Re: [PATCH 5/7] khugepaged: Allow to collapse PTE-mapped compound pages

[Yang Shi <shy828301@gmail.com>]

On Tue, Mar 31, 2020 at 7:08 AM Kirill A. Shutemov <kirill@shutemov.name> wrote:
>
> On Mon, Mar 30, 2020 at 11:50:41AM -0700, Yang Shi wrote:
> > On Sat, Mar 28, 2020 at 5:33 AM Kirill A. Shutemov <kirill@shutemov.name> wrote:
> > >
> > > On Fri, Mar 27, 2020 at 09:17:00PM -0400, Zi Yan wrote:
> > > > > The compound page may be locked here if the function called for the first
> > > > > time for the page and not locked after that (becouse we've unlocked it we
> > > > > saw it the first time). The same with LRU.
> > > > >
> > > >
> > > > For the first time, the compound page is locked and not on LRU, so this VM_BUG_ON passes.
> > > > For the second time and so on, the compound page is unlocked and on the LRU,
> > > > so this VM_BUG_ON still passes.
> > > >
> > > > For base page, VM_BUG_ON passes.
> > > >
> > > > Other unexpected situation (a compound page is locked and on LRU) triggers the VM_BU_ON,
> > > > but your VM_BUG_ON will not detect this situation, right?
> > >
> > > Right. I will rework this code. I've just realized it is racy: after
> > > unlock and putback on LRU the page can be locked by somebody else and this
> > > code can unlock it which completely borken.
> > >
> > > I'll pass down compound_pagelist to release_pte_pages() and handle the
> > > situation there.
> > >
> > > > >>>     if (likely(writable)) {
> > > > >>>             if (likely(referenced)) {
> > > > >>
> > > > >> Do we need a list here? There should be at most one compound page we will see here, right?
> > > > >
> > > > > Um? It's outside the pte loop. We get here once per PMD range.
> > > > >
> > > > > 'page' argument to trace_mm_collapse_huge_page_isolate() is misleading:
> > > > > it's just the last page handled in the loop.
> > > > >
> > > >
> > > > Throughout the pte loop, we should only see at most one compound page, right?
> > >
> > > No. mremap(2) opens a possibility for HPAGE_PMD_NR compound pages for
> > > single PMD range.
> >
> > Do you mean every PTE in the PMD is mapped by a sub page from different THPs?
>
> Yes.
>
> Well, it was harder to archive than I expected, but below is a test case,
> I've come up with. It maps 512 head pages within single PMD range.

Thanks, this is very helpful.

>
> diff --git a/tools/testing/selftests/vm/khugepaged.c b/tools/testing/selftests/vm/khugepaged.c
> index 3a98d5b2d6d8..9ae119234a39 100644
> --- a/tools/testing/selftests/vm/khugepaged.c
> +++ b/tools/testing/selftests/vm/khugepaged.c
> @@ -703,6 +703,63 @@ static void collapse_full_of_compound(void)
>         munmap(p, hpage_pmd_size);
>  }
>
> +static void collapse_compound_extreme(void)
> +{
> +       void *p;
> +       int i;
> +
> +       p = alloc_mapping();
> +       for (i = 0; i < hpage_pmd_nr; i++) {
> +               printf("\rConstruct PTE page table full of different PTE-mapped compound pages %3d/%d...",
> +                               i + 1, hpage_pmd_nr);
> +
> +               madvise(BASE_ADDR, hpage_pmd_size, MADV_HUGEPAGE);
> +               fill_memory(BASE_ADDR, 0, hpage_pmd_size);
> +               if (!check_huge(BASE_ADDR)) {
> +                       printf("Failed to allocate huge page\n");
> +                       exit(EXIT_FAILURE);
> +               }
> +               madvise(BASE_ADDR, hpage_pmd_size, MADV_NOHUGEPAGE);
> +
> +               p = mremap(BASE_ADDR - i * page_size,
> +                               i * page_size + hpage_pmd_size,
> +                               (i + 1) * page_size,
> +                               MREMAP_MAYMOVE | MREMAP_FIXED,
> +                               BASE_ADDR + 2 * hpage_pmd_size);
> +               if (p == MAP_FAILED) {
> +                       perror("mremap+unmap");
> +                       exit(EXIT_FAILURE);
> +               }
> +
> +               p = mremap(BASE_ADDR + 2 * hpage_pmd_size,
> +                               (i + 1) * page_size,
> +                               (i + 1) * page_size + hpage_pmd_size,
> +                               MREMAP_MAYMOVE | MREMAP_FIXED,
> +                               BASE_ADDR - (i + 1) * page_size);
> +               if (p == MAP_FAILED) {
> +                       perror("mremap+alloc");
> +                       exit(EXIT_FAILURE);
> +               }
> +       }
> +
> +       munmap(BASE_ADDR, hpage_pmd_size);
> +       fill_memory(p, 0, hpage_pmd_size);
> +       if (!check_huge(p))
> +               success("OK");
> +       else
> +               fail("Fail");
> +
> +       if (wait_for_scan("Collapse PTE table full of different compound pages", p))
> +               fail("Timeout");
> +       else if (check_huge(p))
> +               success("OK");
> +       else
> +               fail("Fail");
> +
> +       validate_memory(p, 0, hpage_pmd_size);
> +       munmap(p, hpage_pmd_size);
> +}
> +
>  static void collapse_fork(void)
>  {
>         int wstatus;
> @@ -916,6 +973,7 @@ int main(void)
>         collapse_max_ptes_swap();
>         collapse_signle_pte_entry_compound();
>         collapse_full_of_compound();
> +       collapse_compound_extreme();
>         collapse_fork();
>         collapse_fork_compound();
>         collapse_max_ptes_shared();
> --
>  Kirill A. Shutemov