[PATCH v5 6/6] mm: handle userfaults under VMA lock

Suren Baghdasaryan
2023-06-28T17:26:45Z

Enable handle_userfault to operate under VMA lock by releasing VMA lock
instead of mmap_lock and retrying. Note that FAULT_FLAG_RETRY_NOWAIT
should never be used when handling faults under per-VMA lock protection
because that would break the assumption that lock is dropped on retry.

Signed-off-by: Suren Baghdasaryan <surenb@google.com>
---
fs/userfaultfd.c | 34 ++++++++++++++--------------------
include/linux/mm.h | 26 ++++++++++++++++++++++++++
mm/memory.c | 20 +++++++++++---------
3 files changed, 51 insertions(+), 29 deletions(-)

diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index 4e800bb7d2ab..9d61e3e7da7b 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -277,17 +277,16 @@ 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);
+ assert_fault_locked(vmf);

- 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 +307,8 @@ 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 +322,11 @@ 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 +334,7 @@ static inline bool userfaultfd_must_wait(struct userfaultfd_ctx *ctx,
pte_t *pte;
bool ret = true;

- mmap_assert_locked(mm);
+ assert_fault_locked(vmf);

pgd = pgd_offset(mm, address);
if (!pgd_present(*pgd))
@@ -445,7 +442,7 @@ 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);
+ assert_fault_locked(vmf);

ctx = vma->vm_userfaultfd_ctx.ctx;
if (!ctx)
@@ -561,15 +558,12 @@ 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);
+ release_fault_lock(vmf);

if (likely(must_wait && !READ_ONCE(ctx->released))) {
wake_up_poll(&ctx->fd_wqh, EPOLLIN);
diff --git a/include/linux/mm.h b/include/linux/mm.h
index bbaec479bf98..cd5389338def 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -705,6 +705,17 @@ static inline bool vma_try_start_write(struct vm_area_struct *vma)
return true;
}

+static inline void vma_assert_locked(struct vm_area_struct *vma)
+{
+ int mm_lock_seq;
+
+ if (__is_vma_write_locked(vma, &mm_lock_seq))
+ return;
+
+ lockdep_assert_held(&vma->vm_lock->lock);
+ VM_BUG_ON_VMA(!rwsem_is_locked(&vma->vm_lock->lock), vma);
+}
+
static inline void vma_assert_write_locked(struct vm_area_struct *vma)
{
int mm_lock_seq;
@@ -731,6 +742,15 @@ static inline void release_fault_lock(struct vm_fault *vmf)
mmap_read_unlock(vmf->vma->vm_mm);
}

+static inline
+void assert_fault_locked(struct vm_fault *vmf)
+{
+ if (vmf->flags & FAULT_FLAG_VMA_LOCK)
+ vma_assert_locked(vmf->vma);
+ else
+ mmap_assert_locked(vmf->vma->vm_mm);
+}
+
#else /* CONFIG_PER_VMA_LOCK */

static inline void vma_init_lock(struct vm_area_struct *vma) {}
@@ -749,6 +769,12 @@ static inline void release_fault_lock(struct vm_fault *vmf)
mmap_read_unlock(vmf->vma->vm_mm);
}

+static inline
+void assert_fault_locked(struct vm_fault *vmf)
+{
+ mmap_assert_locked(vmf->vma->vm_mm);
+}
+
#endif /* CONFIG_PER_VMA_LOCK */

/*
diff --git a/mm/memory.c b/mm/memory.c
index 4fb8ecfc6d13..672f7383a622 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -5202,6 +5202,17 @@ static vm_fault_t sanitize_fault_flags(struct vm_area_struct *vma,
!is_cow_mapping(vma->vm_flags)))
return VM_FAULT_SIGSEGV;
}
+#ifdef CONFIG_PER_VMA_LOCK
+ /*
+ * Per-VMA locks can't be used with FAULT_FLAG_RETRY_NOWAIT because of
+ * the assumption that lock is dropped on VM_FAULT_RETRY.
+ */
+ if (WARN_ON_ONCE((*flags &
+ (FAULT_FLAG_VMA_LOCK | FAULT_FLAG_RETRY_NOWAIT)) ==
+ (FAULT_FLAG_VMA_LOCK | FAULT_FLAG_RETRY_NOWAIT)))
+ return VM_FAULT_SIGSEGV;
+#endif
+
return 0;
}

@@ -5294,15 +5305,6 @@ 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);