Re: [PATCH v1 16/16] arm64/mm: Defer barriers when updating kernel mappings

[Anshuman Khandual <anshuman.khandual@arm.com>]

On 2/10/25 16:42, Ryan Roberts wrote:
> On 10/02/2025 08:03, Anshuman Khandual wrote:
>>
>>
>> On 2/5/25 20:39, Ryan Roberts wrote:
>>> Because the kernel can't tolerate page faults for kernel mappings, when
>>> setting a valid, kernel space pte (or pmd/pud/p4d/pgd), it emits a
>>> dsb(ishst) to ensure that the store to the pgtable is observed by the
>>> table walker immediately. Additionally it emits an isb() to ensure that
>>> any already speculatively determined invalid mapping fault gets
>>> canceled.> 
>>> We can improve the performance of vmalloc operations by batching these
>>> barriers until the end of a set up entry updates. The newly added
>>> arch_update_kernel_mappings_begin() / arch_update_kernel_mappings_end()
>>> provide the required hooks.
>>>
>>> vmalloc improves by up to 30% as a result.
>>>
>>> Two new TIF_ flags are created; TIF_KMAP_UPDATE_ACTIVE tells us if we
>>> are in the batch mode and can therefore defer any barriers until the end
>>> of the batch. TIF_KMAP_UPDATE_PENDING tells us if barriers are queued to
>>> be emited at the end of the batch.
>>
>> Why cannot this be achieved with a single TIF_KMAP_UPDATE_ACTIVE which is
>> set in __begin(), cleared in __end() and saved across a __switch_to().
> 
> So unconditionally emit the barriers in _end(), and emit them in __switch_to()
> if TIF_KMAP_UPDATE_ACTIVE is set?

Right.

> 
> I guess if calling _begin() then you are definitely going to be setting at least
> 1 PTE. So you can definitely emit the barriers unconditionally. I was trying to
> protect against the case where you get pre-empted (potentially multiple times)
> while in the loop. The TIF_KMAP_UPDATE_PENDING flag ensures you only emit the
> barriers when you definitely need to. Without it, you would have to emit on
> every pre-emption even if no more PTEs got set.
> 
> But I suspect this is a premature optimization. Probably it will never occur. So

Agreed.

> I'll simplify as you suggest.
> 
> Thanks,
> Ryan
> 
>>
>>>
>>> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
>>> ---
>>>  arch/arm64/include/asm/pgtable.h     | 65 +++++++++++++++++++---------
>>>  arch/arm64/include/asm/thread_info.h |  2 +
>>>  arch/arm64/kernel/process.c          | 20 +++++++--
>>>  3 files changed, 63 insertions(+), 24 deletions(-)
>>>
>>> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
>>> index ff358d983583..1ee9b9588502 100644
>>> --- a/arch/arm64/include/asm/pgtable.h
>>> +++ b/arch/arm64/include/asm/pgtable.h
>>> @@ -39,6 +39,41 @@
>>>  #include <linux/mm_types.h>
>>>  #include <linux/sched.h>
>>>  #include <linux/page_table_check.h>
>>> +#include <linux/pgtable_modmask.h>
>>> +
>>> +static inline void emit_pte_barriers(void)
>>> +{
>>> +	dsb(ishst);
>>> +	isb();
>>> +}
>>
>> There are many sequence of these two barriers in this particular header,
>> hence probably a good idea to factor this out into a common helper.
>>>> +
>>> +static inline void queue_pte_barriers(void)
>>> +{
>>> +	if (test_thread_flag(TIF_KMAP_UPDATE_ACTIVE)) {
>>> +		if (!test_thread_flag(TIF_KMAP_UPDATE_PENDING))
>>> +			set_thread_flag(TIF_KMAP_UPDATE_PENDING);
>>> +	} else
>>> +		emit_pte_barriers();
>>> +}
>>> +
>>> +#define arch_update_kernel_mappings_begin arch_update_kernel_mappings_begin
>>> +static inline void arch_update_kernel_mappings_begin(unsigned long start,
>>> +						     unsigned long end)
>>> +{
>>> +	set_thread_flag(TIF_KMAP_UPDATE_ACTIVE);
>>> +}
>>> +
>>> +#define arch_update_kernel_mappings_end arch_update_kernel_mappings_end
>>> +static inline void arch_update_kernel_mappings_end(unsigned long start,
>>> +						   unsigned long end,
>>> +						   pgtbl_mod_mask mask)
>>> +{
>>> +	if (test_thread_flag(TIF_KMAP_UPDATE_PENDING))
>>> +		emit_pte_barriers();
>>> +
>>> +	clear_thread_flag(TIF_KMAP_UPDATE_PENDING);
>>> +	clear_thread_flag(TIF_KMAP_UPDATE_ACTIVE);
>>> +}
>>>  
>>>  #ifdef CONFIG_TRANSPARENT_HUGEPAGE
>>>  #define __HAVE_ARCH_FLUSH_PMD_TLB_RANGE
>>> @@ -323,10 +358,8 @@ static inline void __set_pte_complete(pte_t pte)
>>>  	 * Only if the new pte is valid and kernel, otherwise TLB maintenance
>>>  	 * or update_mmu_cache() have the necessary barriers.
>>>  	 */
>>> -	if (pte_valid_not_user(pte)) {
>>> -		dsb(ishst);
>>> -		isb();
>>> -	}
>>> +	if (pte_valid_not_user(pte))
>>> +		queue_pte_barriers();
>>>  }
>>>  
>>>  static inline void __set_pte(pte_t *ptep, pte_t pte)
>>> @@ -791,10 +824,8 @@ static inline void set_pmd(pmd_t *pmdp, pmd_t pmd)
>>>  
>>>  	WRITE_ONCE(*pmdp, pmd);
>>>  
>>> -	if (pmd_valid_not_user(pmd)) {
>>> -		dsb(ishst);
>>> -		isb();
>>> -	}
>>> +	if (pmd_valid_not_user(pmd))
>>> +		queue_pte_barriers();
>>>  }
>>>  
>>>  static inline void pmd_clear(pmd_t *pmdp)
>>> @@ -869,10 +900,8 @@ static inline void set_pud(pud_t *pudp, pud_t pud)
>>>  
>>>  	WRITE_ONCE(*pudp, pud);
>>>  
>>> -	if (pud_valid_not_user(pud)) {
>>> -		dsb(ishst);
>>> -		isb();
>>> -	}
>>> +	if (pud_valid_not_user(pud))
>>> +		queue_pte_barriers();
>>>  }
>>>  
>>>  static inline void pud_clear(pud_t *pudp)
>>> @@ -960,10 +989,8 @@ static inline void set_p4d(p4d_t *p4dp, p4d_t p4d)
>>>  
>>>  	WRITE_ONCE(*p4dp, p4d);
>>>  
>>> -	if (p4d_valid_not_user(p4d)) {
>>> -		dsb(ishst);
>>> -		isb();
>>> -	}
>>> +	if (p4d_valid_not_user(p4d))
>>> +		queue_pte_barriers();
>>>  }
>>>  
>>>  static inline void p4d_clear(p4d_t *p4dp)
>>> @@ -1098,10 +1125,8 @@ static inline void set_pgd(pgd_t *pgdp, pgd_t pgd)
>>>  
>>>  	WRITE_ONCE(*pgdp, pgd);
>>>  
>>> -	if (pgd_valid_not_user(pgd)) {
>>> -		dsb(ishst);
>>> -		isb();
>>> -	}
>>> +	if (pgd_valid_not_user(pgd))
>>> +		queue_pte_barriers();
>>>  }
>>>  
>>>  static inline void pgd_clear(pgd_t *pgdp)
>>> diff --git a/arch/arm64/include/asm/thread_info.h b/arch/arm64/include/asm/thread_info.h
>>> index 1114c1c3300a..382d2121261e 100644
>>> --- a/arch/arm64/include/asm/thread_info.h
>>> +++ b/arch/arm64/include/asm/thread_info.h
>>> @@ -82,6 +82,8 @@ void arch_setup_new_exec(void);
>>>  #define TIF_SME_VL_INHERIT	28	/* Inherit SME vl_onexec across exec */
>>>  #define TIF_KERNEL_FPSTATE	29	/* Task is in a kernel mode FPSIMD section */
>>>  #define TIF_TSC_SIGSEGV		30	/* SIGSEGV on counter-timer access */
>>> +#define TIF_KMAP_UPDATE_ACTIVE	31	/* kernel map update in progress */
>>> +#define TIF_KMAP_UPDATE_PENDING	32	/* kernel map updated with deferred barriers */
>>>  
>>>  #define _TIF_SIGPENDING		(1 << TIF_SIGPENDING)
>>>  #define _TIF_NEED_RESCHED	(1 << TIF_NEED_RESCHED)
>>> diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
>>> index 42faebb7b712..1367ec6407d1 100644
>>> --- a/arch/arm64/kernel/process.c
>>> +++ b/arch/arm64/kernel/process.c
>>> @@ -680,10 +680,10 @@ struct task_struct *__switch_to(struct task_struct *prev,
>>>  	gcs_thread_switch(next);
>>>  
>>>  	/*
>>> -	 * Complete any pending TLB or cache maintenance on this CPU in case
>>> -	 * the thread migrates to a different CPU.
>>> -	 * This full barrier is also required by the membarrier system
>>> -	 * call.
>>> +	 * Complete any pending TLB or cache maintenance on this CPU in case the
>>> +	 * thread migrates to a different CPU. This full barrier is also
>>> +	 * required by the membarrier system call. Additionally it is required
>>> +	 * for TIF_KMAP_UPDATE_PENDING, see below.
>>>  	 */
>>>  	dsb(ish);
>>>  
>>> @@ -696,6 +696,18 @@ struct task_struct *__switch_to(struct task_struct *prev,
>>>  	/* avoid expensive SCTLR_EL1 accesses if no change */
>>>  	if (prev->thread.sctlr_user != next->thread.sctlr_user)
>>>  		update_sctlr_el1(next->thread.sctlr_user);
>>> +	else if (unlikely(test_thread_flag(TIF_KMAP_UPDATE_PENDING))) {
>>> +		/*
>>> +		 * In unlikely event that a kernel map update is on-going when
>>> +		 * preemption occurs, we must emit_pte_barriers() if pending.
>>> +		 * emit_pte_barriers() consists of "dsb(ishst); isb();". The dsb
>>> +		 * is already handled above. The isb() is handled if
>>> +		 * update_sctlr_el1() was called. So only need to emit isb()
>>> +		 * here if it wasn't called.
>>> +		 */
>>> +		isb();
>>> +		clear_thread_flag(TIF_KMAP_UPDATE_PENDING);
>>> +	}
>>>  
>>>  	/* the actual thread switch */
>>>  	last = cpu_switch_to(prev, next);
>