[PATCH 32/36] thp: reintroduce split_huge_page()

[Kirill A. Shutemov]

This patch adds implementation of split_huge_page() for new
refcountings.

Unlike previous implementation, new split_huge_page() can fail if
somebody holds GUP pin on the page. It also means that pin on page
would prevent it from bening split under you. It makes situation in
many places much cleaner.

The basic scheme of split_huge_page():

  - Check that sum of mapcounts of all subpage is equal to page_count()
    plus one (caller pin). Foll off with -EBUSY. This way we can avoid
    useless PMD-splits.

  - Freeze the page counters by splitting all PMD and setup migration
    PTEs.

  - Re-check sum of mapcounts against page_count(). Page's counts are
    stable now. -EBUSY if page is pinned.

  - Split compound page.

  - Unfreeze the page by removing migration entries.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
Tested-by: Sasha Levin <sasha.levin@xxxxxxxxxx>
---
 include/linux/huge_mm.h |   7 +-
 include/linux/pagemap.h |  13 +-
 mm/huge_memory.c        | 318 ++++++++++++++++++++++++++++++++++++++++++++++++
 mm/internal.h           |  26 +++-
 mm/rmap.c               |  21 ----
 5 files changed, 357 insertions(+), 28 deletions(-)

diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
index 9cfce23ab9d0..89981a042d85 100644
--- a/include/linux/huge_mm.h
+++ b/include/linux/huge_mm.h
@@ -92,8 +92,11 @@ extern bool is_vma_temporary_stack(struct vm_area_struct *vma);
 
 extern unsigned long transparent_hugepage_flags;
 
-#define split_huge_page_to_list(page, list) BUILD_BUG()
-#define split_huge_page(page) BUILD_BUG()
+int split_huge_page_to_list(struct page *page, struct list_head *list);
+static inline int split_huge_page(struct page *page)
+{
+	return split_huge_page_to_list(page, NULL);
+}
 
 void __split_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
 		unsigned long address);
diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index 4779de5ed499..3f0da5cd8cca 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -387,10 +387,21 @@ static inline struct page *read_mapping_page(struct address_space *mapping,
  */
 static inline pgoff_t page_to_pgoff(struct page *page)
 {
+	pgoff_t pgoff;
+
 	if (unlikely(PageHeadHuge(page)))
 		return page->index << compound_order(page);
-	else
+
+	if (likely(!PageTransTail(page)))
 		return page->index << (PAGE_CACHE_SHIFT - PAGE_SHIFT);
+
+	/*
+	 *  We don't initialize ->index for tail pages: calculate based on
+	 *  head page
+	 */
+	pgoff = page->first_page->index << (PAGE_CACHE_SHIFT - PAGE_SHIFT);
+	pgoff += page - page->first_page;
+	return pgoff;
 }
 
 /*
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index affbabaf31bd..5df1b76837c7 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2749,3 +2749,321 @@ void __vma_adjust_trans_huge(struct vm_area_struct *vma,
 			split_huge_pmd_address(next, nstart);
 	}
 }
+
+static void freeze_page_vma(struct vm_area_struct *vma, struct page *page,
+		unsigned long address)
+{
+	spinlock_t *ptl;
+	pgd_t *pgd;
+	pud_t *pud;
+	pmd_t *pmd;
+	pte_t *pte;
+	int i;
+
+	pgd = pgd_offset(vma->vm_mm, address);
+	if (!pgd_present(*pgd))
+		return;
+	pud = pud_offset(pgd, address);
+	if (!pud_present(*pud))
+		return;
+	pmd = pmd_offset(pud, address);
+	ptl = pmd_lock(vma->vm_mm, pmd);
+	if (!pmd_present(*pmd)) {
+		spin_unlock(ptl);
+		return;
+	}
+	if (pmd_trans_huge(*pmd)) {
+		if (page == pmd_page(*pmd))
+			__split_huge_pmd_locked(vma, pmd, address, true);
+		spin_unlock(ptl);
+		return;
+	}
+	spin_unlock(ptl);
+
+	pte = pte_offset_map_lock(vma->vm_mm, pmd, address, &ptl);
+	for (i = 0; i < HPAGE_PMD_NR; i++, address += PAGE_SIZE, page++) {
+		pte_t entry, swp_pte;
+		swp_entry_t swp_entry;
+
+		if (!pte_present(pte[i]))
+			continue;
+		if (page_to_pfn(page) != pte_pfn(pte[i]))
+			continue;
+		flush_cache_page(vma, address, page_to_pfn(page));
+		entry = ptep_clear_flush(vma, address, pte + i);
+		swp_entry = make_migration_entry(page, pte_write(entry));
+		swp_pte = swp_entry_to_pte(swp_entry);
+		if (pte_soft_dirty(entry))
+			swp_pte = pte_swp_mksoft_dirty(swp_pte);
+		set_pte_at(vma->vm_mm, address, pte + i, swp_pte);
+	}
+	pte_unmap_unlock(pte, ptl);
+}
+
+static void freeze_page(struct anon_vma *anon_vma, struct page *page)
+{
+	struct anon_vma_chain *avc;
+	pgoff_t pgoff = page_to_pgoff(page);
+
+	VM_BUG_ON_PAGE(!PageHead(page), page);
+
+	anon_vma_interval_tree_foreach(avc, &anon_vma->rb_root, pgoff,
+			pgoff + HPAGE_PMD_NR - 1) {
+		unsigned long haddr;
+
+		haddr = __vma_address(page, avc->vma) & HPAGE_PMD_MASK;
+		mmu_notifier_invalidate_range_start(avc->vma->vm_mm,
+				haddr, haddr + HPAGE_PMD_SIZE);
+		freeze_page_vma(avc->vma, page, haddr);
+		mmu_notifier_invalidate_range_end(avc->vma->vm_mm,
+				haddr, haddr + HPAGE_PMD_SIZE);
+	}
+}
+
+static void unfreeze_page_vma(struct vm_area_struct *vma, struct page *page,
+		unsigned long address)
+{
+	spinlock_t *ptl;
+	pmd_t *pmd;
+	pte_t *pte, entry;
+	swp_entry_t swp_entry;
+	int i;
+
+	pmd = mm_find_pmd(vma->vm_mm, address);
+	if (!pmd)
+		return;
+	pte = pte_offset_map_lock(vma->vm_mm, pmd, address, &ptl);
+	for (i = 0; i < HPAGE_PMD_NR; i++, address += PAGE_SIZE, page++) {
+		if (!page_mapped(page))
+			continue;
+		if (!is_swap_pte(pte[i]))
+			continue;
+
+		swp_entry = pte_to_swp_entry(pte[i]);
+		if (!is_migration_entry(swp_entry))
+			continue;
+		if (migration_entry_to_page(swp_entry) != page)
+			continue;
+
+		entry = pte_mkold(mk_pte(page, vma->vm_page_prot));
+		if (is_write_migration_entry(swp_entry))
+			entry = maybe_mkwrite(entry, vma);
+
+		flush_dcache_page(page);
+		set_pte_at(vma->vm_mm, address, pte + i, entry);
+
+		/* No need to invalidate - it was non-present before */
+		update_mmu_cache(vma, address, pte + i);
+	}
+	pte_unmap_unlock(pte, ptl);
+}
+
+static void unfreeze_page(struct anon_vma *anon_vma, struct page *page)
+{
+	struct anon_vma_chain *avc;
+	pgoff_t pgoff = page_to_pgoff(page);
+
+	anon_vma_interval_tree_foreach(avc, &anon_vma->rb_root,
+			pgoff, pgoff + HPAGE_PMD_NR - 1) {
+		unsigned long address = __vma_address(page, avc->vma);
+
+		mmu_notifier_invalidate_range_start(avc->vma->vm_mm,
+				address, address + HPAGE_PMD_SIZE);
+		unfreeze_page_vma(avc->vma, page, address);
+		mmu_notifier_invalidate_range_end(avc->vma->vm_mm,
+				address, address + HPAGE_PMD_SIZE);
+	}
+}
+
+static int total_mapcount(struct page *page)
+{
+	int i, ret;
+
+	ret = compound_mapcount(page);
+	for (i = 0; i < HPAGE_PMD_NR; i++)
+		ret += atomic_read(&page[i]._mapcount) + 1;
+
+	if (PageDoubleMap(page))
+		ret -= HPAGE_PMD_NR;
+
+	return ret;
+}
+
+static int __split_huge_page_tail(struct page *head, int tail,
+		struct lruvec *lruvec, struct list_head *list)
+{
+	int mapcount;
+	struct page *page_tail = head + tail;
+
+	mapcount = atomic_read(&page_tail->_mapcount) + 1;
+	VM_BUG_ON_PAGE(atomic_read(&page_tail->_count) != 0, page_tail);
+
+	/*
+	 * tail_page->_count is zero and not changing from under us. But
+	 * get_page_unless_zero() may be running from under us on the
+	 * tail_page. If we used atomic_set() below instead of atomic_add(), we
+	 * would then run atomic_set() concurrently with
+	 * get_page_unless_zero(), and atomic_set() is implemented in C not
+	 * using locked ops. spin_unlock on x86 sometime uses locked ops
+	 * because of PPro errata 66, 92, so unless somebody can guarantee
+	 * atomic_set() here would be safe on all archs (and not only on x86),
+	 * it's safer to use atomic_add().
+	 */
+	atomic_add(mapcount + 1, &page_tail->_count);
+
+	/* after clearing PageTail the gup refcount can be released */
+	smp_mb__after_atomic();
+
+	/*
+	 * retain hwpoison flag of the poisoned tail page:
+	 *   fix for the unsuitable process killed on Guest Machine(KVM)
+	 *   by the memory-failure.
+	 */
+	page_tail->flags &= ~PAGE_FLAGS_CHECK_AT_PREP | __PG_HWPOISON;
+	page_tail->flags |= (head->flags &
+			((1L << PG_referenced) |
+			 (1L << PG_swapbacked) |
+			 (1L << PG_mlocked) |
+			 (1L << PG_uptodate) |
+			 (1L << PG_active) |
+			 (1L << PG_locked) |
+			 (1L << PG_unevictable)));
+	page_tail->flags |= (1L << PG_dirty);
+
+	/* clear PageTail before overwriting first_page */
+	smp_wmb();
+
+	/* ->mapping in first tail page is compound_mapcount */
+	VM_BUG_ON_PAGE(tail != 1 && page_tail->mapping != TAIL_MAPPING,
+			page_tail);
+	page_tail->mapping = head->mapping;
+
+	page_tail->index = head->index + tail;
+	page_cpupid_xchg_last(page_tail, page_cpupid_last(head));
+	lru_add_page_tail(head, page_tail, lruvec, list);
+
+	return mapcount;
+}
+
+static void __split_huge_page(struct page *page, struct list_head *list)
+{
+	struct page *head = compound_head(page);
+	struct zone *zone = page_zone(head);
+	struct lruvec *lruvec;
+	int i, tail_mapcount;
+
+	/* prevent PageLRU to go away from under us, and freeze lru stats */
+	spin_lock_irq(&zone->lru_lock);
+	lruvec = mem_cgroup_page_lruvec(head, zone);
+
+	/* complete memcg works before add pages to LRU */
+	mem_cgroup_split_huge_fixup(head);
+
+	tail_mapcount = 0;
+	for (i = HPAGE_PMD_NR - 1; i >= 1; i--)
+		tail_mapcount += __split_huge_page_tail(head, i, lruvec, list);
+	atomic_sub(tail_mapcount, &head->_count);
+
+	ClearPageCompound(head);
+	spin_unlock_irq(&zone->lru_lock);
+
+	unfreeze_page(page_anon_vma(head), head);
+
+	for (i = 0; i < HPAGE_PMD_NR; i++) {
+		struct page *subpage = head + i;
+		if (subpage == page)
+			continue;
+		unlock_page(subpage);
+
+		/*
+		 * Subpages may be freed if there wasn't any mapping
+		 * like if add_to_swap() is running on a lru page that
+		 * had its mapping zapped. And freeing these pages
+		 * requires taking the lru_lock so we do the put_page
+		 * of the tail pages after the split is complete.
+		 */
+		put_page(subpage);
+	}
+}
+
+/*
+ * This function splits huge page into normal pages. @page can point to any
+ * subpage of huge page to split. Split doesn't change the position of @page.
+ *
+ * Only caller must hold pin on the @page, otherwise split fails with -EBUSY.
+ * The huge page must be locked.
+ *
+ * If @list is null, tail pages will be added to LRU list, otherwise, to @list.
+ *
+ * Both head page and tail pages will inherit mapping, flags, and so on from
+ * the hugepage.
+ *
+ * GUP pin and PG_locked transfered to @page. Rest subpages can be freed if
+ * they are not mapped.
+ *
+ * Returns 0 if the hugepage is split successfully.
+ * Returns -EBUSY if the page is pinned or if anon_vma disappeared from under
+ * us.
+ */
+int split_huge_page_to_list(struct page *page, struct list_head *list)
+{
+	struct page *head = compound_head(page);
+	struct anon_vma *anon_vma;
+	int count, mapcount, ret;
+
+	VM_BUG_ON_PAGE(is_huge_zero_page(page), page);
+	VM_BUG_ON_PAGE(!PageAnon(page), page);
+	VM_BUG_ON_PAGE(!PageLocked(page), page);
+	VM_BUG_ON_PAGE(!PageSwapBacked(page), page);
+	VM_BUG_ON_PAGE(!PageCompound(page), page);
+
+	/*
+	 * The caller does not necessarily hold an mmap_sem that would prevent
+	 * the anon_vma disappearing so we first we take a reference to it
+	 * and then lock the anon_vma for write. This is similar to
+	 * page_lock_anon_vma_read except the write lock is taken to serialise
+	 * against parallel split or collapse operations.
+	 */
+	anon_vma = page_get_anon_vma(head);
+	if (!anon_vma) {
+		ret = -EBUSY;
+		goto out;
+	}
+	anon_vma_lock_write(anon_vma);
+
+	/*
+	 * Racy check if we can split the page, before freeze_page() will
+	 * split PMDs
+	 */
+	if (total_mapcount(head) != page_count(head) - 1) {
+		ret = -EBUSY;
+		goto out_unlock;
+	}
+
+	freeze_page(anon_vma, head);
+	VM_BUG_ON_PAGE(compound_mapcount(head), head);
+
+	count = page_count(head);
+	mapcount = total_mapcount(head);
+	if (mapcount == count - 1) {
+		__split_huge_page(page, list);
+		ret = 0;
+	} else if (IS_ENABLED(CONFIG_DEBUG_VM) && mapcount > count - 1) {
+		pr_alert("total_mapcount: %u, page_count(): %u\n",
+				mapcount, count);
+		if (PageTail(page))
+			dump_page(head, NULL);
+		dump_page(page, "total_mapcount(head) > page_count(head) - 1");
+		BUG();
+	} else {
+		unfreeze_page(anon_vma, head);
+		ret = -EBUSY;
+	}
+
+out_unlock:
+	anon_vma_unlock_write(anon_vma);
+	put_anon_vma(anon_vma);
+out:
+	count_vm_event(!ret ? THP_SPLIT_PAGE : THP_SPLIT_PAGE_FAILED);
+	return ret;
+}
diff --git a/mm/internal.h b/mm/internal.h
index 6aa08db10589..a2e466bc60b2 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -13,6 +13,7 @@
 
 #include <linux/fs.h>
 #include <linux/mm.h>
+#include <linux/pagemap.h>
 
 void free_pgtables(struct mmu_gather *tlb, struct vm_area_struct *start_vma,
 		unsigned long floor, unsigned long ceiling);
@@ -245,10 +246,27 @@ static inline void mlock_migrate_page(struct page *newpage, struct page *page)
 
 extern pmd_t maybe_pmd_mkwrite(pmd_t pmd, struct vm_area_struct *vma);
 
-#ifdef CONFIG_TRANSPARENT_HUGEPAGE
-extern unsigned long vma_address(struct page *page,
-				 struct vm_area_struct *vma);
-#endif
+/*
+ * At what user virtual address is page expected in @vma?
+ */
+static inline unsigned long
+__vma_address(struct page *page, struct vm_area_struct *vma)
+{
+	pgoff_t pgoff = page_to_pgoff(page);
+	return vma->vm_start + ((pgoff - vma->vm_pgoff) << PAGE_SHIFT);
+}
+
+static inline unsigned long
+vma_address(struct page *page, struct vm_area_struct *vma)
+{
+	unsigned long address = __vma_address(page, vma);
+
+	/* page should be within @vma mapping range */
+	VM_BUG_ON_VMA(address < vma->vm_start || address >= vma->vm_end, vma);
+
+	return address;
+}
+
 #else /* !CONFIG_MMU */
 static inline void clear_page_mlock(struct page *page) { }
 static inline void mlock_vma_page(struct page *page) { }
diff --git a/mm/rmap.c b/mm/rmap.c
index bae27e4608b5..89908269fdf4 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -563,27 +563,6 @@ void page_unlock_anon_vma_read(struct anon_vma *anon_vma)
 }
 
 /*
- * At what user virtual address is page expected in @vma?
- */
-static inline unsigned long
-__vma_address(struct page *page, struct vm_area_struct *vma)
-{
-	pgoff_t pgoff = page_to_pgoff(page);
-	return vma->vm_start + ((pgoff - vma->vm_pgoff) << PAGE_SHIFT);
-}
-
-inline unsigned long
-vma_address(struct page *page, struct vm_area_struct *vma)
-{
-	unsigned long address = __vma_address(page, vma);
-
-	/* page should be within @vma mapping range */
-	VM_BUG_ON_VMA(address < vma->vm_start || address >= vma->vm_end, vma);
-
-	return address;
-}
-
-/*
  * At what user virtual address is page expected in vma?
  * Caller should check the page is actually part of the vma.
  */
-- 
2.1.4

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