Re: [PATCH v2 2/2] selftests/mm/hmm-tests: test pagemap reads of PMD device-private entries

[Balbir Singh]

On 5/30/26 18:54, Dev Jain wrote:
> To cover pagemap paths scanning PMD entries, add assertions to check
> whether a device-private PMD entry has the correct pagemap information -
> the PM_SWAP bit must be on in the pagemap entry. Before that, we must
> assert through HMM_DMIRROR_SNAPSHOT snapshot that the leaf entry is
> at PMD level and not PTE level.
> 
> Signed-off-by: Dev Jain <dev.jain@xxxxxxx>
> ---
>  tools/testing/selftests/mm/hmm-tests.c | 29 ++++++++++++++++++++++++++
>  1 file changed, 29 insertions(+)
> 
> diff --git a/tools/testing/selftests/mm/hmm-tests.c b/tools/testing/selftests/mm/hmm-tests.c
> index e1c8a679a4cf3..d09d4a9081de1 100644
> --- a/tools/testing/selftests/mm/hmm-tests.c
> +++ b/tools/testing/selftests/mm/hmm-tests.c
> @@ -2276,8 +2276,11 @@ TEST_F(hmm, migrate_anon_huge_fault)
>  	unsigned long npages;
>  	unsigned long size;
>  	unsigned long i;
> +	unsigned char *m;
> +	uint64_t entry;
>  	void *old_ptr;
>  	void *map;
> +	int pagemap_fd;
>  	int *ptr;
>  	int ret;
>  
> @@ -2318,6 +2321,32 @@ TEST_F(hmm, migrate_anon_huge_fault)
>  	for (i = 0, ptr = buffer->mirror; i < size / sizeof(*ptr); ++i)
>  		ASSERT_EQ(ptr[i], i);
>  
> +	if (!hmm_is_coherent_type(variant->device_number)) {
> +		ret = hmm_dmirror_cmd(self->fd, HMM_DMIRROR_SNAPSHOT,
> +				      buffer, npages);
> +		ASSERT_EQ(ret, 0);
> +		ASSERT_EQ(buffer->cpages, npages);
> +
> +		m = buffer->mirror;
> +		for (i = 0; i < npages; ++i)
> +			ASSERT_EQ(m[i], HMM_DMIRROR_PROT_DEV_PRIVATE_LOCAL |
> +					HMM_DMIRROR_PROT_WRITE |
> +					HMM_DMIRROR_PROT_PMD);

madvise(..., MADV_HUGEPAGE) is not sufficient to guarantee that the allocation
was indeed converted to THP. Might be worth using the kpageflags interface (but that
requires elevated privileges) and then KPF_THP? Otherwise the HMM_DMIRROR_PROT_PMD
can be a miss from time to time. One other option is not to assert, but to check
and inform?

> +
> +		pagemap_fd = open("/proc/self/pagemap", O_RDONLY);
> +		ASSERT_GE(pagemap_fd, 0);
> +
> +		for (i = 0; i < npages; ++i) {
> +			entry = pagemap_get_entry(pagemap_fd,
> +					(char *)buffer->ptr + i * self->page_size);
> +

If this is a THP entry, do we have valid pagemap entries for offset of i * page_size?

> +			ASSERT_NE(entry & PM_SWAP, 0);
> +			ASSERT_EQ(entry & PM_PRESENT, 0);

Nit: You can use PAGEMAP_PRESENT()

> +		}
> +
> +		close(pagemap_fd);
> +	}
> +
>  	/* Fault pages back to system memory and check them. */
>  	for (i = 0, ptr = buffer->ptr; i < size / sizeof(*ptr); ++i)
>  		ASSERT_EQ(ptr[i], i);

Balbir Singh