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

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 <redacted>

+#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.)

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