[PATCH v4] arm64: Enable vmalloc-huge with ptdump

[Dev Jain]

On 04/07/25 11:28 am, Anshuman Khandual wrote:
>
> On 26/06/25 10:55 AM, 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
> Please keep this paragraph separate explaining the current scenario. It
> might be worth adding bit more details such CONFIG_PTDUMP_DEBUGFS config
> dependency in vmalloc()'s arch call backs arch_vmap_pud|pmd_supported().
>   > 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
> Please keep this paragraph separate explaining what is the benefit of
> enabling ptdump with huge vmalloc() mappings. This will establish the
> required motivation to change the current scenario.
>
>> 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.
> Different page tables ? OR different areas in same page table i.e init_mm.

Yes different areas, will clear that.

>
>> For pud_free_pmd_page(), we isolate the PMD table to avoid taking the lock
>> 512 times again via pmd_free_pte_page().
> This talks about subsequent optimization without establishing the base
> solution first.
>   >
>> 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.
> Above write up is convoluted and not very clear. Please rewrite the solution
> description while avoiding too much implementation and code file details that
> can be derived from the patch itself.

Okay.

>
>> 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.
> Please provide some more details about test_vmallo.sh which seems to be a
> custom script.

What do you mean by "custom script"? It is part of selftests/mm and explaining
what a selftest is doing in the patch description, when the name makes it pretty
clear (test_vmalloc) seems redundant.

>
>> Reviewed-by: Ryan Roberts <ryan.roberts at arm.com>
>> Signed-off-by: Dev Jain <dev.jain at arm.com>
>> ---
>> v3->v4:
>>   - Lock-unlock immediately
>>   - Simplify includes
>>
>> 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()
>>
>>   arch/arm64/include/asm/ptdump.h |  2 ++
>>   arch/arm64/mm/mmu.c             | 44 ++++++++++++++++++++++++++++++---
>>   arch/arm64/mm/ptdump.c          |  5 +++-
>>   3 files changed, 46 insertions(+), 5 deletions(-)
>>
>> diff --git a/arch/arm64/include/asm/ptdump.h b/arch/arm64/include/asm/ptdump.h
>> index fded5358641f..5b331f2a7be1 100644
>> --- a/arch/arm64/include/asm/ptdump.h
>> +++ b/arch/arm64/include/asm/ptdump.h
>> @@ -7,6 +7,8 @@
>>   
>>   #include <linux/ptdump.h>
>>   
>> +DECLARE_STATIC_KEY_FALSE(ptdump_lock_key);
>> +
>>   #ifdef CONFIG_PTDUMP
>>   
>>   #include <linux/mm_types.h>
>> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
>> index 00ab1d648db6..9d3be249047c 100644
>> --- a/arch/arm64/mm/mmu.c
>> +++ b/arch/arm64/mm/mmu.c
>> @@ -46,6 +46,8 @@
>>   #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);
>> +ptdump_lock_key sounds too generic even though this locking
> requirement is currently arm64 platform specific. Otherwise
> this might just appear as a locking from the generic ptdump
> Could this be renamed as arm64_ptdump_lock_key instead ?

Sure.

>
>>   enum pgtable_type {
>>   	TABLE_PTE,
>>   	TABLE_PMD,
>> @@ -1267,7 +1269,7 @@ 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)
>>   {
> 'lock' argument needs explanation here in a comment.

Sure.

>
>>   	pte_t *table;
>>   	pmd_t pmd;
>> @@ -1279,13 +1281,24 @@ 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);
> Or here ?

I'll do it above the function declaration, the reader then won't
have to look for the comment.

>
>> +	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)
>>   {
>>   	pmd_t *table;
>> @@ -1301,16 +1314,39 @@ 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)) {
> 		/* case 1 - ptdump comes first
> 		 *
> 		 * mmap_read_lock() here will wait on mmap_write_lock()
> 	 	 * taken in generic ptdump until it has been released.
> 		 */
>
>> +		mmap_read_lock(&init_mm);
>> +		mmap_read_unlock(&init_mm);
>> +	}
> 	} else {
> 		/* case 2 - pud_free_pmd_page() comes first
> 		 *
> 		 * pud_clear() and __flush_tlb_kernel_pgtable() are
> 		 * sufficient for ptdump to observe an empty PUD.
> 		 */	
> 	}> +

Thanks.

>>   	pmdp = table;
>>   	next = addr;
>>   	end = addr + PUD_SIZE;
>>   	do {
>>   		if (pmd_present(pmdp_get(pmdp)))
>> -			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..41c9ea61813b 100644
>> --- a/arch/arm64/mm/ptdump.c
>> +++ b/arch/arm64/mm/ptdump.c
>> @@ -25,7 +25,6 @@
>>   #include <asm/pgtable-hwdef.h>
>>   #include <asm/ptdump.h>
>>   
>> -
> Stray change. Please drop it.

Thanks.

>
>>   #define pt_dump_seq_printf(m, fmt, args...)	\
>>   ({						\
>>   	if (m)					\
>> @@ -311,7 +310,9 @@ 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 +354,9 @@ 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",Although tempting but I guess encapsulating above ptdump_walk_pgd()
> locking sequence inside a new helper arm64_ptdump_walk_pgd() while
> also explaining this locking might be worth ?

Okay.

>
> PUD/PMD vmalloc mappings need to be enabled via these arch callback
> changes via dropping the current CONFIG_PTDUMP_DEBUGFS dependency.
>
> #define arch_vmap_pud_supported arch_vmap_pud_supported
> static inline bool arch_vmap_pud_supported(pgprot_t prot)
> {
> 	return pud_sect_supported()
> }
>
> #define arch_vmap_pmd_supported arch_vmap_pmd_supported
> static inline bool arch_vmap_pmd_supported(pgprot_t prot)
> {
> 	return return.
> }

Thanks for the spot.