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

[Lorenzo Stoakes <ljs@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);

Thanks for this!

though, hmm it really feels like you maybe want to add this as a test and
make this a series :)

Andrew is usually fine with adding tests as part of a fix I believe!

>
>
> 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>

The logic does seem sense, it does seem like something of an oversight here
and I can repro the assert (though I have to fix something else first on my
system, patch incoming for that...!)

> ---
> Applies on mm-unstable (404fb4f38e8f).
>
>  fs/proc/task_mmu.c | 1 -
>  1 file changed, 1 deletion(-)
>
> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> index 1e3a15bf46f4e..58938e62154d9 100644
> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -2129,7 +2129,6 @@ static int pagemap_pmd_range_thp(pmd_t *pmdp, unsigned long addr,
>  			flags |= PM_SOFT_DIRTY;
>  		if (pmd_swp_uffd_wp(pmd))
>  			flags |= PM_UFFD_WP;
> -		VM_WARN_ON_ONCE(!pmd_is_migration_entry(pmd));
>  		page = softleaf_to_page(entry);

>  	}
>
> --
> 2.43.0
>

Cheers, Lorenzo