> 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. > side memory barrier except alpha which has read side memory barrier. Right. Only alpha has data dependency barrier. > > > Regards > Yin, Fengwei > >> >>> >>> Thanks, >>> Miaohe Lin
