Re: [PATCH v2 2/3] LoongArch: Add barrier between set_pte and memory access

[maobibo <maobibo@loongson.cn>]

On 2024/10/15 下午8:27, Huacai Chen wrote:
> On Tue, Oct 15, 2024 at 10:54 AM maobibo <maobibo@loongson.cn> wrote:
>>
>>
>>
>> On 2024/10/14 下午2:31, Huacai Chen wrote:
>>> Hi, Bibo,
>>>
>>> On Mon, Oct 14, 2024 at 11:59 AM Bibo Mao <maobibo@loongson.cn> wrote:
>>>>
>>>> It is possible to return a spurious fault if memory is accessed
>>>> right after the pte is set. For user address space, pte is set
>>>> in kernel space and memory is accessed in user space, there is
>>>> long time for synchronization, no barrier needed. However for
>>>> kernel address space, it is possible that memory is accessed
>>>> right after the pte is set.
>>>>
>>>> Here flush_cache_vmap/flush_cache_vmap_early is used for
>>>> synchronization.
>>>>
>>>> Signed-off-by: Bibo Mao <maobibo@loongson.cn>
>>>> ---
>>>>    arch/loongarch/include/asm/cacheflush.h | 14 +++++++++++++-
>>>>    1 file changed, 13 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/arch/loongarch/include/asm/cacheflush.h b/arch/loongarch/include/asm/cacheflush.h
>>>> index f8754d08a31a..53be231319ef 100644
>>>> --- a/arch/loongarch/include/asm/cacheflush.h
>>>> +++ b/arch/loongarch/include/asm/cacheflush.h
>>>> @@ -42,12 +42,24 @@ void local_flush_icache_range(unsigned long start, unsigned long end);
>>>>    #define flush_cache_dup_mm(mm)                         do { } while (0)
>>>>    #define flush_cache_range(vma, start, end)             do { } while (0)
>>>>    #define flush_cache_page(vma, vmaddr, pfn)             do { } while (0)
>>>> -#define flush_cache_vmap(start, end)                   do { } while (0)
>>>>    #define flush_cache_vunmap(start, end)                 do { } while (0)
>>>>    #define flush_icache_user_page(vma, page, addr, len)   do { } while (0)
>>>>    #define flush_dcache_mmap_lock(mapping)                        do { } while (0)
>>>>    #define flush_dcache_mmap_unlock(mapping)              do { } while (0)
>>>>
>>>> +/*
>>>> + * It is possible for a kernel virtual mapping access to return a spurious
>>>> + * fault if it's accessed right after the pte is set. The page fault handler
>>>> + * does not expect this type of fault. flush_cache_vmap is not exactly the
>>>> + * right place to put this, but it seems to work well enough.
>>>> + */
>>>> +static inline void flush_cache_vmap(unsigned long start, unsigned long end)
>>>> +{
>>>> +       smp_mb();
>>>> +}
>>>> +#define flush_cache_vmap flush_cache_vmap
>>>> +#define flush_cache_vmap_early flush_cache_vmap
>>>   From the history of flush_cache_vmap_early(), It seems only archs with
>>> "virtual cache" (VIVT or VIPT) need this API, so LoongArch can be a
>>> no-op here.
> OK,  flush_cache_vmap_early() also needs smp_mb().
> 
>>
>> Here is usage about flush_cache_vmap_early in file linux/mm/percpu.c,
>> map the page and access it immediately. Do you think it should be noop
>> on LoongArch.
>>
>> rc = __pcpu_map_pages(unit_addr, &pages[unit * unit_pages],
>>                                        unit_pages);
>> if (rc < 0)
>>       panic("failed to map percpu area, err=%d\n", rc);
>>       flush_cache_vmap_early(unit_addr, unit_addr + ai->unit_size);
>>       /* copy static data */
>>       memcpy((void *)unit_addr, __per_cpu_load, ai->static_size);
>> }
>>
>>
>>>
>>> And I still think flush_cache_vunmap() should be a smp_mb(). A
>>> smp_mb() in flush_cache_vmap() prevents subsequent accesses be
>>> reordered before pte_set(), and a smp_mb() in flush_cache_vunmap()
>> smp_mb() in flush_cache_vmap() does not prevent reorder. It is to flush
>> pipeline and let page table walker HW sync with data cache.
>>
>> For the following example.
>>     rb = vmap(pages, nr_meta_pages + 2 * nr_data_pages,
>>                     VM_MAP | VM_USERMAP, PAGE_KERNEL);
>>     if (rb) {
>> <<<<<<<<<<< * the sentence if (rb) can prevent reorder. Otherwise with
>> any API kmalloc/vmap/vmalloc and subsequent memory access, there will be
>> reorder issu. *
>>         kmemleak_not_leak(pages);
>>         rb->pages = pages;
>>         rb->nr_pages = nr_pages;
>>         return rb;
>>     }
>>
>>> prevents preceding accesses be reordered after pte_clear(). This
>> Can you give an example about such usage about flush_cache_vunmap()? and
>> we can continue to talk about it, else it is just guessing.
> Since we cannot reach a consensus, and the flush_cache_* API look very
> strange for this purpose (Yes, I know PowerPC does it like this, but
> ARM64 doesn't). I prefer to still use the ARM64 method which means add
> a dbar in set_pte(). Of course the performance will be a little worse,
> but still better than the old version, and it is more robust.
> 
> I know you are very busy, so if you have no time you don't need to
> send V3, I can just do a small modification on the 3rd patch.
No, I will send V3 by myself. And I will drop the this patch in this 
patchset since by actual test vmalloc_test works well even without this
patch on 3C5000 Dual-way, also weak function kernel_pte_init will be 
replaced with inline function rebased on
 
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-define-general-function-pxd_init.patch

I dislike the copy-paste method without further understanding :(, 
although I also copy and paste code, but as least I try best to 
understand it.

Regards
Bibo Mao
> 
> 
> Huacai
> 
>>
>> Regards
>> Bibo Mao
>>> potential problem may not be seen from experiment, but it is needed in
>>> theory.
>>>
>>> Huacai
>>>
>>>> +
>>>>    #define cache_op(op, addr)                                             \
>>>>           __asm__ __volatile__(                                           \
>>>>           "       cacop   %0, %1                                  \n"     \
>>>> --
>>>> 2.39.3
>>>>
>>>>
>>
>>