[PATCH 1/2] userfaultfd: handle zeropage moves by UFFDIO_MOVE

[Suren Baghdasaryan <surenb@google.com>]

Current implementation of UFFDIO_MOVE fails to move zeropages and returns
EBUSY when it encounters one. We can handle them by mapping a zeropage
at the destination and clearing the mapping at the source. This is done
both for ordinary and for huge zeropages.

Signed-off-by: Suren Baghdasaryan <surenb@google.com>
---
Applies cleanly over mm-unstable branch.

 mm/huge_memory.c | 105 +++++++++++++++++++++++++++--------------------
 mm/userfaultfd.c |  42 +++++++++++++++----
 2 files changed, 96 insertions(+), 51 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index f40feb31b507..5dcc02c25e97 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2190,13 +2190,18 @@ int move_pages_huge_pmd(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd, pm
 	}
 
 	src_page = pmd_page(src_pmdval);
-	if (unlikely(!PageAnonExclusive(src_page))) {
-		spin_unlock(src_ptl);
-		return -EBUSY;
-	}
 
-	src_folio = page_folio(src_page);
-	folio_get(src_folio);
+	if (!is_huge_zero_pmd(src_pmdval)) {
+		if (unlikely(!PageAnonExclusive(src_page))) {
+			spin_unlock(src_ptl);
+			return -EBUSY;
+		}
+
+		src_folio = page_folio(src_page);
+		folio_get(src_folio);
+	} else
+		src_folio = NULL;
+
 	spin_unlock(src_ptl);
 
 	flush_cache_range(src_vma, src_addr, src_addr + HPAGE_PMD_SIZE);
@@ -2204,19 +2209,22 @@ int move_pages_huge_pmd(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd, pm
 				src_addr + HPAGE_PMD_SIZE);
 	mmu_notifier_invalidate_range_start(&range);
 
-	folio_lock(src_folio);
+	if (src_folio) {
+		folio_lock(src_folio);
 
-	/*
-	 * split_huge_page walks the anon_vma chain without the page
-	 * lock. Serialize against it with the anon_vma lock, the page
-	 * lock is not enough.
-	 */
-	src_anon_vma = folio_get_anon_vma(src_folio);
-	if (!src_anon_vma) {
-		err = -EAGAIN;
-		goto unlock_folio;
-	}
-	anon_vma_lock_write(src_anon_vma);
+		/*
+		 * split_huge_page walks the anon_vma chain without the page
+		 * lock. Serialize against it with the anon_vma lock, the page
+		 * lock is not enough.
+		 */
+		src_anon_vma = folio_get_anon_vma(src_folio);
+		if (!src_anon_vma) {
+			err = -EAGAIN;
+			goto unlock_folio;
+		}
+		anon_vma_lock_write(src_anon_vma);
+	} else
+		src_anon_vma = NULL;
 
 	dst_ptl = pmd_lockptr(mm, dst_pmd);
 	double_pt_lock(src_ptl, dst_ptl);
@@ -2225,45 +2233,54 @@ int move_pages_huge_pmd(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd, pm
 		err = -EAGAIN;
 		goto unlock_ptls;
 	}
-	if (folio_maybe_dma_pinned(src_folio) ||
-	    !PageAnonExclusive(&src_folio->page)) {
-		err = -EBUSY;
-		goto unlock_ptls;
-	}
+	if (src_folio) {
+		if (folio_maybe_dma_pinned(src_folio) ||
+		    !PageAnonExclusive(&src_folio->page)) {
+			err = -EBUSY;
+			goto unlock_ptls;
+		}
 
-	if (WARN_ON_ONCE(!folio_test_head(src_folio)) ||
-	    WARN_ON_ONCE(!folio_test_anon(src_folio))) {
-		err = -EBUSY;
-		goto unlock_ptls;
-	}
+		if (WARN_ON_ONCE(!folio_test_head(src_folio)) ||
+		    WARN_ON_ONCE(!folio_test_anon(src_folio))) {
+			err = -EBUSY;
+			goto unlock_ptls;
+		}
 
-	folio_move_anon_rmap(src_folio, dst_vma);
-	WRITE_ONCE(src_folio->index, linear_page_index(dst_vma, dst_addr));
+		folio_move_anon_rmap(src_folio, dst_vma);
+		WRITE_ONCE(src_folio->index, linear_page_index(dst_vma, dst_addr));
 
-	src_pmdval = pmdp_huge_clear_flush(src_vma, src_addr, src_pmd);
-	/* Folio got pinned from under us. Put it back and fail the move. */
-	if (folio_maybe_dma_pinned(src_folio)) {
-		set_pmd_at(mm, src_addr, src_pmd, src_pmdval);
-		err = -EBUSY;
-		goto unlock_ptls;
-	}
+		src_pmdval = pmdp_huge_clear_flush(src_vma, src_addr, src_pmd);
+		/* Folio got pinned from under us. Put it back and fail the move. */
+		if (folio_maybe_dma_pinned(src_folio)) {
+			set_pmd_at(mm, src_addr, src_pmd, src_pmdval);
+			err = -EBUSY;
+			goto unlock_ptls;
+		}
 
-	_dst_pmd = mk_huge_pmd(&src_folio->page, dst_vma->vm_page_prot);
-	/* Follow mremap() behavior and treat the entry dirty after the move */
-	_dst_pmd = pmd_mkwrite(pmd_mkdirty(_dst_pmd), dst_vma);
+		_dst_pmd = mk_huge_pmd(&src_folio->page, dst_vma->vm_page_prot);
+		/* Follow mremap() behavior and treat the entry dirty after the move */
+		_dst_pmd = pmd_mkwrite(pmd_mkdirty(_dst_pmd), dst_vma);
+	} else {
+		src_pmdval = pmdp_huge_clear_flush(src_vma, src_addr, src_pmd);
+		_dst_pmd = mk_huge_pmd(src_page, dst_vma->vm_page_prot);
+	}
 	set_pmd_at(mm, dst_addr, dst_pmd, _dst_pmd);
 
 	src_pgtable = pgtable_trans_huge_withdraw(mm, src_pmd);
 	pgtable_trans_huge_deposit(mm, dst_pmd, src_pgtable);
 unlock_ptls:
 	double_pt_unlock(src_ptl, dst_ptl);
-	anon_vma_unlock_write(src_anon_vma);
-	put_anon_vma(src_anon_vma);
+	if (src_anon_vma) {
+		anon_vma_unlock_write(src_anon_vma);
+		put_anon_vma(src_anon_vma);
+	}
 unlock_folio:
 	/* unblock rmap walks */
-	folio_unlock(src_folio);
+	if (src_folio)
+		folio_unlock(src_folio);
 	mmu_notifier_invalidate_range_end(&range);
-	folio_put(src_folio);
+	if (src_folio)
+		folio_put(src_folio);
 	return err;
 }
 #endif /* CONFIG_USERFAULTFD */
diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index 3548b3e31a97..5fbf4da15c5c 100644
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -959,6 +959,31 @@ static int move_swap_pte(struct mm_struct *mm,
 	return 0;
 }
 
+static int move_zeropage_pte(struct mm_struct *mm,
+			     struct vm_area_struct *dst_vma,
+			     struct vm_area_struct *src_vma,
+			     unsigned long dst_addr, unsigned long src_addr,
+			     pte_t *dst_pte, pte_t *src_pte,
+			     pte_t orig_dst_pte, pte_t orig_src_pte,
+			     spinlock_t *dst_ptl, spinlock_t *src_ptl)
+{
+	pte_t zero_pte;
+
+	double_pt_lock(dst_ptl, src_ptl);
+	if (!pte_same(ptep_get(src_pte), orig_src_pte) ||
+	    !pte_same(ptep_get(dst_pte), orig_dst_pte))
+		return -EAGAIN;
+
+	zero_pte = pte_mkspecial(pfn_pte(my_zero_pfn(dst_addr),
+					 dst_vma->vm_page_prot));
+	ptep_clear_flush(src_vma, src_addr, src_pte);
+	set_pte_at(mm, dst_addr, dst_pte, zero_pte);
+	double_pt_unlock(dst_ptl, src_ptl);
+
+	return 0;
+}
+
+
 /*
  * The mmap_lock for reading is held by the caller. Just move the page
  * from src_pmd to dst_pmd if possible, and return true if succeeded
@@ -1041,6 +1066,14 @@ static int move_pages_pte(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd,
 	}
 
 	if (pte_present(orig_src_pte)) {
+		if (is_zero_pfn(pte_pfn(orig_src_pte))) {
+			err = move_zeropage_pte(mm, dst_vma, src_vma,
+					       dst_addr, src_addr, dst_pte, src_pte,
+					       orig_dst_pte, orig_src_pte,
+					       dst_ptl, src_ptl);
+			goto out;
+		}
+
 		/*
 		 * Pin and lock both source folio and anon_vma. Since we are in
 		 * RCU read section, we can't block, so on contention have to
@@ -1404,19 +1437,14 @@ ssize_t move_pages(struct userfaultfd_ctx *ctx, struct mm_struct *mm,
 				err = -ENOENT;
 				break;
 			}
-			/* Avoid moving zeropages for now */
-			if (is_huge_zero_pmd(*src_pmd)) {
-				spin_unlock(ptl);
-				err = -EBUSY;
-				break;
-			}
 
 			/* Check if we can move the pmd without splitting it. */
 			if (move_splits_huge_pmd(dst_addr, src_addr, src_start + len) ||
 			    !pmd_none(dst_pmdval)) {
 				struct folio *folio = pfn_folio(pmd_pfn(*src_pmd));
 
-				if (!folio || !PageAnonExclusive(&folio->page)) {
+				if (!folio || (!is_huge_zero_page(&folio->page) &&
+					       !PageAnonExclusive(&folio->page))) {
 					spin_unlock(ptl);
 					err = -EBUSY;
 					break;
-- 
2.43.0.429.g432eaa2c6b-goog