numpages,
>   
>   	/*
>   	 * Kernel VA mappings are always live, and splitting live section
> -	 * mappings into page mappings may cause TLB conflicts. This means
> -	 * we have to ensure that changing the permission bits of the range
> -	 * we are operating on does not result in such splitting.
> +	 * mappings into page mappings may cause TLB conflicts on the machines
> +	 * which don't support BBML2_NOABORT.
>   	 *
>   	 * Let's restrict ourselves to mappings created by vmalloc (or vmap).
> -	 * Disallow VM_ALLOW_HUGE_VMAP mappings to guarantee that only page
> -	 * mappings are updated and splitting is never needed.
> +	 * Disallow VM_ALLOW_HUGE_VMAP mappings if the systems don't support
> +	 * BBML2_NOABORT to guarantee that only page mappings are updated and
> +	 * splitting is never needed on those machines.
>   	 *
>   	 * So check whether the [addr, addr + size) interval is entirely
>   	 * covered by precisely one VM area that has the VM_ALLOC flag set.
> @@ -171,7 +171,8 @@ static int change_memory_common(unsigned long addr, int numpages,
>   	area = find_vm_area((void *)addr);
>   	if (!area ||
>   	    end > (unsigned long)kasan_reset_tag(area->addr) + area->size ||
> -	    ((area->flags & (VM_ALLOC | VM_ALLOW_HUGE_VMAP)) != VM_ALLOC))
> +	    !(area->flags & VM_ALLOC) || ((area->flags & VM_ALLOW_HUGE_VMAP) &&
> +	    !system_supports_bbml2_noabort()))
>   		return -EINVAL;
>   
>   	if (!numpages)

This will conflict with my upcoming vmalloc-huge series, so best to leave it to me,
I already have this included :)