Re: [Qemu-devel] [PATCH 08/10] userfaultfd: add new syscall to provide memory externalization

[Andy Lutomirski <luto@amacapital.net>]

On 07/02/2014 09:50 AM, Andrea Arcangeli wrote:
> Once an userfaultfd is created MADV_USERFAULT regions talks through
> the userfaultfd protocol with the thread responsible for doing the
> memory externalization of the process.
> 
> The protocol starts by userland writing the requested/preferred
> USERFAULT_PROTOCOL version into the userfault fd (64bit write), if
> kernel knows it, it will ack it by allowing userland to read 64bit
> from the userfault fd that will contain the same 64bit
> USERFAULT_PROTOCOL version that userland asked. Otherwise userfault
> will read __u64 value -1ULL (aka USERFAULTFD_UNKNOWN_PROTOCOL) and it
> will have to try again by writing an older protocol version if
> suitable for its usage too, and read it back again until it stops
> reading -1ULL. After that the userfaultfd protocol starts.
> 
> The protocol consists in the userfault fd reads 64bit in size
> providing userland the fault addresses. After a userfault address has
> been read and the fault is resolved by userland, the application must
> write back 128bits in the form of [ start, end ] range (64bit each)
> that will tell the kernel such a range has been mapped. Multiple read
> userfaults can be resolved in a single range write. poll() can be used
> to know when there are new userfaults to read (POLLIN) and when there
> are threads waiting a wakeup through a range write (POLLOUT).
> 
> Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>

> +#ifdef CONFIG_PROC_FS
> +static int userfaultfd_show_fdinfo(struct seq_file *m, struct file *f)
> +{
> +	struct userfaultfd_ctx *ctx = f->private_data;
> +	int ret;
> +	wait_queue_t *wq;
> +	struct userfaultfd_wait_queue *uwq;
> +	unsigned long pending = 0, total = 0;
> +
> +	spin_lock(&ctx->fault_wqh.lock);
> +	list_for_each_entry(wq, &ctx->fault_wqh.task_list, task_list) {
> +		uwq = container_of(wq, struct userfaultfd_wait_queue, wq);
> +		if (uwq->pending)
> +			pending++;
> +		total++;
> +	}
> +	spin_unlock(&ctx->fault_wqh.lock);
> +
> +	ret = seq_printf(m, "pending:\t%lu\ntotal:\t%lu\n", pending, total);

This should show the protocol version, too.

> +
> +SYSCALL_DEFINE1(userfaultfd, int, flags)
> +{
> +	int fd, error;
> +	struct file *file;

This looks like it can't be used more than once in a process.  That will
be unfortunate for libraries.  Would it be feasible to either have
userfaultfd claim a range of addresses or for a vma to be explicitly
associated with a userfaultfd?  (In the latter case, giant PROT_NONE
MAP_NORESERVE mappings could be used.)