Re: [PATCH RESEND v2 4/4] selftests: vm: add a hugetlb test case

[Muchun Song]

On Tue, Sep 21, 2021 at 8:29 PM Barry Song <21cnbao@xxxxxxxxx> wrote:
>
> On Tue, Sep 21, 2021 at 2:26 AM Muchun Song <songmuchun@xxxxxxxxxxxxx> wrote:
> >
> > On Sat, Sep 18, 2021 at 1:20 PM Barry Song <21cnbao@xxxxxxxxx> wrote:
> > >
> > > On Sat, Sep 18, 2021 at 12:08 AM Muchun Song <songmuchun@xxxxxxxxxxxxx> 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
>  */

Got it. Will do. Thanks.

> +       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