Message-ID: <cd1c9dd0-dc1a-497c-b78f-d6c6dde898d9@arm.com>
Date: Fri, 2 Jan 2026 11:03:52 +0000
From: Ryan Roberts <ryan.roberts@....com>
To: Yeoreum Yun <yeoreum.yun@....com>, catalin.marinas@....com,
 will@...nel.org, akpm@...ux-foundation.org, david@...nel.org,
 kevin.brodsky@....com, quic_zhenhuah@...cinc.com, dev.jain@....com,
 yang@...amperecomputing.com, chaitanyas.prakash@....com,
 bigeasy@...utronix.de, clrkwllms@...nel.org, rostedt@...dmis.org,
 lorenzo.stoakes@...cle.com, ardb@...nel.org, jackmanb@...gle.com,
 vbabka@...e.cz, mhocko@...e.com
Cc: linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
 linux-rt-devel@...ts.linux.dev
Subject: Re: [PATCH v3 1/2] arm64: mmu: avoid allocating pages while splitting
 the linear mapping

On 18/12/2025 19:47, Yeoreum Yun wrote:
> linear_map_split_to_ptes() currently allocates page tables while
> splitting the linear mapping into PTEs under stop_machine() using GFP_ATOMIC.
> 
> This is fine for non-PREEMPT_RT configurations.
> However, it becomes problematic on PREEMPT_RT, because
> generic memory allocation/free APIs (e.g. pgtable_alloc(), __get_free_pages(), etc.)
> cannot be called from a non-preemptible context, except for the _nolock() variants.
> This is because generic memory allocation/free paths are sleepable,
> as they rely on spin_lock(), which becomes sleepable on PREEMPT_RT.
> 
> In other words, even calling pgtable_alloc() with GFP_ATOMIC is not permitted
> in __linear_map_split_to_pte() when it is executed by the stopper thread,
>  where preemption is disabled on PREEMPT_RT.
> 
> To address this, the required number of page tables is first collected
> and preallocated, and the preallocated page tables are then used
> when splitting the linear mapping in __linear_map_split_to_pte().
> 
> Fixes: 3df6979d222b ("arm64: mm: split linear mapping if BBML2 unsupported on secondary CPUs")
> Signed-off-by: Yeoreum Yun <yeoreum.yun@....com>
> ---
>  arch/arm64/mm/mmu.c | 232 +++++++++++++++++++++++++++++++++++---------
>  1 file changed, 184 insertions(+), 48 deletions(-)
> 
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index 9ae7ce00a7ef..96a9fa505e71 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -527,18 +527,15 @@ static void early_create_pgd_mapping(pgd_t *pgdir, phys_addr_t phys,
>  		panic("Failed to create page tables\n");
>  }
>  
> -static phys_addr_t __pgd_pgtable_alloc(struct mm_struct *mm, gfp_t gfp,
> -				       enum pgtable_type pgtable_type)
> -{
> -	/* Page is zeroed by init_clear_pgtable() so don't duplicate effort. */
> -	struct ptdesc *ptdesc = pagetable_alloc(gfp & ~__GFP_ZERO, 0);
> -	phys_addr_t pa;
> -
> -	if (!ptdesc)
> -		return INVALID_PHYS_ADDR;
> -
> -	pa = page_to_phys(ptdesc_page(ptdesc));
> +static struct ptdesc **split_pgtables;
> +static int split_pgtables_order;
> +static unsigned long split_pgtables_count;
> +static unsigned long split_pgtables_idx;
>  
> +static __always_inline void __pgd_pgtable_init(struct mm_struct *mm,
> +					       struct ptdesc *ptdesc,
> +					       enum pgtable_type pgtable_type)
> +{
>  	switch (pgtable_type) {
>  	case TABLE_PTE:
>  		BUG_ON(!pagetable_pte_ctor(mm, ptdesc));
> @@ -554,19 +551,28 @@ static phys_addr_t __pgd_pgtable_alloc(struct mm_struct *mm, gfp_t gfp,
>  		break;
>  	}
>  

nit: no need for this empty line

> -	return pa;
>  }
>  
> -static phys_addr_t
> -pgd_pgtable_alloc_init_mm_gfp(enum pgtable_type pgtable_type, gfp_t gfp)
> +static phys_addr_t __pgd_pgtable_alloc(struct mm_struct *mm, gfp_t gfp,

nit: all remaining callers pass gfp=GFP_PGTABLE_KERNEL so you could remove the
param?

> +				       enum pgtable_type pgtable_type)
>  {
> -	return __pgd_pgtable_alloc(&init_mm, gfp, pgtable_type);
> +	/* Page is zeroed by init_clear_pgtable() so don't duplicate effort. */
> +	struct ptdesc *ptdesc = pagetable_alloc(gfp & ~__GFP_ZERO, 0);
> +
> +	if (!ptdesc)
> +		return INVALID_PHYS_ADDR;
> +
> +	__pgd_pgtable_init(mm, ptdesc, pgtable_type);
> +
> +	return page_to_phys(ptdesc_page(ptdesc));
>  }
>  
> +typedef phys_addr_t (split_pgtable_alloc_fn)(enum pgtable_type);

This type is used more generally than just for splitting. Perhaps simply call it
"pgtable_alloc_fn"?

We also pass this type around in __create_pgd_mapping() and friends; perhaps we
should have a preparatory patch to define this type and consistently use it
everywhere instead of passing around "phys_addr_t (*pgtable_alloc)(enum
pgtable_type)"?

> +
>  static phys_addr_t __maybe_unused

This is no longer __maybe_unused; you can drop the decorator.

>  pgd_pgtable_alloc_init_mm(enum pgtable_type pgtable_type)
>  {
> -	return pgd_pgtable_alloc_init_mm_gfp(pgtable_type, GFP_PGTABLE_KERNEL);
> +	return __pgd_pgtable_alloc(&init_mm, GFP_PGTABLE_KERNEL, pgtable_type);
>  }
>  
>  static phys_addr_t
> @@ -575,6 +581,23 @@ pgd_pgtable_alloc_special_mm(enum pgtable_type pgtable_type)
>  	return  __pgd_pgtable_alloc(NULL, GFP_PGTABLE_KERNEL, pgtable_type);
>  }
>  
> +static phys_addr_t
> +pgd_pgtable_get_preallocated(enum pgtable_type pgtable_type)
> +{
> +	struct ptdesc *ptdesc;
> +
> +	if (WARN_ON(split_pgtables_idx >= split_pgtables_count))
> +		return INVALID_PHYS_ADDR;
> +
> +	ptdesc = split_pgtables[split_pgtables_idx++];
> +	if (!ptdesc)
> +		return INVALID_PHYS_ADDR;
> +
> +	__pgd_pgtable_init(&init_mm, ptdesc, pgtable_type);
> +
> +	return page_to_phys(ptdesc_page(ptdesc));
> +}
> +
>  static void split_contpte(pte_t *ptep)
>  {
>  	int i;
> @@ -584,7 +607,9 @@ static void split_contpte(pte_t *ptep)
>  		__set_pte(ptep, pte_mknoncont(__ptep_get(ptep)));
>  }
>  
> -static int split_pmd(pmd_t *pmdp, pmd_t pmd, gfp_t gfp, bool to_cont)
> +static int split_pmd(pmd_t *pmdp, pmd_t pmd,
> +		     split_pgtable_alloc_fn *pgtable_alloc_fn,

nit: I believe the * has no effect when passing function pointers and the usual
convention in Linux is to not use the *. Existing functions are also calling it
"pgtable_alloc" so perhaps this is a bit more consistent:

pgtable_alloc_fn pgtable_alloc

(same nitty comment for all uses below :) )

> +		     bool to_cont)
>  {
>  	pmdval_t tableprot = PMD_TYPE_TABLE | PMD_TABLE_UXN | PMD_TABLE_AF;
>  	unsigned long pfn = pmd_pfn(pmd);
> @@ -593,7 +618,7 @@ static int split_pmd(pmd_t *pmdp, pmd_t pmd, gfp_t gfp, bool to_cont)
>  	pte_t *ptep;
>  	int i;
>  
> -	pte_phys = pgd_pgtable_alloc_init_mm_gfp(TABLE_PTE, gfp);
> +	pte_phys = pgtable_alloc_fn(TABLE_PTE);
>  	if (pte_phys == INVALID_PHYS_ADDR)
>  		return -ENOMEM;
>  	ptep = (pte_t *)phys_to_virt(pte_phys);
> @@ -628,7 +653,9 @@ static void split_contpmd(pmd_t *pmdp)
>  		set_pmd(pmdp, pmd_mknoncont(pmdp_get(pmdp)));
>  }
>  
> -static int split_pud(pud_t *pudp, pud_t pud, gfp_t gfp, bool to_cont)
> +static int split_pud(pud_t *pudp, pud_t pud,
> +		     split_pgtable_alloc_fn *pgtable_alloc_fn,
> +		     bool to_cont)
>  {
>  	pudval_t tableprot = PUD_TYPE_TABLE | PUD_TABLE_UXN | PUD_TABLE_AF;
>  	unsigned int step = PMD_SIZE >> PAGE_SHIFT;
> @@ -638,7 +665,7 @@ static int split_pud(pud_t *pudp, pud_t pud, gfp_t gfp, bool to_cont)
>  	pmd_t *pmdp;
>  	int i;
>  
> -	pmd_phys = pgd_pgtable_alloc_init_mm_gfp(TABLE_PMD, gfp);
> +	pmd_phys = pgtable_alloc_fn(TABLE_PMD);
>  	if (pmd_phys == INVALID_PHYS_ADDR)
>  		return -ENOMEM;
>  	pmdp = (pmd_t *)phys_to_virt(pmd_phys);
> @@ -707,7 +734,7 @@ static int split_kernel_leaf_mapping_locked(unsigned long addr)
>  	if (!pud_present(pud))
>  		goto out;
>  	if (pud_leaf(pud)) {
> -		ret = split_pud(pudp, pud, GFP_PGTABLE_KERNEL, true);
> +		ret = split_pud(pudp, pud, pgd_pgtable_alloc_init_mm, true);
>  		if (ret)
>  			goto out;
>  	}
> @@ -732,7 +759,7 @@ static int split_kernel_leaf_mapping_locked(unsigned long addr)
>  		 */
>  		if (ALIGN_DOWN(addr, PMD_SIZE) == addr)
>  			goto out;
> -		ret = split_pmd(pmdp, pmd, GFP_PGTABLE_KERNEL, true);
> +		ret = split_pmd(pmdp, pmd, pgd_pgtable_alloc_init_mm, true);
>  		if (ret)
>  			goto out;
>  	}
> @@ -831,34 +858,35 @@ int split_kernel_leaf_mapping(unsigned long start, unsigned long end)
>  static int split_to_ptes_pud_entry(pud_t *pudp, unsigned long addr,
>  				   unsigned long next, struct mm_walk *walk)
>  {
> -	gfp_t gfp = *(gfp_t *)walk->private;
> +	split_pgtable_alloc_fn *pgtable_alloc_fn = walk->private;
>  	pud_t pud = pudp_get(pudp);
> -	int ret = 0;
>  
> -	if (pud_leaf(pud))
> -		ret = split_pud(pudp, pud, gfp, false);
> +	if (!pud_leaf(pud))
> +		return 0;
>  
> -	return ret;
> +	return split_pud(pudp, pud, pgtable_alloc_fn, false);

why are you changing the layout of this function? Seems like unneccessary churn.
Just pass pgtable_alloc to split_pud() instead of gfp.

>  }
>  
>  static int split_to_ptes_pmd_entry(pmd_t *pmdp, unsigned long addr,
>  				   unsigned long next, struct mm_walk *walk)
>  {
> -	gfp_t gfp = *(gfp_t *)walk->private;
> +	split_pgtable_alloc_fn *pgtable_alloc_fn = walk->private;
>  	pmd_t pmd = pmdp_get(pmdp);
> -	int ret = 0;
> +	int ret;
>  
> -	if (pmd_leaf(pmd)) {
> -		if (pmd_cont(pmd))
> -			split_contpmd(pmdp);
> -		ret = split_pmd(pmdp, pmd, gfp, false);
> +	if (!pmd_leaf(pmd))
> +		return 0;
>  
> -		/*
> -		 * We have split the pmd directly to ptes so there is no need to
> -		 * visit each pte to check if they are contpte.
> -		 */
> -		walk->action = ACTION_CONTINUE;
> -	}
> +	if (pmd_cont(pmd))
> +		split_contpmd(pmdp);
> +
> +	ret = split_pmd(pmdp, pmd, pgtable_alloc_fn, false);
> +
> +	/*
> +	 * We have split the pmd directly to ptes so there is no need to
> +	 * visit each pte to check if they are contpte.
> +	 */
> +	walk->action = ACTION_CONTINUE;

Same comment; no need to change the layout of the function.

>  
>  	return ret;
>  }
> @@ -880,13 +908,15 @@ static const struct mm_walk_ops split_to_ptes_ops = {
>  	.pte_entry	= split_to_ptes_pte_entry,
>  };
>  
> -static int range_split_to_ptes(unsigned long start, unsigned long end, gfp_t gfp)
> +static int range_split_to_ptes(unsigned long start, unsigned long end,
> +			       split_pgtable_alloc_fn *pgtable_alloc_fn)
>  {
>  	int ret;
>  
>  	arch_enter_lazy_mmu_mode();
>  	ret = walk_kernel_page_table_range_lockless(start, end,
> -					&split_to_ptes_ops, NULL, &gfp);
> +						    &split_to_ptes_ops, NULL,
> +						    pgtable_alloc_fn);
>  	arch_leave_lazy_mmu_mode();
>  
>  	return ret;
> @@ -903,6 +933,105 @@ static void __init init_idmap_kpti_bbml2_flag(void)
>  	smp_mb();
>  }
>  
> +static int __init
> +collect_to_split_pud_entry(pud_t *pudp, unsigned long addr,
> +			   unsigned long next, struct mm_walk *walk)
> +{
> +	pud_t pud = pudp_get(pudp);
> +
> +	if (pud_leaf(pud))
> +		split_pgtables_count += 1 + PTRS_PER_PMD;

I think you probably want:

	walk->action = ACTION_CONTINUE;

Likely you will end up with the same behaviour regardless. But you should at
least we consistent with collect_to_split_pmd_entry().

> +
> +	return 0;
> +}
> +
> +static int __init
> +collect_to_split_pmd_entry(pmd_t *pmdp, unsigned long addr,
> +			   unsigned long next, struct mm_walk *walk)
> +{
> +	pmd_t pmd = pmdp_get(pmdp);
> +
> +	if (pmd_leaf(pmd))
> +		split_pgtables_count++;
> +
> +	walk->action = ACTION_CONTINUE;
> +
> +	return 0;
> +}
> +
> +static void __init linear_map_free_split_pgtables(void)
> +{
> +	int i;
> +
> +	if (!split_pgtables_count || !split_pgtables)
> +		goto skip_free;
> +
> +	for (i = split_pgtables_idx; i < split_pgtables_count; i++) {
> +		if (split_pgtables[i])
> +			pagetable_free(split_pgtables[i]);
> +	}
> +
> +	free_pages((unsigned long)split_pgtables, split_pgtables_order);
> +
> +skip_free:
> +	split_pgtables = NULL;
> +	split_pgtables_count = 0;
> +	split_pgtables_idx = 0;
> +	split_pgtables_order = 0;
> +}
> +
> +static int __init linear_map_prealloc_split_pgtables(void)
> +{
> +	int ret, i;
> +	unsigned long lstart = _PAGE_OFFSET(vabits_actual);
> +	unsigned long lend = PAGE_END;
> +	unsigned long kstart = (unsigned long)lm_alias(_stext);
> +	unsigned long kend = (unsigned long)lm_alias(__init_begin);
> +
> +	const struct mm_walk_ops collect_to_split_ops = {
> +		.pud_entry	= collect_to_split_pud_entry,
> +		.pmd_entry	= collect_to_split_pmd_entry
> +	};
> +
> +	split_pgtables_idx = 0;
> +	split_pgtables_count = 0;
> +
> +	ret = walk_kernel_page_table_range_lockless(lstart, kstart,
> +						    &collect_to_split_ops,
> +						    NULL, NULL);
> +	if (!ret)
> +		ret = walk_kernel_page_table_range_lockless(kend, lend,
> +							    &collect_to_split_ops,
> +							    NULL, NULL);
> +	if (ret || !split_pgtables_count)
> +		goto error;
> +
> +	ret = -ENOMEM;
> +
> +	split_pgtables_order =
> +		order_base_2(PAGE_ALIGN(split_pgtables_count *
> +					sizeof(struct ptdesc *)) >> PAGE_SHIFT);

Wouldn't this be simpler?

	split_pgtables_order = get_order(split_pgtables_count *
					 sizeof(struct ptdesc *));

> +
> +	split_pgtables = (struct ptdesc **) __get_free_pages(GFP_KERNEL | __GFP_ZERO,
> +							     split_pgtables_order);

Do you really need the cast? (I'm not sure).

> +	if (!split_pgtables)
> +		goto error;
> +
> +	for (i = 0; i < split_pgtables_count; i++) {
> +		split_pgtables[i] = pagetable_alloc(GFP_KERNEL, 0);

For consistency with other code, perhaps spell it out?:

	/* Page is zeroed by init_clear_pgtable() so don't duplicate effort. */
	gfp_t gfp = GFP_PGTABLE_KERNEL & ~__GFP_ZERO;

	for (i = 0; i < split_pgtables_count; i++) {
		split_pgtables[i] = pagetable_alloc(gfp, 0);

> +		if (!split_pgtables[i])
> +			goto error;
> +	}
> +
> +	ret = 0;
> +
> +error:
> +	if (ret)
> +		linear_map_free_split_pgtables();
> +
> +	return ret;
> +}

I wonder if there is value in generalizing this a bit to separate out the
determination of the number of pages from the actual pre-allocation and free
functions? Then you have a reusable pre-allocation function that you could also
use for the KPTI case instead of having it have yet another private pre-allocator?

> +
>  static int __init linear_map_split_to_ptes(void *__unused)
>  {
>  	/*
> @@ -928,9 +1057,9 @@ static int __init linear_map_split_to_ptes(void *__unused)
>  		 * PTE. The kernel alias remains static throughout runtime so
>  		 * can continue to be safely mapped with large mappings.
>  		 */
> -		ret = range_split_to_ptes(lstart, kstart, GFP_ATOMIC);
> +		ret = range_split_to_ptes(lstart, kstart, pgd_pgtable_get_preallocated);
>  		if (!ret)
> -			ret = range_split_to_ptes(kend, lend, GFP_ATOMIC);
> +			ret = range_split_to_ptes(kend, lend, pgd_pgtable_get_preallocated);
>  		if (ret)
>  			panic("Failed to split linear map\n");
>  		flush_tlb_kernel_range(lstart, lend);
> @@ -963,10 +1092,16 @@ static int __init linear_map_split_to_ptes(void *__unused)
>  
>  void __init linear_map_maybe_split_to_ptes(void)
>  {
> -	if (linear_map_requires_bbml2 && !system_supports_bbml2_noabort()) {
> -		init_idmap_kpti_bbml2_flag();
> -		stop_machine(linear_map_split_to_ptes, NULL, cpu_online_mask);
> -	}
> +	if (!linear_map_requires_bbml2 || system_supports_bbml2_noabort())
> +		return;
> +
> +	if (linear_map_prealloc_split_pgtables())
> +		panic("Failed to split linear map\n");
> +
> +	init_idmap_kpti_bbml2_flag();
> +	stop_machine(linear_map_split_to_ptes, NULL, cpu_online_mask);
> +
> +	linear_map_free_split_pgtables();
>  }
>  
>  /*
> @@ -1088,6 +1223,7 @@ bool arch_kfence_init_pool(void)
>  	unsigned long end = start + KFENCE_POOL_SIZE;
>  	int ret;
>  
> +

nit: Remove extra empty line.

This is looking much cleaner now; nearly there!

Thanks,
Ryan


>  	/* Exit early if we know the linear map is already pte-mapped. */
>  	if (!split_leaf_mapping_possible())
>  		return true;
> @@ -1097,7 +1233,7 @@ bool arch_kfence_init_pool(void)
>  		return true;
>  
>  	mutex_lock(&pgtable_split_lock);
> -	ret = range_split_to_ptes(start, end, GFP_PGTABLE_KERNEL);
> +	ret = range_split_to_ptes(start, end, pgd_pgtable_alloc_init_mm);
>  	mutex_unlock(&pgtable_split_lock);
>  
>  	/*

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