[PATCH v3 8/8] mm: handle userfaults under VMA lock

[Suren Baghdasaryan <surenb@google.com>]

Enable handle_userfault to operate under VMA lock by releasing VMA lock
instead of mmap_lock and retrying.

Signed-off-by: Suren Baghdasaryan <surenb@google.com>
---
 fs/userfaultfd.c | 42 ++++++++++++++++++++++--------------------
 mm/memory.c      |  9 ---------
 2 files changed, 22 insertions(+), 29 deletions(-)

diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index 4e800bb7d2ab..b88632c404b6 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -277,17 +277,17 @@ static inline struct uffd_msg userfault_msg(unsigned long address,
  * hugepmd ranges.
  */
 static inline bool userfaultfd_huge_must_wait(struct userfaultfd_ctx *ctx,
-					 struct vm_area_struct *vma,
-					 unsigned long address,
-					 unsigned long flags,
-					 unsigned long reason)
+					      struct vm_fault *vmf,
+					      unsigned long reason)
 {
+	struct vm_area_struct *vma = vmf->vma;
 	pte_t *ptep, pte;
 	bool ret = true;
 
-	mmap_assert_locked(ctx->mm);
+	if (!(vmf->flags & FAULT_FLAG_VMA_LOCK))
+		mmap_assert_locked(ctx->mm);
 
-	ptep = hugetlb_walk(vma, address, vma_mmu_pagesize(vma));
+	ptep = hugetlb_walk(vma, vmf->address, vma_mmu_pagesize(vma));
 	if (!ptep)
 		goto out;
 
@@ -308,10 +308,8 @@ static inline bool userfaultfd_huge_must_wait(struct userfaultfd_ctx *ctx,
 }
 #else
 static inline bool userfaultfd_huge_must_wait(struct userfaultfd_ctx *ctx,
-					 struct vm_area_struct *vma,
-					 unsigned long address,
-					 unsigned long flags,
-					 unsigned long reason)
+					      struct vm_fault *vmf,
+					      unsigned long reason)
 {
 	return false;	/* should never get here */
 }
@@ -325,11 +323,11 @@ static inline bool userfaultfd_huge_must_wait(struct userfaultfd_ctx *ctx,
  * threads.
  */
 static inline bool userfaultfd_must_wait(struct userfaultfd_ctx *ctx,
-					 unsigned long address,
-					 unsigned long flags,
+					 struct vm_fault *vmf,
 					 unsigned long reason)
 {
 	struct mm_struct *mm = ctx->mm;
+	unsigned long address = vmf->address;
 	pgd_t *pgd;
 	p4d_t *p4d;
 	pud_t *pud;
@@ -337,7 +335,8 @@ static inline bool userfaultfd_must_wait(struct userfaultfd_ctx *ctx,
 	pte_t *pte;
 	bool ret = true;
 
-	mmap_assert_locked(mm);
+	if (!(vmf->flags & FAULT_FLAG_VMA_LOCK))
+		mmap_assert_locked(mm);
 
 	pgd = pgd_offset(mm, address);
 	if (!pgd_present(*pgd))
@@ -445,7 +444,8 @@ vm_fault_t handle_userfault(struct vm_fault *vmf, unsigned long reason)
 	 * Coredumping runs without mmap_lock so we can only check that
 	 * the mmap_lock is held, if PF_DUMPCORE was not set.
 	 */
-	mmap_assert_locked(mm);
+	if (!(vmf->flags & FAULT_FLAG_VMA_LOCK))
+		mmap_assert_locked(mm);
 
 	ctx = vma->vm_userfaultfd_ctx.ctx;
 	if (!ctx)
@@ -561,15 +561,17 @@ vm_fault_t handle_userfault(struct vm_fault *vmf, unsigned long reason)
 	spin_unlock_irq(&ctx->fault_pending_wqh.lock);
 
 	if (!is_vm_hugetlb_page(vma))
-		must_wait = userfaultfd_must_wait(ctx, vmf->address, vmf->flags,
-						  reason);
+		must_wait = userfaultfd_must_wait(ctx, vmf, reason);
 	else
-		must_wait = userfaultfd_huge_must_wait(ctx, vma,
-						       vmf->address,
-						       vmf->flags, reason);
+		must_wait = userfaultfd_huge_must_wait(ctx, vmf, reason);
 	if (is_vm_hugetlb_page(vma))
 		hugetlb_vma_unlock_read(vma);
-	mmap_read_unlock(mm);
+	if (vmf->flags & FAULT_FLAG_VMA_LOCK) {
+		/* WARNING: VMA can't be used after this */
+		vma_end_read(vma);
+	} else
+		mmap_read_unlock(mm);
+	vmf->flags |= FAULT_FLAG_LOCK_DROPPED;
 
 	if (likely(must_wait && !READ_ONCE(ctx->released))) {
 		wake_up_poll(&ctx->fd_wqh, EPOLLIN);
diff --git a/mm/memory.c b/mm/memory.c
index bdf46fdc58d6..923c1576bd14 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -5316,15 +5316,6 @@ struct vm_area_struct *lock_vma_under_rcu(struct mm_struct *mm,
 	if (!vma_start_read(vma))
 		goto inval;
 
-	/*
-	 * Due to the possibility of userfault handler dropping mmap_lock, avoid
-	 * it for now and fall back to page fault handling under mmap_lock.
-	 */
-	if (userfaultfd_armed(vma)) {
-		vma_end_read(vma);
-		goto inval;
-	}
-
 	/* Check since vm_start/vm_end might change before we lock the VMA */
 	if (unlikely(address < vma->vm_start || address >= vma->vm_end)) {
 		vma_end_read(vma);
-- 
2.41.0.178.g377b9f9a00-goog