Re: [PATCH] fs/proc/task_mmu: do not warn on seeing non-migration pmd entry

["Oscar Salvador (SUSE)" <osalvador@kernel.org>]

On Fri, May 29, 2026 at 11:17:03AM +0000, Dev Jain wrote:
> pagemap_pmd_range_thp() warns if a non-present PMD is not a migration
> entry. This became false once device-private entries at the PMD level were
> added.
> 
> One can hit the warning by patching hmm-tests.c with the following:
> 
> diff --git a/tools/testing/selftests/mm/hmm-tests.c b/tools/testing/selftests/mm/hmm-tests.c
> index e1c8a679a4cf3..7f0a3384f3c5f 100644
> --- a/tools/testing/selftests/mm/hmm-tests.c
> +++ b/tools/testing/selftests/mm/hmm-tests.c
> @@ -209,6 +209,37 @@ static int hmm_dmirror_cmd(int fd,
>  	return 0;
>  }
> 
> +static int hmm_read_self_pagemap(void *addr, unsigned long npages,
> +				 unsigned long page_size)
> +{
> +	const size_t entry_size = sizeof(uint64_t);
> +	const off_t offset = ((uintptr_t)addr / page_size) * entry_size;
> +	uint64_t *entries;
> +	ssize_t nread;
> +	int fd;
> +
> +	entries = malloc(npages * entry_size);
> +	if (!entries)
> +		return -ENOMEM;
> +
> +	fd = open("/proc/self/pagemap", O_RDONLY);
> +	if (fd < 0) {
> +		free(entries);
> +		return -errno;
> +	}
> +
> +	nread = pread(fd, entries, npages * entry_size, offset);
> +	close(fd);
> +	free(entries);
> +
> +	if (nread < 0)
> +		return -errno;
> +	if ((size_t)nread != npages * entry_size)
> +		return -EIO;
> +
> +	return 0;
> +}
> +
>  static void hmm_buffer_free(struct hmm_buffer *buffer)
>  {
>  	if (buffer == NULL)
> @@ -2314,6 +2345,10 @@ TEST_F(hmm, migrate_anon_huge_fault)
>  	ASSERT_EQ(ret, 0);
>  	ASSERT_EQ(buffer->cpages, npages);
> 
> +	/* Exercise pagemap on a PMD device-private entry. */
> +	ret = hmm_read_self_pagemap(buffer->ptr, npages, self->page_size);
> +	ASSERT_EQ(ret, 0);
> +
>  	/* Check what the device read. */
>  	for (i = 0, ptr = buffer->mirror; i < size / sizeof(*ptr); ++i)
>  		ASSERT_EQ(ptr[i], i);
> 
> 
> Therefore, remove the stale migration-only assertion.
> 
> Fixes: a30b48bf1b24 ("mm/migrate_device: implement THP migration of zone device pages")
> Cc: stable@vger.kernel.org
> Signed-off-by: Dev Jain <dev.jain@arm.com>

LGTM,

Reviewed-by: Oscar Salvador (SUSE) <osalvador@kernel.org>



-- 
Oscar Salvador
SUSE Labs