[PATCHv11 37/37] thp: allow mlocked THP again

[Kirill A. Shutemov]

Before THP refcounting rework, THP was not allowed to cross VMA boundary.
So, if we have THP and we split it, PG_mlocked can be safely transfered to
small pages.

With new THP refcounting and naive approach to mlocking we can end up with
this scenario:
 1. we have a mlocked THP, which belong to one VM_LOCKED VMA.
 2. the process does munlock() on the *part* of the THP:
      - the VMA is split into two, one of them VM_LOCKED;
      - huge PMD split into PTE table;
      - THP is still mlocked;
 3. split_huge_page():
      - it transfers PG_mlocked to *all* small pages regrardless if it
	blong to any VM_LOCKED VMA.

We probably could munlock() all small pages on split_huge_page(), but I
think we have accounting issue already on step two.

Instead of forbidding mlocked pages altogether, we just avoid mlocking
PTE-mapped THPs and munlock THPs on split_huge_pmd().

This means PTE-mapped THPs will be on normal lru lists and will be
split under memory pressure by vmscan. After the split vmscan will
detect unevictable small pages and mlock them.

With this approach we shouldn't hit situation like described above.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
---
 mm/gup.c         |  6 ++++--
 mm/huge_memory.c | 37 +++++++++++++++++++++++++++-------
 mm/memory.c      |  3 +--
 mm/mlock.c       | 61 +++++++++++++++++++++++++++++++++++++-------------------
 4 files changed, 75 insertions(+), 32 deletions(-)

diff --git a/mm/gup.c b/mm/gup.c
index 70d65e4015a4..e95b0cb6ed81 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -143,6 +143,10 @@ retry:
 		mark_page_accessed(page);
 	}
 	if ((flags & FOLL_MLOCK) && (vma->vm_flags & VM_LOCKED)) {
+		/* Do not mlock pte-mapped THP */
+		if (PageTransCompound(page))
+			goto out;
+
 		/*
 		 * The preliminary mapping check is mainly to avoid the
 		 * pointless overhead of lock_page on the ZERO_PAGE
@@ -920,8 +924,6 @@ long populate_vma_page_range(struct vm_area_struct *vma,
 	gup_flags = FOLL_TOUCH | FOLL_POPULATE | FOLL_MLOCK;
 	if (vma->vm_flags & VM_LOCKONFAULT)
 		gup_flags &= ~FOLL_POPULATE;
-	if (vma->vm_flags & VM_LOCKED)
-		gup_flags |= FOLL_SPLIT;
 	/*
 	 * We want to touch writable mappings with a write fault in order
 	 * to break COW, except for shared mappings because these don't COW
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 8730e24bfdfb..1d6190c40dc7 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -904,8 +904,6 @@ int do_huge_pmd_anonymous_page(struct mm_struct *mm, struct vm_area_struct *vma,
 
 	if (haddr < vma->vm_start || haddr + HPAGE_PMD_SIZE > vma->vm_end)
 		return VM_FAULT_FALLBACK;
-	if (vma->vm_flags & VM_LOCKED)
-		return VM_FAULT_FALLBACK;
 	if (unlikely(anon_vma_prepare(vma)))
 		return VM_FAULT_OOM;
 	if (unlikely(khugepaged_enter(vma, vma->vm_flags)))
@@ -1374,7 +1372,20 @@ struct page *follow_trans_huge_pmd(struct vm_area_struct *vma,
 			update_mmu_cache_pmd(vma, addr, pmd);
 	}
 	if ((flags & FOLL_MLOCK) && (vma->vm_flags & VM_LOCKED)) {
-		if (page->mapping && trylock_page(page)) {
+		/*
+		 * We don't mlock() pte-mapped THPs. This way we can avoid
+		 * leaking mlocked pages into non-VM_LOCKED VMAs.
+		 *
+		 * In most cases the pmd is the only mapping of the page as we
+		 * break COW for the mlock() -- see gup_flags |= FOLL_WRITE for
+		 * writable private mappings in populate_vma_page_range().
+		 *
+		 * The only scenario when we have the page shared here is if we
+		 * mlocking read-only mapping shared over fork(). We skip
+		 * mlocking such pages.
+		 */
+		if (compound_mapcount(page) == 1 && !PageDoubleMap(page) &&
+				page->mapping && trylock_page(page)) {
 			lru_add_drain();
 			if (page->mapping)
 				mlock_vma_page(page);
@@ -2274,8 +2285,6 @@ static bool hugepage_vma_check(struct vm_area_struct *vma)
 	if ((!(vma->vm_flags & VM_HUGEPAGE) && !khugepaged_always()) ||
 	    (vma->vm_flags & VM_NOHUGEPAGE))
 		return false;
-	if (vma->vm_flags & VM_LOCKED)
-		return false;
 	if (!vma->anon_vma || vma->vm_ops)
 		return false;
 	if (is_vma_temporary_stack(vma))
@@ -2941,14 +2950,28 @@ void __split_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
 {
 	spinlock_t *ptl;
 	struct mm_struct *mm = vma->vm_mm;
+	struct page *page = NULL;
 	unsigned long haddr = address & HPAGE_PMD_MASK;
 
 	mmu_notifier_invalidate_range_start(mm, haddr, haddr + HPAGE_PMD_SIZE);
 	ptl = pmd_lock(mm, pmd);
-	if (likely(pmd_trans_huge(*pmd)))
-		__split_huge_pmd_locked(vma, pmd, haddr, false);
+	if (unlikely(!pmd_trans_huge(*pmd)))
+		goto out;
+	page = pmd_page(*pmd);
+	__split_huge_pmd_locked(vma, pmd, haddr, false);
+	if (PageMlocked(page))
+		get_page(page);
+	else
+		page = NULL;
+out:
 	spin_unlock(ptl);
 	mmu_notifier_invalidate_range_end(mm, haddr, haddr + HPAGE_PMD_SIZE);
+	if (page) {
+		lock_page(page);
+		munlock_vma_page(page);
+		unlock_page(page);
+		put_page(page);
+	}
 }
 
 static void split_huge_pmd_address(struct vm_area_struct *vma,
diff --git a/mm/memory.c b/mm/memory.c
index d69e9ae023ce..58b61b241560 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -2155,8 +2155,7 @@ static int wp_page_copy(struct mm_struct *mm, struct vm_area_struct *vma,
 
 	pte_unmap_unlock(page_table, ptl);
 	mmu_notifier_invalidate_range_end(mm, mmun_start, mmun_end);
-	/* THP pages are never mlocked */
-	if (old_page && !PageTransCompound(old_page)) {
+	if (old_page) {
 		/*
 		 * Don't let another task, with possibly unlocked vma,
 		 * keep the mlocked page.
diff --git a/mm/mlock.c b/mm/mlock.c
index ef5fafd934b6..0147b57f9704 100644
--- a/mm/mlock.c
+++ b/mm/mlock.c
@@ -82,6 +82,9 @@ void mlock_vma_page(struct page *page)
 	/* Serialize with page migration */
 	BUG_ON(!PageLocked(page));
 
+	VM_BUG_ON_PAGE(PageTail(page), page);
+	VM_BUG_ON_PAGE(PageCompound(page) && PageDoubleMap(page), page);
+
 	if (!TestSetPageMlocked(page)) {
 		mod_zone_page_state(page_zone(page), NR_MLOCK,
 				    hpage_nr_pages(page));
@@ -178,6 +181,8 @@ unsigned int munlock_vma_page(struct page *page)
 	/* For try_to_munlock() and to serialize with page migration */
 	BUG_ON(!PageLocked(page));
 
+	VM_BUG_ON_PAGE(PageTail(page), page);
+
 	/*
 	 * Serialize with any parallel __split_huge_page_refcount() which
 	 * might otherwise copy PageMlocked to part of the tail pages before
@@ -443,29 +448,43 @@ void munlock_vma_pages_range(struct vm_area_struct *vma,
 		page = follow_page_mask(vma, start, FOLL_GET | FOLL_DUMP,
 				&page_mask);
 
-		if (page && !IS_ERR(page) && !PageTransCompound(page)) {
-			/*
-			 * Non-huge pages are handled in batches via
-			 * pagevec. The pin from follow_page_mask()
-			 * prevents them from collapsing by THP.
-			 */
-			pagevec_add(&pvec, page);
-			zone = page_zone(page);
-			zoneid = page_zone_id(page);
+		if (page && !IS_ERR(page)) {
+			if (PageTransTail(page)) {
+				VM_BUG_ON_PAGE(PageMlocked(page), page);
+				put_page(page); /* follow_page_mask() */
+			} else if (PageTransHuge(page)) {
+				lock_page(page);
+				/*
+				 * Any THP page found by follow_page_mask() may
+				 * have gotten split before reaching
+				 * munlock_vma_page(), so we need to recompute
+				 * the page_mask here.
+				 */
+				page_mask = munlock_vma_page(page);
+				unlock_page(page);
+				put_page(page); /* follow_page_mask() */
+			} else {
+				/*
+				 * Non-huge pages are handled in batches via
+				 * pagevec. The pin from follow_page_mask()
+				 * prevents them from collapsing by THP.
+				 */
+				pagevec_add(&pvec, page);
+				zone = page_zone(page);
+				zoneid = page_zone_id(page);
 
-			/*
-			 * Try to fill the rest of pagevec using fast
-			 * pte walk. This will also update start to
-			 * the next page to process. Then munlock the
-			 * pagevec.
-			 */
-			start = __munlock_pagevec_fill(&pvec, vma,
-					zoneid, start, end);
-			__munlock_pagevec(&pvec, zone);
-			goto next;
+				/*
+				 * Try to fill the rest of pagevec using fast
+				 * pte walk. This will also update start to
+				 * the next page to process. Then munlock the
+				 * pagevec.
+				 */
+				start = __munlock_pagevec_fill(&pvec, vma,
+						zoneid, start, end);
+				__munlock_pagevec(&pvec, zone);
+				goto next;
+			}
 		}
-		/* It's a bug to munlock in the middle of a THP page */
-		VM_BUG_ON((start >> PAGE_SHIFT) & page_mask);
 		page_increm = 1 + page_mask;
 		start += page_increm * PAGE_SIZE;
 next:
-- 
2.5.1

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