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

[Peter Xu]

On Mon, Jun 26, 2023 at 09:23:21PM -0700, Suren Baghdasaryan wrote:
> Enable handle_userfault to operate under VMA lock by releasing VMA lock
> instead of mmap_lock and retrying.

This mostly good to me (besides the new DROP flag.. of course), thanks.
Still some nitpicks below.

> 
> Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>
> ---
>  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);

Maybe we can have a helper asserting proper vma protector locks (mmap for
!VMA_LOCK and vma read lock for VMA_LOCK)?  It basically tells the context
the vma is still safe to access.

>  
> -	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);

(the assert helper can also be used here)

>  
>  	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);

I also think maybe we should have a helper mm_release_fault_lock() just
release different locks for with/without VMA_LOCK.  It can also be used in
the other patch of folio_lock_or_retry().

> +	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
> 

-- 
Peter Xu