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. Maybe we could do a test: add large delay between reset_struct_page() and set_pte_at? Regards Yin, Fengwei > >> side memory barrier except alpha which has read side memory barrier. > > Right. Only alpha has data dependency barrier. > >> >> >> Regards >> Yin, Fengwei >> >>> >>>> >>>> Thanks, >>>> Miaohe Lin >
