Re: [PATCH v4] mm: introduce reference pages

[David Hildenbrand <david@redhat.com>]

> +SYSCALL_DEFINE2(refpage_create, const void *__user, content, unsigned long,
> +		flags)
> +{
> +	unsigned long content_addr = (unsigned long)content;
> +	struct page *userpage;
> +	struct refpage_private_data *private_data;
> +	int fd;
> +
> +	if (flags != 0)
> +		return -EINVAL;
> +
> +	if ((content_addr & (PAGE_SIZE - 1)) != 0 ||
> +	    get_user_pages(content_addr, 1, 0, &userpage, 0) != 1)
> +		return -EFAULT;
> +
> +	private_data = kzalloc(sizeof(struct refpage_private_data), GFP_KERNEL);
> +	if (!private_data) {
> +		put_page(userpage);
> +		return -ENOMEM;
> +	}
> +
> +	private_data->refpage = alloc_page(GFP_KERNEL);
> +	if (!private_data->refpage) {
> +		kfree(private_data);
> +		put_page(userpage);
> +		return -ENOMEM;
> +	}
> +
> +	copy_highpage(private_data->refpage, userpage);
> +	arch_prep_refpage_private_data(private_data);
> +	put_page(userpage);
> +
> +	fd = anon_inode_getfd("[refpage]", &refpage_file_operations,
> +			      private_data, O_RDONLY | O_CLOEXEC);
> +	if (fd < 0)
> +		put_refpage_private_data(private_data);
> +
> +	return fd;
> +}
> 

Hi,

some questions:

1. is there any upper limit or can a simple process effectively flood 
the system with alloc_page(GFP_KERNEL)? (does ulimit -n apply or is it 
/proc/sys/fs/file-max)

2. Shouldn't there be a GFP_ACCOUNT or am I missing something important?

3. How does this interact with MADV_DONTNEED? I assume we'll be able to 
zap the mapped refpage and on refault, we'll map in the refpage again, 
correct?

-- 
Thanks,

David / dhildenb