linux-kernel - Re: [PATCH 10/23] userfaultfd: add new syscall to provide memory externalization

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Date:	Tue, 23 Jun 2015 12:00:19 -0700
From:	Dave Hansen <dave.hansen@...el.com>
To:	Andrea Arcangeli <aarcange@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	qemu-devel@...gnu.org, kvm@...r.kernel.org,
	linux-api@...r.kernel.org
CC:	Pavel Emelyanov <xemul@...allels.com>,
	Sanidhya Kashyap <sanidhya.gatech@...il.com>,
	zhang.zhanghailiang@...wei.com,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Kirill A. Shutemov" <kirill@...temov.name>,
	Andres Lagar-Cavilla <andreslc@...gle.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Rik van Riel <riel@...hat.com>, Mel Gorman <mgorman@...e.de>,
	Andy Lutomirski <luto@...capital.net>,
	Hugh Dickins <hughd@...gle.com>,
	Peter Feiner <pfeiner@...gle.com>,
	"Dr. David Alan Gilbert" <dgilbert@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	"Huangpeng (Peter)" <peter.huangpeng@...wei.com>
Subject: Re: [PATCH 10/23] userfaultfd: add new syscall to provide memory
 externalization

On 05/14/2015 10:31 AM, Andrea Arcangeli wrote:
> +static int userfaultfd_wake_function(wait_queue_t *wq, unsigned mode,
> +				     int wake_flags, void *key)
> +{
> +	struct userfaultfd_wake_range *range = key;
> +	int ret;
> +	struct userfaultfd_wait_queue *uwq;
> +	unsigned long start, len;
> +
> +	uwq = container_of(wq, struct userfaultfd_wait_queue, wq);
> +	ret = 0;
> +	/* don't wake the pending ones to avoid reads to block */
> +	if (uwq->pending && !ACCESS_ONCE(uwq->ctx->released))
> +		goto out;
> +	/* len == 0 means wake all */
> +	start = range->start;
> +	len = range->len;
> +	if (len && (start > uwq->address || start + len <= uwq->address))
> +		goto out;
> +	ret = wake_up_state(wq->private, mode);
> +	if (ret)
> +		/* wake only once, autoremove behavior */
> +		list_del_init(&wq->task_list);
> +out:
> +	return ret;
> +}
...
> +static __always_inline int validate_range(struct mm_struct *mm,
> +					  __u64 start, __u64 len)
> +{
> +	__u64 task_size = mm->task_size;
> +
> +	if (start & ~PAGE_MASK)
> +		return -EINVAL;
> +	if (len & ~PAGE_MASK)
> +		return -EINVAL;
> +	if (!len)
> +		return -EINVAL;
> +	if (start < mmap_min_addr)
> +		return -EINVAL;
> +	if (start >= task_size)
> +		return -EINVAL;
> +	if (len > task_size - start)
> +		return -EINVAL;
> +	return 0;
> +}

Hey Andrea,

Down in userfaultfd_wake_function(), it looks like you intended for a
len=0 to mean "wake all".  But the validate_range() that we do from
userspace has a !len check in it, which keeps us from passing a len=0 in
from userspace.

Was that "wake all" for some internal use, or is the check too strict?

I was trying to use the wake ioctl after an madvise() (as opposed to
filling things in using a userfd copy).
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