Re: [PATCH 4/6] mm: hugetlb_vmemmap: add missing smp_wmb() before set_pte_at()

[Miaohe Lin]

On 2022/8/19 11:19, Muchun Song wrote:
> 
> 
>> On Aug 18, 2022, at 20:58, Miaohe Lin <linmiaohe@xxxxxxxxxx> wrote:
>>
>> On 2022/8/18 17:18, Muchun Song wrote:
>>>
>>>
>>>> On Aug 18, 2022, at 16:54, Yin, Fengwei <fengwei.yin@xxxxxxxxx> wrote:
>>>>
>>>>
>>>>
>>>> On 8/18/2022 4:40 PM, Muchun Song wrote:
>>>>>
>>>>>
>>>>>> On Aug 18, 2022, at 16:32, Yin, Fengwei <fengwei.yin@xxxxxxxxx> wrote:
>>>>>>
>>>>>>
>>>>>>
>>>>>> On 8/18/2022 3:59 PM, Muchun Song wrote:
>>>>>>>
>>>>>>>
>>>>>>>> On Aug 18, 2022, at 15:52, Miaohe Lin <linmiaohe@xxxxxxxxxx> wrote:
>>>>>>>>
>>>>>>>> On 2022/8/18 10:47, Muchun Song wrote:
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>> On Aug 18, 2022, at 10:00, Yin, Fengwei <fengwei.yin@xxxxxxxxx> wrote:
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> On 8/18/2022 9:55 AM, Miaohe Lin wrote:
>>>>>>>>>>>>>> 	/*
>>>>>>>>>>>>>> 	 * The memory barrier inside __SetPageUptodate makes sure that
>>>>>>>>>>>>>> 	 * preceding stores to the page contents become visible before
>>>>>>>>>>>>>> 	 * the set_pte_at() write.
>>>>>>>>>>>>>> 	 */
>>>>>>>>>>>>>> 	__SetPageUptodate(page);
>>>>>>>>>>>>> IIUC, the case here we should make sure others (CPUs) can see new page’s
>>>>>>>>>>>>> contents after they have saw PG_uptodate is set. I think commit 0ed361dec369
>>>>>>>>>>>>> can tell us more details.
>>>>>>>>>>>>>
>>>>>>>>>>>>> I also looked at commit 52f37629fd3c to see why we need a barrier before
>>>>>>>>>>>>> set_pte_at(), but I didn’t find any info to explain why. I guess we want
>>>>>>>>>>>>> to make sure the order between the page’s contents and subsequent memory
>>>>>>>>>>>>> accesses using the corresponding virtual address, do you agree with this?
>>>>>>>>>>>> This is my understanding also. Thanks.
>>>>>>>>>>> That's also my understanding. Thanks both.
>>>>>>>>>> I have an unclear thing (not related with this patch directly): Who is response
>>>>>>>>>> for the read barrier in the read side in this case?
>>>>>>>>>>
>>>>>>>>>> For SetPageUptodate, there are paring write/read memory barrier.
>>>>>>>>>>
>>>>>>>>>
>>>>>>>>> I have the same question. So I think the example proposed by Miaohe is a little
>>>>>>>>> difference from the case (hugetlb_vmemmap) here.
>>>>>>>>
>>>>>>>> Per my understanding, memory barrier in PageUptodate() is needed because user might access the
>>>>>>>> page contents using page_address() (corresponding pagetable entry already exists) soon. But for
>>>>>>>> the above proposed case, if user wants to access the page contents, the corresponding pagetable
>>>>>>>> should be visible first or the page contents can't be accessed. So there should be a data dependency
>>>>>>>> acting as memory barrier between pagetable entry is loaded and page contents is accessed.
>>>>>>>> Or am I miss something?
>>>>>>>
>>>>>>> Yep, it is a data dependency. The difference between hugetlb_vmemmap and PageUptodate() is that
>>>>>>> the page table (a pointer to the mapped page frame) is loaded by MMU while PageUptodate() is
>>>>>>> loaded by CPU. Seems like the data dependency should be inserted between the MMU access and the CPU
>>>>>>> access. Maybe it is hardware’s guarantee?
>>>>>> I just found the comment in pmd_install() explained why most arch has no read
>>>>>
>>>>> I think pmd_install() is a little different as well. We should make sure the
>>>>> page table walker (like GUP) see the correct PTE entry after they see the pmd
>>>>> entry.
>>>>
>>>> The difference I can see is that pmd/pte thing has both hardware page walker and
>>>> software page walker (like GUP) as read side. While the case here only has hardware
>>>> page walker as read side. But I suppose the memory barrier requirement still apply
>>>> here.
>>>
>>> I am not against this change. Just in order to make me get a better understanding of
>>> hardware behavior.
>>>
>>>>
>>>> Maybe we could do a test: add large delay between reset_struct_page() and set_pte_at?
>>>
>>> Hi Miaohe,
>>>
>>> Would you mind doing this test? One thread do vmemmap_restore_pte(), another thread
>>> detect if it can see a tail page with PG_head after the previous thread has executed
>>> set_pte_at().
>>
>> Will it be easier to construct the memory reorder manually like below?
>>
>> vmemmap_restore_pte()
>> 	...
>> 	set_pte_at(&init_mm, addr, pte, mk_pte(page, pgprot));
>> 	/* might a delay. */
>> 	copy_page(to, (void *)walk->reuse_addr);
>> 	reset_struct_pages(to);
> 
> 
> Well, you have changed the code ordering. I thought we don’t change the code
> ordering. Just let the hardware do reordering. The ideal scenario would be
> as follows.
> 
> 
> CPU0:						CPU1:
> 
> vmemmap_restore_pte()
> 	copy_page(to, (void *)walk->reuse_addr);
>         reset_struct_pages(to); // clear the tail page’s PG_head
> 	set_pte_at(&init_mm, addr, pte, mk_pte(page, pgprot));
> 						// Detect if it can see a tail page with PG_head.
> 
> I should admit it is a little difficult to construct the scenario. After more
> thought, I think here should be inserted a barrier. So:
> 
> Reviewed-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>

Many thanks both for review and discussion. :)

Thanks,
Miaohe Lin