Re: [PATCH] hmm-tests: Fix migrate_dirty_page test

["Mika Penttilä" <mpenttil@redhat.com>]

On 13.9.2022 8.22, Alistair Popple wrote:
> As noted by John Hubbard the original test relied on side effects of the
> implementation of migrate_vma_setup() to detect if pages had been
> swapped to disk or not. This is subject to change in future so
> explicitly check for swap entries via pagemap instead. Fix a spelling
> mistake while we're at it.
> 
> Signed-off-by: Alistair Popple <apopple@nvidia.com>
> Fixes: 5cc88e844e87 ("selftests/hmm-tests: add test for dirty bits")
> ---
>   tools/testing/selftests/vm/hmm-tests.c | 50 +++++++++++++++++++++++---
>   1 file changed, 46 insertions(+), 4 deletions(-)
> 
> diff --git a/tools/testing/selftests/vm/hmm-tests.c b/tools/testing/selftests/vm/hmm-tests.c
> index 70fdb49b59ed..b5f6a7dc1f12 100644
> --- a/tools/testing/selftests/vm/hmm-tests.c
> +++ b/tools/testing/selftests/vm/hmm-tests.c
> @@ -1261,9 +1261,47 @@ static int destroy_cgroup(void)
>   	return 0;
>   }
>   
> +static uint64_t get_pfn(int fd, uint64_t ptr)
> +{
> +	uint64_t pfn;
> +	int ret;
> +
> +	ret = pread(fd, &pfn, sizeof(ptr),
> +		(uint64_t) ptr / getpagesize() * sizeof(ptr));
> +	if (ret != sizeof(ptr))
> +		return 0;
> +
> +	return pfn;
> +}
> +
> +#define PAGEMAP_SWAPPED (1ULL << 62)
> +
> +/* Returns true if at least one page in the range is on swap */
> +static bool pages_swapped(void *ptr, unsigned long pages)
> +{
> +	uint64_t pfn;
> +	int fd = open("/proc/self/pagemap", O_RDONLY);
> +	unsigned long i;
> +
> +	if (fd < 0)
> +		return false;
> +
> +	for (i = 0; i < pages; i++) {
> +		pfn = get_pfn(fd, (uint64_t) ptr + i * getpagesize());
> +
> +		if (pfn & PAGEMAP_SWAPPED) {
> +			close(fd);
> +			return true;
> +		}
> +	}
> +
> +	close(fd);
> +	return false;
> +}
> +
>   /*
>    * Try and migrate a dirty page that has previously been swapped to disk. This
> - * checks that we don't loose dirty bits.
> + * checks that we don't lose dirty bits.
>    */
>   TEST_F(hmm, migrate_dirty_page)
>   {
> @@ -1300,6 +1338,10 @@ TEST_F(hmm, migrate_dirty_page)
>   
>   	ASSERT_FALSE(write_cgroup_param(cgroup, "memory.reclaim", 1UL<<30));
>   
> +	/* Make sure at least some pages got paged to disk. */
> +	if (!pages_swapped(buffer->ptr, npages))
> +		SKIP(return, "Pages weren't swapped when they should have been");
> +
>   	/* Fault pages back in from swap as clean pages */
>   	for (i = 0, ptr = buffer->ptr; i < size / sizeof(*ptr); ++i)
>   		tmp += ptr[i];
> @@ -1309,10 +1351,10 @@ TEST_F(hmm, migrate_dirty_page)
>   		ptr[i] = i;
>   
>   	/*
> -	 * Attempt to migrate memory to device, which should fail because
> -	 * hopefully some pages are backed by swap storage.
> +	 * Attempt to migrate memory to device. This might fail if some pages
> +	 * are/were backed by swap but that's ok.
>   	 */
> -	ASSERT_TRUE(hmm_migrate_sys_to_dev(self->fd, buffer, npages));
> +	hmm_migrate_sys_to_dev(self->fd, buffer, npages);
>   
>   	ASSERT_FALSE(write_cgroup_param(cgroup, "memory.reclaim", 1UL<<30));
>   


Reviewed-by: Mika Penttilä <mpenttil@redhat.com>