[PATCH 05/10] mm/hugetlb: Make userfaultfd_huge_must_wait() safe to pmd unshare

Peter Xu
2022-11-29T19:38:36Z

We can take the hugetlb walker lock, here taking vma lock directly.

Signed-off-by: Peter Xu <peterx@redhat.com>
---
fs/userfaultfd.c | 18 ++++++++++++++----
1 file changed, 14 insertions(+), 4 deletions(-)

diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index 07c81ab3fd4d..a602f008dde5 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -376,7 +376,8 @@ static inline unsigned int userfaultfd_get_blocking_state(unsigned int flags)
*/
vm_fault_t handle_userfault(struct vm_fault *vmf, unsigned long reason)
{
- struct mm_struct *mm = vmf->vma->vm_mm;
+ struct vm_area_struct *vma = vmf->vma;
+ struct mm_struct *mm = vma->vm_mm;
struct userfaultfd_ctx *ctx;
struct userfaultfd_wait_queue uwq;
vm_fault_t ret = VM_FAULT_SIGBUS;
@@ -403,7 +404,7 @@ vm_fault_t handle_userfault(struct vm_fault *vmf, unsigned long reason)
*/
mmap_assert_locked(mm);

- ctx = vmf->vma->vm_userfaultfd_ctx.ctx;
+ ctx = vma->vm_userfaultfd_ctx.ctx;
if (!ctx)
goto out;

@@ -493,6 +494,13 @@ vm_fault_t handle_userfault(struct vm_fault *vmf, unsigned long reason)

blocking_state = userfaultfd_get_blocking_state(vmf->flags);

+ /*
+ * This stablizes pgtable for hugetlb on e.g. pmd unsharing. Need
+ * to be before setting current state.
+ */
+ if (is_vm_hugetlb_page(vma))
+ hugetlb_vma_lock_read(vma);
+
spin_lock_irq(&ctx->fault_pending_wqh.lock);
/*
* After the __add_wait_queue the uwq is visible to userland
@@ -507,13 +515,15 @@ vm_fault_t handle_userfault(struct vm_fault *vmf, unsigned long reason)
set_current_state(blocking_state);
spin_unlock_irq(&ctx->fault_pending_wqh.lock);

- if (!is_vm_hugetlb_page(vmf->vma))
+ if (!is_vm_hugetlb_page(vma))
must_wait = userfaultfd_must_wait(ctx, vmf->address, vmf->flags,
reason);
else
- must_wait = userfaultfd_huge_must_wait(ctx, vmf->vma,
+ must_wait = userfaultfd_huge_must_wait(ctx, vma,
vmf->address,
vmf->flags, reason);
+ if (is_vm_hugetlb_page(vma))
+ hugetlb_vma_unlock_read(vma);
mmap_read_unlock(mm);

if (likely(must_wait && !READ_ONCE(ctx->released))) {
--
2.37.3