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

[Ackerley Tng]

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);

  	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? 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.