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@xxxxxxxxx>
---
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
