Message-ID: <6e51f756-786f-4f15-a50a-73a482cc73a2@arm.com>
Date: Sat, 19 Jul 2025 19:22:20 +0530
From: Dev Jain <dev.jain@....com>
To: akpm@...ux-foundation.org, david@...hat.com, catalin.marinas@....com,
 will@...nel.org
Cc: lorenzo.stoakes@...cle.com, Liam.Howlett@...cle.com, vbabka@...e.cz,
 rppt@...nel.org, surenb@...gle.com, mhocko@...e.com, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, suzuki.poulose@....com, steven.price@....com,
 gshan@...hat.com, linux-arm-kernel@...ts.infradead.org,
 yang@...amperecomputing.com, ryan.roberts@....com, anshuman.khandual@....com
Subject: Re: [PATCH v4] arm64: Enable permission change on arm64 kernel block
 mappings

Gentle ping

On 03/07/25 8:44 pm, Dev Jain wrote:
> This patch paves the path to enable huge mappings in vmalloc space and
> linear map space by default on arm64. For this we must ensure that we can
> handle any permission games on the kernel (init_mm) pagetable. Currently,
> __change_memory_common() uses apply_to_page_range() which does not support
> changing permissions for block mappings. We attempt to move away from this
> by using the pagewalk API, similar to what riscv does right now; however,
> it is the responsibility of the caller to ensure that we do not pass a
> range overlapping a partial block mapping or cont mapping; in such a case,
> the system must be able to support range splitting.
>
> This patch is tied with Yang Shi's attempt [1] at using huge mappings
> in the linear mapping in case the system supports BBML2, in which case
> we will be able to split the linear mapping if needed without
> break-before-make. Thus, Yang's series, IIUC, will be one such user of my
> patch; suppose we are changing permissions on a range of the linear map
> backed by PMD-hugepages, then the sequence of operations should look
> like the following:
>
> split_range(start)
> split_range(end);
> __change_memory_common(start, end);
>
> However, this patch can be used independently of Yang's; since currently
> permission games are being played only on pte mappings (due to
> apply_to_page_range not supporting otherwise), this patch provides the
> mechanism for enabling huge mappings for various kernel mappings
> like linear map and vmalloc.
>
> ---------------------
> Implementation
> ---------------------
>
> arm64 currently changes permissions on vmalloc objects locklessly, via
> apply_to_page_range, whose limitation is to deny changing permissions for
> block mappings. Therefore, we move away to use the generic pagewalk API,
> thus paving the path for enabling huge mappings by default on kernel space
> mappings, thus leading to more efficient TLB usage. However, the API
> currently enforces the init_mm.mmap_lock to be held. To avoid the
> unnecessary bottleneck of the mmap_lock for our usecase, this patch
> extends this generic API to be used locklessly, so as to retain the
> existing behaviour for changing permissions. Apart from this reason, it is
> noted at [2] that KFENCE can manipulate kernel pgtable entries during
> softirqs. It does this by calling set_memory_valid() -> __change_memory_common().
> This being a non-sleepable context, we cannot take the init_mm mmap lock.
>
> Add comments to highlight the conditions under which we can use the
> lockless variant - no underlying VMA, and the user having exclusive control
> over the range, thus guaranteeing no concurrent access.
>
> We require that the start and end of a given range do not partially overlap
> block mappings, or cont mappings. Return -EINVAL in case a partial block
> mapping is detected in any of the PGD/P4D/PUD/PMD levels; add a
> corresponding comment in update_range_prot() to warn that eliminating
> such a condition is the responsibility of the caller.
>
> Note that, the pte level callback may change permissions for a whole
> contpte block, and that will be done one pte at a time, as opposed to
> an atomic operation for the block mappings. This is fine as any access
> will decode either the old or the new permission until the TLBI.
>
> apply_to_page_range() currently performs all pte level callbacks while in
> lazy mmu mode. Since arm64 can optimize performance by batching barriers
> when modifying kernel pgtables in lazy mmu mode, we would like to continue
> to benefit from this optimisation. Unfortunately walk_kernel_page_table_range()
> does not use lazy mmu mode. However, since the pagewalk framework is not
> allocating any memory, we can safely bracket the whole operation inside
> lazy mmu mode ourselves. Therefore, wrap the call to
> walk_kernel_page_table_range() with the lazy MMU helpers.
>
> [1] https://lore.kernel.org/all/20250304222018.615808-1-yang@os.amperecomputing.com/
> [2] https://lore.kernel.org/linux-arm-kernel/89d0ad18-4772-4d8f-ae8a-7c48d26a927e@arm.com/
>
> Signed-off-by: Dev Jain <dev.jain@....com>
> ---
>
> v3->v4:
>   - Drop patch 2, will add it back when we actually enable vmalloc block
>     mappings by default (Ryan)
>   - Rename ___change_memory_common -> update_range_prot (Mike Rapoport)
>   - Explain why changing live kernel mappings mapped with PTE_CONT
>     pte-by-pte is safe
>
> v2->v3:
>   - Drop adding PGWALK_NOLOCK, instead have a new lockless helper
>   - Merge patch 1 and 2 from v2
>   - Add a patch *actually* enabling vmalloc-huge permission change
>
> v1->v2:
>   - Squash patch 2 and 3
>   - Add comment describing the responsibility of the caller to ensure no
>     partial overlap with block mapping
>   - Add comments and return -EINVAL at relevant places to document the usage
>     of PGWALK_NOLOCK (Lorenzo)
>   - Nest walk_kernel_page_table_range() with lazy_mmu calls, instead of
>     doing so only per PTE level, fix bug in the PTE callback, introduce
>     callbacks for all pagetable levels, use ptdesc_t instead of unsigned
>     long, introduce ___change_memory_common and use them for direct map
>     permission change functions (Ryan)
>
> v1:
> https://lore.kernel.org/all/20250530090407.19237-1-dev.jain@arm.com/
>
>   arch/arm64/mm/pageattr.c | 155 +++++++++++++++++++++++++++++++--------
>   include/linux/pagewalk.h |   3 +
>   mm/pagewalk.c            |  24 ++++++
>   3 files changed, 150 insertions(+), 32 deletions(-)
>
> diff --git a/arch/arm64/mm/pageattr.c b/arch/arm64/mm/pageattr.c
> index 04d4a8f676db..c6a85000fa0e 100644
> --- a/arch/arm64/mm/pageattr.c
> +++ b/arch/arm64/mm/pageattr.c
> @@ -8,6 +8,7 @@
>   #include <linux/mem_encrypt.h>
>   #include <linux/sched.h>
>   #include <linux/vmalloc.h>
> +#include <linux/pagewalk.h>
>   
>   #include <asm/cacheflush.h>
>   #include <asm/pgtable-prot.h>
> @@ -20,6 +21,99 @@ struct page_change_data {
>   	pgprot_t clear_mask;
>   };
>   
> +static ptdesc_t set_pageattr_masks(ptdesc_t val, struct mm_walk *walk)
> +{
> +	struct page_change_data *masks = walk->private;
> +
> +	val &= ~(pgprot_val(masks->clear_mask));
> +	val |= (pgprot_val(masks->set_mask));
> +
> +	return val;
> +}
> +
> +static int pageattr_pgd_entry(pgd_t *pgd, unsigned long addr,
> +			      unsigned long next, struct mm_walk *walk)
> +{
> +	pgd_t val = pgdp_get(pgd);
> +
> +	if (pgd_leaf(val)) {
> +		if (WARN_ON_ONCE((next - addr) != PGDIR_SIZE))
> +			return -EINVAL;
> +		val = __pgd(set_pageattr_masks(pgd_val(val), walk));
> +		set_pgd(pgd, val);
> +		walk->action = ACTION_CONTINUE;
> +	}
> +
> +	return 0;
> +}
> +
> +static int pageattr_p4d_entry(p4d_t *p4d, unsigned long addr,
> +			      unsigned long next, struct mm_walk *walk)
> +{
> +	p4d_t val = p4dp_get(p4d);
> +
> +	if (p4d_leaf(val)) {
> +		if (WARN_ON_ONCE((next - addr) != P4D_SIZE))
> +			return -EINVAL;
> +		val = __p4d(set_pageattr_masks(p4d_val(val), walk));
> +		set_p4d(p4d, val);
> +		walk->action = ACTION_CONTINUE;
> +	}
> +
> +	return 0;
> +}
> +
> +static int pageattr_pud_entry(pud_t *pud, unsigned long addr,
> +			      unsigned long next, struct mm_walk *walk)
> +{
> +	pud_t val = pudp_get(pud);
> +
> +	if (pud_leaf(val)) {
> +		if (WARN_ON_ONCE((next - addr) != PUD_SIZE))
> +			return -EINVAL;
> +		val = __pud(set_pageattr_masks(pud_val(val), walk));
> +		set_pud(pud, val);
> +		walk->action = ACTION_CONTINUE;
> +	}
> +
> +	return 0;
> +}
> +
> +static int pageattr_pmd_entry(pmd_t *pmd, unsigned long addr,
> +			      unsigned long next, struct mm_walk *walk)
> +{
> +	pmd_t val = pmdp_get(pmd);
> +
> +	if (pmd_leaf(val)) {
> +		if (WARN_ON_ONCE((next - addr) != PMD_SIZE))
> +			return -EINVAL;
> +		val = __pmd(set_pageattr_masks(pmd_val(val), walk));
> +		set_pmd(pmd, val);
> +		walk->action = ACTION_CONTINUE;
> +	}
> +
> +	return 0;
> +}
> +
> +static int pageattr_pte_entry(pte_t *pte, unsigned long addr,
> +			      unsigned long next, struct mm_walk *walk)
> +{
> +	pte_t val = __ptep_get(pte);
> +
> +	val = __pte(set_pageattr_masks(pte_val(val), walk));
> +	__set_pte(pte, val);
> +
> +	return 0;
> +}
> +
> +static const struct mm_walk_ops pageattr_ops = {
> +	.pgd_entry	= pageattr_pgd_entry,
> +	.p4d_entry	= pageattr_p4d_entry,
> +	.pud_entry	= pageattr_pud_entry,
> +	.pmd_entry	= pageattr_pmd_entry,
> +	.pte_entry	= pageattr_pte_entry,
> +};
> +
>   bool rodata_full __ro_after_init = IS_ENABLED(CONFIG_RODATA_FULL_DEFAULT_ENABLED);
>   
>   bool can_set_direct_map(void)
> @@ -37,33 +131,35 @@ bool can_set_direct_map(void)
>   		arm64_kfence_can_set_direct_map() || is_realm_world();
>   }
>   
> -static int change_page_range(pte_t *ptep, unsigned long addr, void *data)
> +static int update_range_prot(unsigned long start, unsigned long size,
> +			     pgprot_t set_mask, pgprot_t clear_mask)
>   {
> -	struct page_change_data *cdata = data;
> -	pte_t pte = __ptep_get(ptep);
> +	struct page_change_data data;
> +	int ret;
>   
> -	pte = clear_pte_bit(pte, cdata->clear_mask);
> -	pte = set_pte_bit(pte, cdata->set_mask);
> +	data.set_mask = set_mask;
> +	data.clear_mask = clear_mask;
>   
> -	__set_pte(ptep, pte);
> -	return 0;
> +	arch_enter_lazy_mmu_mode();
> +
> +	/*
> +	 * The caller must ensure that the range we are operating on does not
> +	 * partially overlap a block mapping, or a cont mapping. Any such case
> +	 * must be eliminated by splitting the mapping.
> +	 */
> +	ret = walk_kernel_page_table_range_lockless(start, start + size,
> +						    &pageattr_ops, &data);
> +	arch_leave_lazy_mmu_mode();
> +
> +	return ret;
>   }
>   
> -/*
> - * This function assumes that the range is mapped with PAGE_SIZE pages.
> - */
>   static int __change_memory_common(unsigned long start, unsigned long size,
> -				pgprot_t set_mask, pgprot_t clear_mask)
> +				  pgprot_t set_mask, pgprot_t clear_mask)
>   {
> -	struct page_change_data data;
>   	int ret;
>   
> -	data.set_mask = set_mask;
> -	data.clear_mask = clear_mask;
> -
> -	ret = apply_to_page_range(&init_mm, start, size, change_page_range,
> -					&data);
> -
> +	ret = update_range_prot(start, size, set_mask, clear_mask);
>   	/*
>   	 * If the memory is being made valid without changing any other bits
>   	 * then a TLBI isn't required as a non-valid entry cannot be cached in
> @@ -71,6 +167,7 @@ static int __change_memory_common(unsigned long start, unsigned long size,
>   	 */
>   	if (pgprot_val(set_mask) != PTE_VALID || pgprot_val(clear_mask))
>   		flush_tlb_kernel_range(start, start + size);
> +
>   	return ret;
>   }
>   
> @@ -174,32 +271,26 @@ int set_memory_valid(unsigned long addr, int numpages, int enable)
>   
>   int set_direct_map_invalid_noflush(struct page *page)
>   {
> -	struct page_change_data data = {
> -		.set_mask = __pgprot(0),
> -		.clear_mask = __pgprot(PTE_VALID),
> -	};
> +	pgprot_t clear_mask = __pgprot(PTE_VALID);
> +	pgprot_t set_mask = __pgprot(0);
>   
>   	if (!can_set_direct_map())
>   		return 0;
>   
> -	return apply_to_page_range(&init_mm,
> -				   (unsigned long)page_address(page),
> -				   PAGE_SIZE, change_page_range, &data);
> +	return update_range_prot((unsigned long)page_address(page),
> +				 PAGE_SIZE, set_mask, clear_mask);
>   }
>   
>   int set_direct_map_default_noflush(struct page *page)
>   {
> -	struct page_change_data data = {
> -		.set_mask = __pgprot(PTE_VALID | PTE_WRITE),
> -		.clear_mask = __pgprot(PTE_RDONLY),
> -	};
> +	pgprot_t set_mask = __pgprot(PTE_VALID | PTE_WRITE);
> +	pgprot_t clear_mask = __pgprot(PTE_RDONLY);
>   
>   	if (!can_set_direct_map())
>   		return 0;
>   
> -	return apply_to_page_range(&init_mm,
> -				   (unsigned long)page_address(page),
> -				   PAGE_SIZE, change_page_range, &data);
> +	return update_range_prot((unsigned long)page_address(page),
> +				 PAGE_SIZE, set_mask, clear_mask);
>   }
>   
>   static int __set_memory_enc_dec(unsigned long addr,
> diff --git a/include/linux/pagewalk.h b/include/linux/pagewalk.h
> index 682472c15495..8212e8f2d2d5 100644
> --- a/include/linux/pagewalk.h
> +++ b/include/linux/pagewalk.h
> @@ -134,6 +134,9 @@ int walk_page_range(struct mm_struct *mm, unsigned long start,
>   int walk_kernel_page_table_range(unsigned long start,
>   		unsigned long end, const struct mm_walk_ops *ops,
>   		pgd_t *pgd, void *private);
> +int walk_kernel_page_table_range_lockless(unsigned long start,
> +		unsigned long end, const struct mm_walk_ops *ops,
> +		void *private);
>   int walk_page_range_vma(struct vm_area_struct *vma, unsigned long start,
>   			unsigned long end, const struct mm_walk_ops *ops,
>   			void *private);
> diff --git a/mm/pagewalk.c b/mm/pagewalk.c
> index 648038247a8d..18a675ab87cf 100644
> --- a/mm/pagewalk.c
> +++ b/mm/pagewalk.c
> @@ -633,6 +633,30 @@ int walk_kernel_page_table_range(unsigned long start, unsigned long end,
>   	return walk_pgd_range(start, end, &walk);
>   }
>   
> +/*
> + * Use this function to walk the kernel page tables locklessly. It should be
> + * guaranteed that the caller has exclusive access over the range they are
> + * operating on - that there should be no concurrent access, for example,
> + * changing permissions for vmalloc objects.
> + */
> +int walk_kernel_page_table_range_lockless(unsigned long start, unsigned long end,
> +		const struct mm_walk_ops *ops, void *private)
> +{
> +	struct mm_walk walk = {
> +		.ops		= ops,
> +		.mm		= &init_mm,
> +		.private	= private,
> +		.no_vma		= true
> +	};
> +
> +	if (start >= end)
> +		return -EINVAL;
> +	if (!check_ops_valid(ops))
> +		return -EINVAL;
> +
> +	return walk_pgd_range(start, end, &walk);
> +}
> +
>   /**
>    * walk_page_range_debug - walk a range of pagetables not backed by a vma
>    * @mm:		mm_struct representing the target process of page table walk

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