Re: [PATCH v6 15/20] mm: Change failure of MAP_FIXED to restoring the gap on failure

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

On Tue, Aug 20, 2024 at 07:57:24PM GMT, Liam R. Howlett wrote:
> From: "Liam R. Howlett" <Liam.Howlett@Oracle.com>
>
> Prior to call_mmap(), the vmas that will be replaced need to clear the
> way for what may happen in the call_mmap().  This clean up work includes
> clearing the ptes and calling the close() vm_ops.  Some users do more
> setup than can be restored by calling the vm_ops open() function.  It is
> safer to store the gap in the vma tree in these cases.
>
> That is to say that the failure scenario that existed before the
> MAP_FIXED gap exposure is restored as it is safer than trying to undo a
> partial mapping.
>
> There is also a secondary failure that may occur if there is not enough
> memory to store the gap.  In this case, the vmas are reattached and
> resources freed.  If the system cannot complete the call_mmap() and
> fails to allocate with GFP_KERNEL, then the system will print a warning
> about the failure.
>
> Signed-off-by: Liam R. Howlett <Liam.Howlett@Oracle.com>

Just some nits etc. below, otherwise:

Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>

> ---
>  mm/mmap.c |  3 +--
>  mm/vma.h  | 62 +++++++++++++++++++++++++++++++++++++------------------
>  2 files changed, 43 insertions(+), 22 deletions(-)
>
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 6550d9470d3a..c1b3d17f97be 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -1622,8 +1622,7 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
>  		vm_unacct_memory(charged);
>
>  abort_munmap:
> -	if (vms.nr_pages)
> -		abort_munmap_vmas(&mas_detach, vms.closed_vm_ops);
> +	vms_abort_munmap_vmas(&vms, &mas_detach, vms.closed_vm_ops);
>  	validate_mm(mm);
>  	return error;
>  }
> diff --git a/mm/vma.h b/mm/vma.h
> index 756dd42a6ec4..7618ddbfd2b2 100644
> --- a/mm/vma.h
> +++ b/mm/vma.h
> @@ -82,6 +82,22 @@ int vma_expand(struct vma_iterator *vmi, struct vm_area_struct *vma,
>  int vma_shrink(struct vma_iterator *vmi, struct vm_area_struct *vma,
>  	       unsigned long start, unsigned long end, pgoff_t pgoff);
>
> +static inline int vma_iter_store_gfp(struct vma_iterator *vmi,
> +			struct vm_area_struct *vma, gfp_t gfp)
> +
> +{
> +	if (vmi->mas.status != ma_start &&
> +	    ((vmi->mas.index > vma->vm_start) || (vmi->mas.last < vma->vm_start)))
> +		vma_iter_invalidate(vmi);
> +
> +	__mas_set_range(&vmi->mas, vma->vm_start, vma->vm_end - 1);
> +	mas_store_gfp(&vmi->mas, vma, gfp);
> +	if (unlikely(mas_is_err(&vmi->mas)))
> +		return -ENOMEM;
> +
> +	return 0;
> +}
> +
>  /*
>   * init_vma_munmap() - Initializer wrapper for vma_munmap_struct
>   * @vms: The vma munmap struct
> @@ -136,15 +152,37 @@ static inline void abort_munmap_vmas(struct ma_state *mas_detach, bool closed)
>  	struct vm_area_struct *vma;
>
>  	mas_set(mas_detach, 0);
> -	mas_for_each(mas_detach, vma, ULONG_MAX) {
> +	mas_for_each(mas_detach, vma, ULONG_MAX)
>  		vma_mark_detached(vma, false);
> -		if (closed && vma->vm_ops && vma->vm_ops->open)
> -			vma->vm_ops->open(vma);
> -	}

Yes.

>
>  	__mt_destroy(mas_detach->tree);
>  }
>
> +static inline void vms_abort_munmap_vmas(struct vma_munmap_struct *vms,
> +		struct ma_state *mas_detach, bool closed)

Nitty, but seems strange to comment abort_munmap_vmas() and say that it
undoes any munmap work + frees resources, but not to comment this function.

Also I wonder if, with these changes, abort_munmap_vmas() needs a rename?
Something like reattach_vmas() or something?

> +{
> +	if (!vms->nr_pages)
> +		return;
> +
> +	if (vms->clear_ptes)
> +		return abort_munmap_vmas(mas_detach, vms->closed_vm_ops);
> +
> +	/*
> +	 * Aborting cannot just call the vm_ops open() because they are often
> +	 * not symmetrical and state data has been lost.  Resort to the old
> +	 * failure method of leaving a gap where the MAP_FIXED mapping failed.
> +	 */
> +	if (unlikely(vma_iter_store_gfp(vms->vmi, NULL, GFP_KERNEL))) {
> +		pr_warn_once("%s: (%d) Unable to abort munmap() operation\n",
> +			     current->comm, current->pid);
> +		/* Leaving vmas detached and in-tree may hamper recovery */
> +		abort_munmap_vmas(mas_detach, vms->closed_vm_ops);

OK so I guess rather than trying to do some clever preallocation, we just
warn + get out?

> +	} else {
> +		/* Clean up the insertion of unfortunate the gap */

I'm not sure what this means :P 'unfortunate, the gap, isn't it?'

> +		vms_complete_munmap_vmas(vms, mas_detach);
> +	}
> +}
> +
>  int
>  do_vmi_align_munmap(struct vma_iterator *vmi, struct vm_area_struct *vma,
>  		    struct mm_struct *mm, unsigned long start,
> @@ -297,22 +335,6 @@ static inline struct vm_area_struct *vma_prev_limit(struct vma_iterator *vmi,
>  	return mas_prev(&vmi->mas, min);
>  }
>
> -static inline int vma_iter_store_gfp(struct vma_iterator *vmi,
> -			struct vm_area_struct *vma, gfp_t gfp)
> -{
> -	if (vmi->mas.status != ma_start &&
> -	    ((vmi->mas.index > vma->vm_start) || (vmi->mas.last < vma->vm_start)))
> -		vma_iter_invalidate(vmi);
> -
> -	__mas_set_range(&vmi->mas, vma->vm_start, vma->vm_end - 1);
> -	mas_store_gfp(&vmi->mas, vma, gfp);
> -	if (unlikely(mas_is_err(&vmi->mas)))
> -		return -ENOMEM;
> -
> -	return 0;
> -}
> -
> -
>  /*
>   * These three helpers classifies VMAs for virtual memory accounting.
>   */
> --
> 2.43.0
>