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    SubjectRe: [PATCH RESEND v2 4/4] selftests: vm: add a hugetlb test case
    On Tue, Sep 21, 2021 at 2:26 AM Muchun Song <songmuchun@bytedance.com> wrote:
    >
    > On Sat, Sep 18, 2021 at 1:20 PM Barry Song <21cnbao@gmail.com> wrote:
    > >
    > > On Sat, Sep 18, 2021 at 12:08 AM Muchun Song <songmuchun@bytedance.com> wrote:
    > > >
    > > > Since the head vmemmap page frame associated with each HugeTLB page is
    > > > reused, we should hide the PG_head flag of tail struct page from the
    > > > user. Add a tese case to check whether it is work properly.
    > > >
    > >
    > > TBH, I am a bit confused. I was thinking about some kernel unit tests to make
    > > sure those kernel APIs touched by this patchset are still working as before.
    > > This userspace test, while certainly useful for checking the content of page
    > > frames as expected, doesn't directly prove things haven't changed.
    > >
    > > In patch 1/4, a couple of APIs have the fixup for the fake head issue.
    > > Do you think a test like the below would be more sensible?
    > > 1. alloc 2MB hugeTLB
    >
    > It is done in main().
    >
    > > 2. get each page frame
    > > 3. apply those APIs in each page frame
    > > 4. Those APIs work completely the same as before.
    >
    > Reading the flags of a page by /proc/kpageflags is done
    > in stable_page_flags(), which has invoked PageHead(),
    > PageTail(), PageCompound() and compound_head().
    > If those APIs work properly, the head page must have
    > 15 and 17 bits set. And tail pages must have 16 and 17
    > bits set but 15 unset.
    >
    > So I think check_page_flags() has done the step 2 to 4.
    > What do you think?

    yes. Thanks for your explanation. thereby, I think we just need some doc
    here to explain what it is checking. something like
    /*
    * pages other than the first page must be tail and shouldn't be head;
    * this also verifies kernel has correctly set the fake page_head to tail
    * while hugetlb_free_vmemmap is enabled
    */
    + for (i = 1; i < MAP_LENGTH / PAGE_SIZE; i++) {
    + read(fd, &pageflags, sizeof(pageflags));
    + if ((pageflags & TAIL_PAGE_FLAGS) != TAIL_PAGE_FLAGS ||
    + (pageflags & HEAD_PAGE_FLAGS) == HEAD_PAGE_FLAGS) {
    + close(fd);
    + printf("Tail page flags (%lx) is invalid\n", pageflags);
    + return -1;
    + }
    + }
    >
    > Thanks.

    Thanks
    barry

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