linux-kernel - Re: [PATCH v2 1/4] mm: introduce get_user_pages

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Date:   Tue, 21 Nov 2017 15:11:14 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Christoph Hellwig <hch@....de>, stable@...r.kernel.org,
        linux-nvdimm@...ts.01.org
Subject: Re: [PATCH v2 1/4] mm: introduce get_user_pages_longterm

On Tue, 14 Nov 2017 11:56:34 -0800 Dan Williams <dan.j.williams@...el.com> wrote:

> Until there is a solution to the dma-to-dax vs truncate problem it is
> not safe to allow long standing memory registrations against
> filesytem-dax vmas. Device-dax vmas do not have this problem and are
> explicitly allowed.
> 
> This is temporary until a "memory registration with layout-lease"
> mechanism can be implemented for the affected sub-systems (RDMA and
> V4L2).
> 
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -1095,6 +1095,70 @@ long get_user_pages(unsigned long start, unsigned long nr_pages,
>  }
>  EXPORT_SYMBOL(get_user_pages);
>  
> +#ifdef CONFIG_FS_DAX
> +/*
> + * This is the same as get_user_pages() in that it assumes we are
> + * operating on the current task's mm, but it goes further to validate
> + * that the vmas associated with the address range are suitable for
> + * longterm elevated page reference counts. For example, filesystem-dax
> + * mappings are subject to the lifetime enforced by the filesystem and
> + * we need guarantees that longterm users like RDMA and V4L2 only
> + * establish mappings that have a kernel enforced revocation mechanism.
> + *
> + * "longterm" == userspace controlled elevated page count lifetime.
> + * Contrast this to iov_iter_get_pages() usages which are transient.
> + */
> +long get_user_pages_longterm(unsigned long start, unsigned long nr_pages,
> +		unsigned int gup_flags, struct page **pages,
> +		struct vm_area_struct **vmas_arg)
> +{
> +	struct vm_area_struct **vmas = vmas_arg;
> +	struct vm_area_struct *vma_prev = NULL;
> +	long rc, i;
> +
> +	if (!pages)
> +		return -EINVAL;
> +
> +	if (!vmas) {
> +		vmas = kzalloc(sizeof(struct vm_area_struct *) * nr_pages,
> +				GFP_KERNEL);
> +		if (!vmas)
> +			return -ENOMEM;
> +	}
>
> ...
>

I'll do this:

--- a/mm/gup.c~mm-introduce-get_user_pages_longterm-fix
+++ a/mm/gup.c
@@ -1120,8 +1120,8 @@ long get_user_pages_longterm(unsigned lo
 		return -EINVAL;
 
 	if (!vmas) {
-		vmas = kzalloc(sizeof(struct vm_area_struct *) * nr_pages,
-				GFP_KERNEL);
+		vmas = kcalloc(nr_pages, sizeof(struct vm_area_struct *),
+			       GFP_KERNEL);
 		if (!vmas)
 			return -ENOMEM;
 	}
_