To support the zap functionally for COW-ed PTE, we need to zap the
entire PTE table each time instead of partially zapping pages.
Therefore, if the zap range covers the entire PTE table, we can
handle de-account, remove the rmap, etc. However we shouldn't modify
the entries when there are still someone references to the COW-ed
PTE. Otherwise, if only the zapped process references to this COW-ed
PTE, we just reuse it and do the normal zapping.
Signed-off-by: Chih-En Lin <shiyn.lin@xxxxxxxxx>
---
mm/memory.c | 92 ++++++++++++++++++++++++++++++++++++++++++++++++++---
1 file changed, 87 insertions(+), 5 deletions(-)
diff --git a/mm/memory.c b/mm/memory.c
index f8a87a0fc382..7908e20f802a 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -192,6 +192,12 @@ static inline void free_pmd_range(struct mmu_gather *tlb, pud_t *pud,
pmd = pmd_offset(pud, addr);
do {
next = pmd_addr_end(addr, end);
+#ifdef CONFIG_COW_PTE
+ if (test_bit(MMF_COW_PTE, &tlb->mm->flags)) {
+ if (!pmd_none(*pmd) && !pmd_write(*pmd))
+ VM_WARN_ON(cow_pte_count(pmd) != 1);
+ }
+#endif
if (pmd_none_or_clear_bad(pmd))
continue;
free_pte_range(tlb, pmd, addr);
@@ -1656,6 +1662,7 @@ zap_install_uffd_wp_if_needed(struct vm_area_struct *vma,
#define ZAP_PTE_INIT 0x0000
#define ZAP_PTE_FORCE_FLUSH 0x0001
+#define ZAP_PTE_IS_SHARED 0x0002
struct zap_pte_details {
pte_t **pte;
@@ -1681,9 +1688,13 @@ zap_present_pte(struct mmu_gather *tlb, struct vm_area_struct *vma,
if (unlikely(!should_zap_page(details, page)))
return 0;
- ptent = ptep_get_and_clear_full(mm, addr, pte, tlb->fullmm);
+ if (pte_details->flags & ZAP_PTE_IS_SHARED)
+ ptent = ptep_get(pte);
+ else
+ ptent = ptep_get_and_clear_full(mm, addr, pte, tlb->fullmm);
tlb_remove_tlb_entry(tlb, pte, addr);
- zap_install_uffd_wp_if_needed(vma, addr, pte, details, ptent);
+ if (!(pte_details->flags & ZAP_PTE_IS_SHARED))
+ zap_install_uffd_wp_if_needed(vma, addr, pte, details, ptent);
if (unlikely(!page))
return 0;
@@ -1767,8 +1778,10 @@ zap_nopresent_pte(struct mmu_gather *tlb, struct vm_area_struct *vma,
/* We should have covered all the swap entry types */
WARN_ON_ONCE(1);
}
- pte_clear_not_present_full(mm, addr, pte, tlb->fullmm);
- zap_install_uffd_wp_if_needed(vma, addr, pte, details, ptent);
+ if (!(pte_details->flags & ZAP_PTE_IS_SHARED)) {
+ pte_clear_not_present_full(mm, addr, pte, tlb->fullmm);
+ zap_install_uffd_wp_if_needed(vma, addr, pte, details, ptent);
+ }
}
static unsigned long zap_pte_range(struct mmu_gather *tlb,
@@ -1785,6 +1798,36 @@ static unsigned long zap_pte_range(struct mmu_gather *tlb,
.flags = ZAP_PTE_INIT,
.pte = &pte,
};
+#ifdef CONFIG_COW_PTE
+ unsigned long orig_addr = addr;
+
+ if (test_bit(MMF_COW_PTE, &mm->flags) && !pmd_write(*pmd)) {
+ if (!range_in_vma(vma, addr & PMD_MASK,
+ (addr + PMD_SIZE) & PMD_MASK)) {
+ /*
+ * We cannot promise this COW-ed PTE will also be zap
+ * with the rest of VMAs. So, break COW PTE here.
+ */
+ break_cow_pte(vma, pmd, addr);
+ } else {
+ /*
+ * We free the batched memory before we handle
+ * COW-ed PTE.
+ */
+ tlb_flush_mmu(tlb);
+ end = (addr + PMD_SIZE) & PMD_MASK;
+ addr = addr & PMD_MASK;
+ start_pte = pte_offset_map_lock(mm, pmd, addr, &ptl);
+ if (cow_pte_count(pmd) == 1) {
+ /* Reuse COW-ed PTE */
+ pmd_t new = pmd_mkwrite(*pmd);
+ set_pmd_at(tlb->mm, addr, pmd, new);
+ } else
+ pte_details.flags |= ZAP_PTE_IS_SHARED;
+ pte_unmap_unlock(start_pte, ptl);
+ }
+ }
+#endif
tlb_change_page_size(tlb, PAGE_SIZE);
again:
@@ -1828,7 +1871,16 @@ static unsigned long zap_pte_range(struct mmu_gather *tlb,
*/
if (pte_details.flags & ZAP_PTE_FORCE_FLUSH) {
pte_details.flags &= ~ZAP_PTE_FORCE_FLUSH;
- tlb_flush_mmu(tlb);
+ /*
+ * With COW-ed PTE, we defer freeing the batched memory until
+ * after we have actually cleared the COW-ed PTE's pmd entry.
+ * Since, if we are the only ones still referencing the COW-ed
+ * PTe table after we have freed the batched memory, the page
+ * table check will report a bug with anon_map_count != 0 in
+ * page_table_check_zero().
+ */
+ if (!(pte_details.flags & ZAP_PTE_IS_SHARED))
+ tlb_flush_mmu(tlb);
}
if (addr != end) {
@@ -1836,6 +1888,36 @@ static unsigned long zap_pte_range(struct mmu_gather *tlb,
goto again;
}
+#ifdef CONFIG_COW_PTE
+ if (pte_details.flags & ZAP_PTE_IS_SHARED) {
+ start_pte = pte_offset_map_lock(mm, pmd, orig_addr, &ptl);
+ if (!pmd_put_pte(pmd)) {
+ pmd_t new = pmd_mkwrite(*pmd);
+ set_pmd_at(tlb->mm, addr, pmd, new);
+ /*
+ * We are the only ones who still referencing this.
+ * Clear the page table check before we free the
+ * batched memory.
+ */
+ page_table_check_pte_clear_range(mm, orig_addr, *pmd);
+ pte_unmap_unlock(start_pte, ptl);
+ /* free the batched memory and flush the TLB. */
+ tlb_flush_mmu(tlb);
+ free_pte_range(tlb, pmd, addr);
+ } else {
+ pmd_clear(pmd);
+ pte_unmap_unlock(start_pte, ptl);
+ mm_dec_nr_ptes(tlb->mm);
+ /*
+ * Someone still referencing to the table,
+ * we just flush TLB here.
+ */
+ flush_tlb_range(vma, addr & PMD_MASK,
+ (addr + PMD_SIZE) & PMD_MASK);
+ }
+ }
+#endif
+
return addr;
}
--
2.34.1
