Re: [PATCH v3] arm64: Enable vmalloc-huge with ptdump

[Dev Jain <dev.jain@arm.com>]

On 25/06/25 4:05 pm, Ryan Roberts wrote:
> On 16/06/2025 11:33, Dev Jain wrote:
>> arm64 disables vmalloc-huge when kernel page table dumping is enabled,
>> because an intermediate table may be removed, potentially causing the
>> ptdump code to dereference an invalid address. We want to be able to
>> analyze block vs page mappings for kernel mappings with ptdump, so to
>> enable vmalloc-huge with ptdump, synchronize between page table removal in
>> pmd_free_pte_page()/pud_free_pmd_page() and ptdump pagetable walking. We
>> use mmap_read_lock and not write lock because we don't need to synchronize
>> between two different vm_structs; two vmalloc objects running this same
>> code path will point to different page tables, hence there is no race.
>>
>> For pud_free_pmd_page(), we isolate the PMD table to avoid taking the lock
>> 512 times again via pmd_free_pte_page().
>>
>> We implement the locking mechanism using static keys, since the chance
>> of a race is very small. Observe that the synchronization is needed
>> to avoid the following race:
>>
>> CPU1							CPU2
>> 						take reference of PMD table
>> pud_clear()
>> pte_free_kernel()
>> 						walk freed PMD table
>>
>> and similar race between pmd_free_pte_page and ptdump_walk_pgd.
>>
>> Therefore, there are two cases: if ptdump sees the cleared PUD, then
>> we are safe. If not, then the patched-in read and write locks help us
>> avoid the race.
>>
>> To implement the mechanism, we need the static key access from mmu.c and
>> ptdump.c. Note that in case !CONFIG_PTDUMP_DEBUGFS, ptdump.o won't be a
>> target in the Makefile, therefore we cannot initialize the key there, as
>> is being done, for example, in the static key implementation of
>> hugetlb-vmemmap. Therefore, include asm/cpufeature.h, which includes
>> the jump_label mechanism. Declare the key there and define the key to false
>> in mmu.c.
>>
>> No issues were observed with mm-selftests. No issues were observed while
>> parallelly running test_vmalloc.sh and dumping the kernel pagetable through
>> sysfs in a loop.
>>
>> v2->v3:
>>   - Use static key mechanism
>>
>> v1->v2:
>>   - Take lock only when CONFIG_PTDUMP_DEBUGFS is on
>>   - In case of pud_free_pmd_page(), isolate the PMD table to avoid taking
>>     the lock 512 times again via pmd_free_pte_page()
>>
>> Signed-off-by: Dev Jain <dev.jain@arm.com>
>> ---
>>   arch/arm64/include/asm/cpufeature.h |  1 +
>>   arch/arm64/mm/mmu.c                 | 51 ++++++++++++++++++++++++++---
>>   arch/arm64/mm/ptdump.c              |  5 +++
>>   3 files changed, 53 insertions(+), 4 deletions(-)
>>
>> diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
>> index c4326f1cb917..3e386563b587 100644
>> --- a/arch/arm64/include/asm/cpufeature.h
>> +++ b/arch/arm64/include/asm/cpufeature.h
>> @@ -26,6 +26,7 @@
>>   #include <linux/kernel.h>
>>   #include <linux/cpumask.h>
>>   
>> +DECLARE_STATIC_KEY_FALSE(ptdump_lock_key);
> Is this really the correct header file for this declaration? Perhaps it would be
> better in arch/arm64/include/asm/ptdump.h ?

I tried a lot of things; this didn't work. I get the following:

ld: arch/arm64/mm/mmu.o:(__jump_table+0x8): undefined reference to `ptdump_lock_key'
ld: arch/arm64/mm/mmu.o:(__jump_table+0x48): undefined reference to `ptdump_lock_key'

in case of !CONFIG_PTDUMP_DEBUGFS.

>
>>   /*
>>    * CPU feature register tracking
>>    *
>> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
>> index 8fcf59ba39db..e242ba428820 100644
>> --- a/arch/arm64/mm/mmu.c
>> +++ b/arch/arm64/mm/mmu.c
>> @@ -41,11 +41,14 @@
>>   #include <asm/tlbflush.h>
>>   #include <asm/pgalloc.h>
>>   #include <asm/kfence.h>
>> +#include <asm/cpufeature.h>
>>   
>>   #define NO_BLOCK_MAPPINGS	BIT(0)
>>   #define NO_CONT_MAPPINGS	BIT(1)
>>   #define NO_EXEC_MAPPINGS	BIT(2)	/* assumes FEAT_HPDS is not used */
>>   
>> +DEFINE_STATIC_KEY_FALSE(ptdump_lock_key);
>> +
>>   enum pgtable_type {
>>   	TABLE_PTE,
>>   	TABLE_PMD,
>> @@ -1267,8 +1270,9 @@ int pmd_clear_huge(pmd_t *pmdp)
>>   	return 1;
>>   }
>>   
>> -int pmd_free_pte_page(pmd_t *pmdp, unsigned long addr)
>> +static int __pmd_free_pte_page(pmd_t *pmdp, unsigned long addr, bool lock)
>>   {
>> +	bool lock_taken = false;
> As David commented, no need for this.
>
>>   	pte_t *table;
>>   	pmd_t pmd;
>>   
>> @@ -1279,15 +1283,29 @@ int pmd_free_pte_page(pmd_t *pmdp, unsigned long addr)
>>   		return 1;
>>   	}
>>   
>> +	/* See comment in pud_free_pmd_page for static key logic */
>>   	table = pte_offset_kernel(pmdp, addr);
>>   	pmd_clear(pmdp);
>>   	__flush_tlb_kernel_pgtable(addr);
>> +	if (static_branch_unlikely(&ptdump_lock_key) && lock) {
>> +		mmap_read_lock(&init_mm);
>> +		lock_taken = true;
>> +	}
>> +	if (unlikely(lock_taken))
>> +		mmap_read_unlock(&init_mm);
>> +
> As per David's comment this can just be:
>
> if (static_branch_unlikely(&ptdump_lock_key) && lock) {
> 	mmap_read_lock(&init_mm);
> 	mmap_read_unlock(&init_mm);
> }
>
>>   	pte_free_kernel(NULL, table);
>>   	return 1;
>>   }
>>   
>> +int pmd_free_pte_page(pmd_t *pmdp, unsigned long addr)
>> +{
>> +	return __pmd_free_pte_page(pmdp, addr, true);
>> +}
>> +
>>   int pud_free_pmd_page(pud_t *pudp, unsigned long addr)
>>   {
>> +	bool lock_taken = false;
> Same comment.
>
>>   	pmd_t *table;
>>   	pmd_t *pmdp;
>>   	pud_t pud;
>> @@ -1301,15 +1319,40 @@ int pud_free_pmd_page(pud_t *pudp, unsigned long addr)
>>   	}
>>   
>>   	table = pmd_offset(pudp, addr);
>> +	/*
>> +	 * Isolate the PMD table; in case of race with ptdump, this helps
>> +	 * us to avoid taking the lock in __pmd_free_pte_page().
>> +	 *
>> +	 * Static key logic:
>> +	 *
>> +	 * Case 1: If ptdump does static_branch_enable(), and after that we
>> +	 * execute the if block, then this patches in the read lock, ptdump has
>> +	 * the write lock patched in, therefore ptdump will never read from
>> +	 * a potentially freed PMD table.
>> +	 *
>> +	 * Case 2: If the if block starts executing before ptdump's
>> +	 * static_branch_enable(), then no locking synchronization
>> +	 * will be done. However, pud_clear() + the dsb() in
>> +	 * __flush_tlb_kernel_pgtable will ensure that ptdump observes an
>> +	 * empty PUD. Thus, it will never walk over a potentially freed
>> +	 * PMD table.
>> +	 */
>> +	pud_clear(pudp);
>> +	__flush_tlb_kernel_pgtable(addr);
>> +	if (static_branch_unlikely(&ptdump_lock_key)) {
>> +		mmap_read_lock(&init_mm);
>> +		lock_taken = true;
>> +	}
>> +	if (unlikely(lock_taken))
>> +		mmap_read_unlock(&init_mm);
> Same comment.
>
>> +
>>   	pmdp = table;
>>   	next = addr;
>>   	end = addr + PUD_SIZE;
>>   	do {
>> -		pmd_free_pte_page(pmdp, next);
>> +		__pmd_free_pte_page(pmdp, next, false);
>>   	} while (pmdp++, next += PMD_SIZE, next != end);
>>   
>> -	pud_clear(pudp);
>> -	__flush_tlb_kernel_pgtable(addr);
>>   	pmd_free(NULL, table);
>>   	return 1;
>>   }
>> diff --git a/arch/arm64/mm/ptdump.c b/arch/arm64/mm/ptdump.c
>> index 421a5de806c6..f75e12a1d068 100644
>> --- a/arch/arm64/mm/ptdump.c
>> +++ b/arch/arm64/mm/ptdump.c
>> @@ -25,6 +25,7 @@
>>   #include <asm/pgtable-hwdef.h>
>>   #include <asm/ptdump.h>
>>   
>> +#include <asm/cpufeature.h>
>>   
>>   #define pt_dump_seq_printf(m, fmt, args...)	\
>>   ({						\
>> @@ -311,7 +312,9 @@ void ptdump_walk(struct seq_file *s, struct ptdump_info *info)
>>   		}
>>   	};
>>   
>> +	static_branch_enable(&ptdump_lock_key);
>>   	ptdump_walk_pgd(&st.ptdump, info->mm, NULL);
>> +	static_branch_disable(&ptdump_lock_key);
>>   }
>>   
>>   static void __init ptdump_initialize(void)
>> @@ -353,7 +356,9 @@ bool ptdump_check_wx(void)
>>   		}
>>   	};
>>   
>> +	static_branch_enable(&ptdump_lock_key);
>>   	ptdump_walk_pgd(&st.ptdump, &init_mm, NULL);
>> +	static_branch_disable(&ptdump_lock_key);
>>   
>>   	if (st.wx_pages || st.uxn_pages) {
>>   		pr_warn("Checked W+X mappings: FAILED, %lu W+X pages found, %lu non-UXN pages found\n",
>
> With the improvements as suggested, LGTM:
>
> Reviewed-by: Ryan Roberts <ryan.roberts@arm.com>
>
>