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