Re: [PATCH v2 4/6] mm: lock vma explicitly before doing vm_flags_reset and vm_flags_reset_once

["Liam R. Howlett" <Liam.Howlett@Oracle.com>]

* Suren Baghdasaryan <surenb@google.com> [230801 18:07]:
> Implicit vma locking inside vm_flags_reset() and vm_flags_reset_once() is
> not obvious and makes it hard to understand where vma locking is happening.
> Also in some cases (like in dup_userfaultfd()) vma should be locked earlier
> than vma_flags modification. To make locking more visible, change these
> functions to assert that the vma write lock is taken and explicitly lock
> the vma beforehand. Fix userfaultfd functions which should lock the vma
> earlier.
> 
> Suggested-by: Linus Torvalds <torvalds@linuxfoundation.org>
> Signed-off-by: Suren Baghdasaryan <surenb@google.com>

Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com>

> ---
>  arch/powerpc/kvm/book3s_hv_uvmem.c    |  1 +
>  drivers/infiniband/hw/hfi1/file_ops.c |  1 +
>  fs/userfaultfd.c                      |  6 ++++++
>  include/linux/mm.h                    | 10 +++++++---
>  mm/madvise.c                          |  5 ++---
>  mm/mlock.c                            |  3 ++-
>  mm/mprotect.c                         |  1 +
>  7 files changed, 20 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/powerpc/kvm/book3s_hv_uvmem.c b/arch/powerpc/kvm/book3s_hv_uvmem.c
> index 709ebd578394..e2d6f9327f77 100644
> --- a/arch/powerpc/kvm/book3s_hv_uvmem.c
> +++ b/arch/powerpc/kvm/book3s_hv_uvmem.c
> @@ -410,6 +410,7 @@ static int kvmppc_memslot_page_merge(struct kvm *kvm,
>  			ret = H_STATE;
>  			break;
>  		}
> +		vma_start_write(vma);
>  		/* Copy vm_flags to avoid partial modifications in ksm_madvise */
>  		vm_flags = vma->vm_flags;
>  		ret = ksm_madvise(vma, vma->vm_start, vma->vm_end,
> diff --git a/drivers/infiniband/hw/hfi1/file_ops.c b/drivers/infiniband/hw/hfi1/file_ops.c
> index a5ab22cedd41..5920bfc1e1c5 100644
> --- a/drivers/infiniband/hw/hfi1/file_ops.c
> +++ b/drivers/infiniband/hw/hfi1/file_ops.c
> @@ -344,6 +344,7 @@ static int hfi1_file_mmap(struct file *fp, struct vm_area_struct *vma)
>  		goto done;
>  	}
>  
> +	vma_start_write(vma);
>  	/*
>  	 * vm_pgoff is used as a buffer selector cookie.  Always mmap from
>  	 * the beginning.
> diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
> index 7cecd49e078b..6cde95533dcd 100644
> --- a/fs/userfaultfd.c
> +++ b/fs/userfaultfd.c
> @@ -667,6 +667,7 @@ static void userfaultfd_event_wait_completion(struct userfaultfd_ctx *ctx,
>  		mmap_write_lock(mm);
>  		for_each_vma(vmi, vma) {
>  			if (vma->vm_userfaultfd_ctx.ctx == release_new_ctx) {
> +				vma_start_write(vma);
>  				vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;
>  				userfaultfd_set_vm_flags(vma,
>  							 vma->vm_flags & ~__VM_UFFD_FLAGS);
> @@ -702,6 +703,7 @@ int dup_userfaultfd(struct vm_area_struct *vma, struct list_head *fcs)
>  
>  	octx = vma->vm_userfaultfd_ctx.ctx;
>  	if (!octx || !(octx->features & UFFD_FEATURE_EVENT_FORK)) {
> +		vma_start_write(vma);
>  		vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;
>  		userfaultfd_set_vm_flags(vma, vma->vm_flags & ~__VM_UFFD_FLAGS);
>  		return 0;
> @@ -783,6 +785,7 @@ void mremap_userfaultfd_prep(struct vm_area_struct *vma,
>  		atomic_inc(&ctx->mmap_changing);
>  	} else {
>  		/* Drop uffd context if remap feature not enabled */
> +		vma_start_write(vma);
>  		vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;
>  		userfaultfd_set_vm_flags(vma, vma->vm_flags & ~__VM_UFFD_FLAGS);
>  	}
> @@ -940,6 +943,7 @@ static int userfaultfd_release(struct inode *inode, struct file *file)
>  			prev = vma;
>  		}
>  
> +		vma_start_write(vma);
>  		userfaultfd_set_vm_flags(vma, new_flags);
>  		vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;
>  	}
> @@ -1502,6 +1506,7 @@ static int userfaultfd_register(struct userfaultfd_ctx *ctx,
>  		 * the next vma was merged into the current one and
>  		 * the current one has not been updated yet.
>  		 */
> +		vma_start_write(vma);
>  		userfaultfd_set_vm_flags(vma, new_flags);
>  		vma->vm_userfaultfd_ctx.ctx = ctx;
>  
> @@ -1685,6 +1690,7 @@ static int userfaultfd_unregister(struct userfaultfd_ctx *ctx,
>  		 * the next vma was merged into the current one and
>  		 * the current one has not been updated yet.
>  		 */
> +		vma_start_write(vma);
>  		userfaultfd_set_vm_flags(vma, new_flags);
>  		vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;
>  
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 262b5f44101d..2c720c9bb1ae 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -780,18 +780,22 @@ static inline void vm_flags_init(struct vm_area_struct *vma,
>  	ACCESS_PRIVATE(vma, __vm_flags) = flags;
>  }
>  
> -/* Use when VMA is part of the VMA tree and modifications need coordination */
> +/*
> + * Use when VMA is part of the VMA tree and modifications need coordination
> + * Note: vm_flags_reset and vm_flags_reset_once do not lock the vma and
> + * it should be locked explicitly beforehand.
> + */
>  static inline void vm_flags_reset(struct vm_area_struct *vma,
>  				  vm_flags_t flags)
>  {
> -	vma_start_write(vma);
> +	vma_assert_write_locked(vma);
>  	vm_flags_init(vma, flags);
>  }
>  
>  static inline void vm_flags_reset_once(struct vm_area_struct *vma,
>  				       vm_flags_t flags)
>  {
> -	vma_start_write(vma);
> +	vma_assert_write_locked(vma);
>  	WRITE_ONCE(ACCESS_PRIVATE(vma, __vm_flags), flags);
>  }
>  
> diff --git a/mm/madvise.c b/mm/madvise.c
> index bfe0e06427bd..507b1d299fec 100644
> --- a/mm/madvise.c
> +++ b/mm/madvise.c
> @@ -173,9 +173,8 @@ static int madvise_update_vma(struct vm_area_struct *vma,
>  	}
>  
>  success:
> -	/*
> -	 * vm_flags is protected by the mmap_lock held in write mode.
> -	 */
> +	/* vm_flags is protected by the mmap_lock held in write mode. */
> +	vma_start_write(vma);
>  	vm_flags_reset(vma, new_flags);
>  	if (!vma->vm_file || vma_is_anon_shmem(vma)) {
>  		error = replace_anon_vma_name(vma, anon_name);
> diff --git a/mm/mlock.c b/mm/mlock.c
> index 479e09d0994c..06bdfab83b58 100644
> --- a/mm/mlock.c
> +++ b/mm/mlock.c
> @@ -387,6 +387,7 @@ static void mlock_vma_pages_range(struct vm_area_struct *vma,
>  	 */
>  	if (newflags & VM_LOCKED)
>  		newflags |= VM_IO;
> +	vma_start_write(vma);
>  	vm_flags_reset_once(vma, newflags);
>  
>  	lru_add_drain();
> @@ -461,9 +462,9 @@ static int mlock_fixup(struct vma_iterator *vmi, struct vm_area_struct *vma,
>  	 * It's okay if try_to_unmap_one unmaps a page just after we
>  	 * set VM_LOCKED, populate_vma_page_range will bring it back.
>  	 */
> -
>  	if ((newflags & VM_LOCKED) && (oldflags & VM_LOCKED)) {
>  		/* No work to do, and mlocking twice would be wrong */
> +		vma_start_write(vma);
>  		vm_flags_reset(vma, newflags);
>  	} else {
>  		mlock_vma_pages_range(vma, start, end, newflags);
> diff --git a/mm/mprotect.c b/mm/mprotect.c
> index 3aef1340533a..362e190a8f81 100644
> --- a/mm/mprotect.c
> +++ b/mm/mprotect.c
> @@ -657,6 +657,7 @@ mprotect_fixup(struct vma_iterator *vmi, struct mmu_gather *tlb,
>  	 * vm_flags and vm_page_prot are protected by the mmap_lock
>  	 * held in write mode.
>  	 */
> +	vma_start_write(vma);
>  	vm_flags_reset(vma, newflags);
>  	if (vma_wants_manual_pte_write_upgrade(vma))
>  		mm_cp_flags |= MM_CP_TRY_CHANGE_WRITABLE;
> -- 
> 2.41.0.585.gd2178a4bd4-goog
>