[CRIU] [RFC PATCH] Try to include userfaultfd with criu

[Adrian Reber]

On Tue, Nov 10, 2015 at 03:34:07PM +0300, Pavel Emelyanov wrote:
[...]
> >>> As soon as the process is restored it blocks on the first memory access
> >>> and waits for pages being transferred by userfaultfd.
> >>>
> >>> To handle the required pages a new criu command has been added. The restore
> >>> works now like this:
> >>>
> >>>   criu restore -D /tmp/3 -j -v4
> >>>
> >>> This hangs after the restored process is running and needs:
> >>>
> >>>   criu uffd -v4 -D /tmp/3/
> >>
> >> So the plan for uffd action is to start a daemon that would read
> >> pages from images and put them into some uffd descriptor, right?
> > 
> > Yes, I have the normal criu restore process running now like this:
> > 
> > $ ./criu restore -D /tmp/3 -j -v4 --lazy-page
> 
> Cool :)
> 
> > and it hangs on the uffd file descriptor until criu uffd daemon is
> > running:
> > 
> > $ ./criu uffd -v4 -D /tmp/3/
> 
> And how does criu restore "knows" where to look for the uffd ... server?

That is currently hard-coded to: /tmp/userfault.socket

> >>> +
> >>> +static int ud_open()
> >>> +{
> >>> +	int udfd;
> >>> +	int newfd;
> >>> +
> >>> +	if ((udfd = client_conn()) < 0) {
> >>> +		pr_err("unix domain socket connection error");
> >>> +		return (-1);
> >>> +	}
> >>> +
> >>> +	newfd = recv_fd(udfd);
> >>> +	close(udfd);
> >>> +
> >>> +	return newfd;
> >>> +}
> >>> +
> >>> +
> >>> +static unsigned long find_page(unsigned long addr)
> >>
> >> There's a seek_pagemap_page() call for this type of operation. One problem with
> >> it is that it only works sequentially, i.e. when subsequent calls to it pass
> >> constantly increasing addresses.
> >>
> >> I'd suggest fixing this seek_pagemap_page() to overcome this limitation and use
> >> one here. This would also be much faster than readdir() and open_page_read() for
> >> every single page.
> > 
> > I think I almost have this figured out. I am now doing:
> > 
> > pr->get_pagemap(pr, &iov);
> > seek_pagemap_page(pr, addr, true);
> > pr->read_page(pr, addr, buf);
> > 
> > As this only works for the first page I introduced a rewind_pagemap()
> > function with:
> > 
> > lseek(img_raw_fd(pr->pi), 0, SEEK_SET);
> > 
> > Unfortunately the next call to get_pagemap() now returns 0 instead of
> > the usual 1. So somehow I am missing something which is needed to rewind
> > the pagemap reading. Any idea how to do this better/correctly?
> 
> The get_pagemap() reads pagemap entry from the image file which shows
> where in the virtual memory the corresponding pages should placed. Then
> ->read_page() reads the page.
> 
> Your aim is to read arbitrary page by vaddr. With the current code the
> only way to do it is by scanning the pagemap starting from zero or last
> address.
> 
> > I can also post my current uffd patch if that makes discussion easier...
> 
> Sure!

I am sending out the current patch in the next few minutes.

		Adrian