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    SubjectRe: [PATCH 3/3] mm: rmap: Fix CONT-PTE/PMD size hugetlb issue when unmapping
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    On 4/30/2022 4:02 AM, Gerald Schaefer wrote:
    > On Fri, 29 Apr 2022 16:14:43 +0800
    > Baolin Wang <baolin.wang@linux.alibaba.com> wrote:
    >
    >> On some architectures (like ARM64), it can support CONT-PTE/PMD size
    >> hugetlb, which means it can support not only PMD/PUD size hugetlb:
    >> 2M and 1G, but also CONT-PTE/PMD size: 64K and 32M if a 4K page
    >> size specified.
    >>
    >> When unmapping a hugetlb page, we will get the relevant page table
    >> entry by huge_pte_offset() only once to nuke it. This is correct
    >> for PMD or PUD size hugetlb, since they always contain only one
    >> pmd entry or pud entry in the page table.
    >>
    >> However this is incorrect for CONT-PTE and CONT-PMD size hugetlb,
    >> since they can contain several continuous pte or pmd entry with
    >> same page table attributes, so we will nuke only one pte or pmd
    >> entry for this CONT-PTE/PMD size hugetlb page.
    >>
    >> And now we only use try_to_unmap() to unmap a poisoned hugetlb page,
    >> which means now we will unmap only one pte entry for a CONT-PTE or
    >> CONT-PMD size poisoned hugetlb page, and we can still access other
    >> subpages of a CONT-PTE or CONT-PMD size poisoned hugetlb page,
    >> which will cause serious issues possibly.
    >>
    >> So we should change to use huge_ptep_clear_flush() to nuke the
    >> hugetlb page table to fix this issue, which already considered
    >> CONT-PTE and CONT-PMD size hugetlb.
    >>
    >> Note we've already used set_huge_swap_pte_at() to set a poisoned
    >> swap entry for a poisoned hugetlb page.
    >>
    >> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
    >> ---
    >> mm/rmap.c | 34 +++++++++++++++++-----------------
    >> 1 file changed, 17 insertions(+), 17 deletions(-)
    >>
    >> diff --git a/mm/rmap.c b/mm/rmap.c
    >> index 7cf2408..1e168d7 100644
    >> --- a/mm/rmap.c
    >> +++ b/mm/rmap.c
    >> @@ -1564,28 +1564,28 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
    >> break;
    >> }
    >> }
    >> + pteval = huge_ptep_clear_flush(vma, address, pvmw.pte);
    >
    > Unlike in your patch 2/3, I do not see that this (huge) pteval would later
    > be used again with set_huge_pte_at() instead of set_pte_at(). Not sure if
    > this (huge) pteval could end up at a set_pte_at() later, but if yes, then
    > this would be broken on s390, and you'd need to use set_huge_pte_at()
    > instead of set_pte_at() like in your patch 2/3.

    IIUC, As I said in the commit message, we will only unmap a poisoned
    hugetlb page by try_to_unmap(), and the poisoned hugetlb page will be
    remapped with a poisoned entry by set_huge_swap_pte_at() in
    try_to_unmap_one(). So I think no need change to use set_huge_pte_at()
    instead of set_pte_at() for other cases, since the hugetlb page will not
    hit other cases.

    if (PageHWPoison(subpage) && !(flags & TTU_IGNORE_HWPOISON)) {
    pteval = swp_entry_to_pte(make_hwpoison_entry(subpage));
    if (folio_test_hugetlb(folio)) {
    hugetlb_count_sub(folio_nr_pages(folio), mm);
    set_huge_swap_pte_at(mm, address, pvmw.pte, pteval,
    vma_mmu_pagesize(vma));
    } else {
    dec_mm_counter(mm, mm_counter(&folio->page));
    set_pte_at(mm, address, pvmw.pte, pteval);
    }

    }

    >
    > Please note that huge_ptep_get functions do not return valid PTEs on s390,
    > and such PTEs must never be set directly with set_pte_at(), but only with
    > set_huge_pte_at().
    >
    > Background is that, for hugetlb pages, we are of course not really dealing
    > with PTEs at this level, but rather PMDs or PUDs, depending on hugetlb size.
    > On s390, the layout is quite different for PTEs and PMDs / PUDs, and
    > unfortunately the hugetlb code is not properly reflecting this by using
    > PMD or PUD types, like the THP code does.
    >
    > So, as work-around, on s390, the huge_ptep_xxx functions will return
    > only fake PTEs, which must be converted again to a proper PMD or PUD,
    > before writing them to the page table, which is what happens in
    > set_huge_pte_at(), but not in set_pte_at().

    Thanks for your explanation. As I said as above, I think we've already
    handled the hugetlb with set_huge_swap_pte_at() in try_to_unmap_one().

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