Re: [PATCH v7 00/42] guest_memfd: In-place conversion support

[Sean Christopherson]

On Wed, Jun 10, 2026, Ackerley Tng wrote:
> Sean Christopherson <seanjc@xxxxxxxxxx> writes:
> 
> > On Thu, Jun 04, 2026, Ackerley Tng wrote:
> >> Sean Christopherson <seanjc@xxxxxxxxxx> writes:
> >> >> + KVM: selftests: Test conversion with elevated page refcount
> >> >>     + Askar pointed out that soon vmsplice may not pin pages. Should I
> >> >>       pin pages through CONFIG_GUP_TEST like in [2]? I prefer not to
> >> >>       take a dependency on CONFIG_GUP_TEST.
> >> >
> >> > I'm not exactly excited about taking a dependency on CONFIG_GUP_TEST either, but
> >> > it probably is the least awful choice.  E.g. KVM also pins pages is certain flows,
> >> > but we're _also_ actively working to remove the need to pin.
> >> >
> >> > Hmm, maybe IORING_REGISTER_PBUF_RING?  AFAICT, it's almost literally a "pin user
> >> > memory" syscall.
> >> >
> >>
> >> Hmm that takes a dependency on io_uring, which isn't always compiled
> >> in. Between CONFIG_IO_URING and CONFIG_GUP_TEST, I'd rather
> >> CONFIG_GUP_TEST.
> >
> > Or try both?  If it's not a ridiculous amount of work.
> 
> CONFIG_GUP_TEST was tried in [1]
> 
> [1] https://lore.kernel.org/all/baa8838f623102931e755cf34c86314b305af49c.1747264138.git.ackerleytng@xxxxxxxxxx/
> 
> It looks like this
> 
>   static void pin_pages(void *vaddr, uint64_t size)
>   {
>   	const struct pin_longterm_test args = {
>   		.addr = (uint64_t)vaddr,
>   		.size = size,
>   		.flags = PIN_LONGTERM_TEST_FLAG_USE_WRITE,
>   	};
> 
>   	gup_test_fd = open("/sys/kernel/debug/gup_test", O_RDWR);
>   	TEST_REQUIRE(gup_test_fd > 0);

Use __open_path_or_exit().  I also think it makes sent to make these available
to all KVM selftests, there are probably other testcases that could utilize page
pinning.

>   	TEST_ASSERT_EQ(ioctl(gup_test_fd, PIN_LONGTERM_TEST_START, &args), 0);
>   }
> 
>   static void unpin_pages(void)
>   {
>   	TEST_ASSERT_EQ(ioctl(gup_test_fd, PIN_LONGTERM_TEST_STOP), 0);
>   }
> 
> So in the test I'll call pin_pages(), then try to convert, see that it
> fails with EAGAIN and reports the expected error_offset, then I call
> unpin_pages(), then I convert again and expect success.
> 
> Are you uncomfortable with the CONFIG_GUP_TEST interface?

No, my concern is/was the potential for leaking pages if the test fails/crashes,
but it looks gup_test_release() ensures all pins are dropped when the file is
released, so that should be a non-issue.

> What would you like me to try with CONFIG_IO_URING? I'm thinking that the
> main difference between the two is just down to which non-default CONFIG
> option we want to take for guest_memfd tests.