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    Subject[PATCH v3 05/14] mm/khugepaged: Break COW PTE before scanning pte
    Date
    We should not allow THP to collapse COW-ed PTE. So, break COW PTE
    before collapse_pte_mapped_thp() collapse to THP. Also, break COW
    PTE before khugepaged_scan_pmd() scan PTE.

    Signed-off-by: Chih-En Lin <shiyn.lin@gmail.com>
    ---
    include/trace/events/huge_memory.h | 1 +
    mm/khugepaged.c | 23 +++++++++++++++++++++++
    2 files changed, 24 insertions(+)

    diff --git a/include/trace/events/huge_memory.h b/include/trace/events/huge_memory.h
    index 760455dfa8600..881553aa0f2f2 100644
    --- a/include/trace/events/huge_memory.h
    +++ b/include/trace/events/huge_memory.h
    @@ -13,6 +13,7 @@
    EM( SCAN_PMD_NULL, "pmd_null") \
    EM( SCAN_PMD_NONE, "pmd_none") \
    EM( SCAN_PMD_MAPPED, "page_pmd_mapped") \
    + EM( SCAN_COW_PTE, "cowed_pte") \
    EM( SCAN_EXCEED_NONE_PTE, "exceed_none_pte") \
    EM( SCAN_EXCEED_SWAP_PTE, "exceed_swap_pte") \
    EM( SCAN_EXCEED_SHARED_PTE, "exceed_shared_pte") \
    diff --git a/mm/khugepaged.c b/mm/khugepaged.c
    index a8d5ef2a77d24..106e1ce3931f7 100644
    --- a/mm/khugepaged.c
    +++ b/mm/khugepaged.c
    @@ -31,6 +31,7 @@ enum scan_result {
    SCAN_PMD_NULL,
    SCAN_PMD_NONE,
    SCAN_PMD_MAPPED,
    + SCAN_COW_PTE,
    SCAN_EXCEED_NONE_PTE,
    SCAN_EXCEED_SWAP_PTE,
    SCAN_EXCEED_SHARED_PTE,
    @@ -1030,6 +1031,9 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
    if (result != SCAN_SUCCEED)
    goto out_up_write;

    + /* We should already handled COW-ed PTE. */
    + VM_WARN_ON(test_bit(MMF_COW_PTE, &mm->flags) && !pmd_write(*pmd));
    +
    anon_vma_lock_write(vma->anon_vma);

    mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, NULL, mm,
    @@ -1139,6 +1143,16 @@ static int hpage_collapse_scan_pmd(struct mm_struct *mm,

    memset(cc->node_load, 0, sizeof(cc->node_load));
    nodes_clear(cc->alloc_nmask);
    +
    + /*
    + * Before we scan each pte entry, we should first check PTE
    + * could be modified. So, we break COW if PTE is COW-ed.
    + */
    + if (break_cow_pte(vma, pmd, address) < 0) {
    + result = SCAN_COW_PTE;
    + goto out;
    + }
    +
    pte = pte_offset_map_lock(mm, pmd, address, &ptl);
    for (_address = address, _pte = pte; _pte < pte + HPAGE_PMD_NR;
    _pte++, _address += PAGE_SIZE) {
    @@ -1197,6 +1211,10 @@ static int hpage_collapse_scan_pmd(struct mm_struct *mm,
    goto out_unmap;
    }

    + /*
    + * If we only trigger the break COW PTE, the page usually
    + * still in COW mapping, which it still be shared.
    + */
    if (page_mapcount(page) > 1) {
    ++shared;
    if (cc->is_khugepaged &&
    @@ -1472,6 +1490,11 @@ int collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr,
    goto drop_hpage;
    }

    + /* We shouldn't let COW-ed PTE collapse. */
    + if (break_cow_pte(vma, pmd, haddr) < 0)
    + goto drop_hpage;
    + VM_WARN_ON(test_bit(MMF_COW_PTE, &mm->flags) && !pmd_write(*pmd));
    +
    start_pte = pte_offset_map_lock(mm, pmd, haddr, &ptl);
    result = SCAN_FAIL;

    --
    2.37.3
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