[PATCH v2 13/13] mm: don't split THP page when syscall is called

[Minchan Kim]

We don't need to split THP page when MADV_FREE syscall is called.
It could be done when VM decide to free it in reclaim path when
memory pressure is heavy so we could avoid unnecessary THP split.

For that, this patch changes two things

1. __split_huge_page_map

It does pte_mkdirty to subpages only if pmd_dirty is true.

2. __split_huge_page_refcount

It removes marking PG_dirty to subpages unconditionally.

Cc: Kirill A. Shutemov <kirill@xxxxxxxxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx>
---

I don't understand why it added both SetPageDirty and pte_mkdirty
to subpages unconditionally in split code of THP from the beginning.
I guess at that time, there was no MADV_FREE so it was no problem
and would be more safe to mark both but I might be missing something.
Anyway, this patch doesn't have any problem so far for a few days
of my test.

I just added pmd_dirty respect code in split part of THP, not collapse
intentionally because I don't see how it is helpful in real practice.
Anyway, it could be done in future if it is needed, I think.

 include/linux/huge_mm.h |  3 +++
 mm/huge_memory.c        | 46 ++++++++++++++++++++++++++++++++++++++++++----
 mm/madvise.c            | 12 +++++++++++-
 3 files changed, 56 insertions(+), 5 deletions(-)

diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
index ecb080d6ff42..e9db238a75c1 100644
--- a/include/linux/huge_mm.h
+++ b/include/linux/huge_mm.h
@@ -19,6 +19,9 @@ extern struct page *follow_trans_huge_pmd(struct vm_area_struct *vma,
 					  unsigned long addr,
 					  pmd_t *pmd,
 					  unsigned int flags);
+extern int madvise_free_huge_pmd(struct mmu_gather *tlb,
+			struct vm_area_struct *vma,
+			pmd_t *pmd, unsigned long addr);
 extern int zap_huge_pmd(struct mmu_gather *tlb,
 			struct vm_area_struct *vma,
 			pmd_t *pmd, unsigned long addr);
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index bbac913f96bc..b8c9b44af864 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1453,6 +1453,41 @@ int do_huge_pmd_numa_page(struct mm_struct *mm, struct vm_area_struct *vma,
 	return 0;
 }
 
+int madvise_free_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
+		pmd_t *pmd, unsigned long addr)
+
+{
+	spinlock_t *ptl;
+	pmd_t orig_pmd;
+	struct page *page;
+	struct mm_struct *mm = tlb->mm;
+
+	if (__pmd_trans_huge_lock(pmd, vma, &ptl) != 1)
+		return 1;
+
+	orig_pmd = *pmd;
+	if (is_huge_zero_pmd(orig_pmd))
+		goto out;
+
+	page = pmd_page(orig_pmd);
+	if (PageActive(page))
+		deactivate_page(page);
+
+	if (pmd_young(orig_pmd) || pmd_dirty(orig_pmd)) {
+		orig_pmd = pmdp_huge_get_and_clear_full(tlb->mm, addr, pmd,
+			tlb->fullmm);
+		orig_pmd = pmd_mkold(orig_pmd);
+		orig_pmd = pmd_mkclean(orig_pmd);
+
+		set_pmd_at(mm, addr, pmd, orig_pmd);
+		tlb_remove_pmd_tlb_entry(tlb, pmd, addr);
+	}
+out:
+	spin_unlock(ptl);
+
+	return 0;
+}
+
 int zap_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
 		 pmd_t *pmd, unsigned long addr)
 {
@@ -1752,8 +1787,8 @@ static void __split_huge_page_refcount(struct page *page,
 				      (1L << PG_mlocked) |
 				      (1L << PG_uptodate) |
 				      (1L << PG_active) |
-				      (1L << PG_unevictable)));
-		page_tail->flags |= (1L << PG_dirty);
+				      (1L << PG_unevictable) |
+				      (1L << PG_dirty)));
 
 		/* clear PageTail before overwriting first_page */
 		smp_wmb();
@@ -1787,7 +1822,6 @@ static void __split_huge_page_refcount(struct page *page,
 
 		BUG_ON(!PageAnon(page_tail));
 		BUG_ON(!PageUptodate(page_tail));
-		BUG_ON(!PageDirty(page_tail));
 		BUG_ON(!PageSwapBacked(page_tail));
 
 		lru_add_page_tail(page, page_tail, lruvec, list);
@@ -1831,10 +1865,12 @@ static int __split_huge_page_map(struct page *page,
 	int ret = 0, i;
 	pgtable_t pgtable;
 	unsigned long haddr;
+	bool dirty;
 
 	pmd = page_check_address_pmd(page, mm, address,
 			PAGE_CHECK_ADDRESS_PMD_SPLITTING_FLAG, &ptl);
 	if (pmd) {
+		dirty = pmd_dirty(*pmd);
 		pgtable = pgtable_trans_huge_withdraw(mm, pmd);
 		pmd_populate(mm, &_pmd, pgtable);
 		if (pmd_write(*pmd))
@@ -1850,7 +1886,9 @@ static int __split_huge_page_map(struct page *page,
 			 * permissions across VMAs.
 			 */
 			entry = mk_pte(page + i, vma->vm_page_prot);
-			entry = maybe_mkwrite(pte_mkdirty(entry), vma);
+			if (dirty)
+				entry = pte_mkdirty(entry);
+			entry = maybe_mkwrite(entry, vma);
 			if (!pmd_write(*pmd))
 				entry = pte_wrprotect(entry);
 			if (!pmd_young(*pmd))
diff --git a/mm/madvise.c b/mm/madvise.c
index 4e67ba0b1104..27ed057c0bd7 100644
--- a/mm/madvise.c
+++ b/mm/madvise.c
@@ -271,8 +271,17 @@ static int madvise_free_pte_range(pmd_t *pmd, unsigned long addr,
 	pte_t *pte, ptent;
 	struct page *page;
 	int nr_swap = 0;
+	unsigned long next;
+
+	next = pmd_addr_end(addr, end);
+	if (pmd_trans_huge(*pmd)) {
+		if (next - addr != HPAGE_PMD_SIZE)
+			split_huge_page_pmd(vma, addr, pmd);
+		else if (!madvise_free_huge_pmd(tlb, vma, pmd, addr))
+			goto next;
+		/* fall through */
+	}
 
-	split_huge_page_pmd(vma, addr, pmd);
 	if (pmd_trans_unstable(pmd))
 		return 0;
 
@@ -355,6 +364,7 @@ static int madvise_free_pte_range(pmd_t *pmd, unsigned long addr,
 	arch_leave_lazy_mmu_mode();
 	pte_unmap_unlock(pte - 1, ptl);
 	cond_resched();
+next:
 	return 0;
 }
 
-- 
1.9.1

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