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Subject[RFC PATCH v2 17/30] mm: thp: PUD THP COW splits PUD page and falls back to PMD page.
Date
From: Zi Yan <ziy@nvidia.com>

COW on PUD THPs has the same behavior as COW on PMD THPs to avoid high
COW overhead. As a result, do_huge_pmd_wp will see PMD-mapped PUD THPs,
thus needs to count PUD mappings in total mapcount when calling
page_trans_huge_map_swapcount in reuse_swap_page to avoid false positive.
Change page_trans_huge_map_swapcount to get it right.

Signed-off-by: Zi Yan <ziy@nvidia.com>
---
include/linux/huge_mm.h | 5 +++++
mm/huge_memory.c | 13 +++++++++++++
mm/memory.c | 3 +--
mm/swapfile.c | 7 ++++++-
4 files changed, 25 insertions(+), 3 deletions(-)

diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
index e5c68e680907..589e5af5a1c2 100644
--- a/include/linux/huge_mm.h
+++ b/include/linux/huge_mm.h
@@ -19,6 +19,7 @@ extern int copy_huge_pud(struct mm_struct *dst_mm, struct mm_struct *src_mm,
#ifdef CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD
extern void huge_pud_set_accessed(struct vm_fault *vmf, pud_t orig_pud);
extern int do_huge_pud_anonymous_page(struct vm_fault *vmf);
+extern vm_fault_t do_huge_pud_wp_page(struct vm_fault *vmf, pud_t orig_pud);
#else
static inline void huge_pud_set_accessed(struct vm_fault *vmf, pud_t orig_pud)
{
@@ -27,6 +28,10 @@ extern int do_huge_pud_anonymous_page(struct vm_fault *vmf)
{
return VM_FAULT_FALLBACK;
}
+extern vm_fault_t do_huge_pud_wp_page(struct vm_fault *vmf, pud_t orig_pud)
+{
+ return VM_FAULT_FALLBACK;
+}
#endif

extern vm_fault_t do_huge_pmd_wp_page(struct vm_fault *vmf, pmd_t orig_pmd);
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 4a899e856088..9aa19aa643cd 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1335,6 +1335,19 @@ void huge_pud_set_accessed(struct vm_fault *vmf, pud_t orig_pud)
unlock:
spin_unlock(vmf->ptl);
}
+
+vm_fault_t do_huge_pud_wp_page(struct vm_fault *vmf, pud_t orig_pud)
+{
+ struct vm_area_struct *vma = vmf->vma;
+
+ /*
+ * split pud directly. a whole pud page is not swappable, so there is
+ * no need to try reuse_swap_page
+ */
+ __split_huge_pud(vma, vmf->pud, vmf->address, false, NULL);
+ return VM_FAULT_FALLBACK;
+}
+
#endif /* CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD */

void huge_pmd_set_accessed(struct vm_fault *vmf, pmd_t orig_pmd)
diff --git a/mm/memory.c b/mm/memory.c
index e0e0459c0caf..ab80d13807aa 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -4141,9 +4141,8 @@ static vm_fault_t create_huge_pud(struct vm_fault *vmf)
static vm_fault_t wp_huge_pud(struct vm_fault *vmf, pud_t orig_pud)
{
#ifdef CONFIG_TRANSPARENT_HUGEPAGE
- /* No support for anonymous transparent PUD pages yet */
if (vma_is_anonymous(vmf->vma))
- return VM_FAULT_FALLBACK;
+ return do_huge_pud_wp_page(vmf, orig_pud);
if (vmf->vma->vm_ops->huge_fault)
return vmf->vma->vm_ops->huge_fault(vmf, PE_SIZE_PUD);
#endif /* CONFIG_TRANSPARENT_HUGEPAGE */
diff --git a/mm/swapfile.c b/mm/swapfile.c
index 495ecdbd7859..a6989b0c4d44 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -1635,7 +1635,12 @@ static int page_trans_huge_map_swapcount(struct page *page, int *total_mapcount,
/* hugetlbfs shouldn't call it */
VM_BUG_ON_PAGE(PageHuge(page), page);

- if (!IS_ENABLED(CONFIG_THP_SWAP) || likely(!PageTransCompound(page))) {
+ if (!IS_ENABLED(CONFIG_THP_SWAP) || likely(!PageTransCompound(page)) ||
+ /*
+ * PMD-mapped PUD THP need to take PUD mappings into account by
+ * using page_trans_huge_mapcount
+ */
+ unlikely(thp_order(page) == HPAGE_PUD_ORDER)) {
mapcount = page_trans_huge_mapcount(page, total_mapcount);
if (PageSwapCache(page))
swapcount = page_swapcount(page);
--
2.28.0
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