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: [email protected]
> Signed-off-by: Dev Jain <[email protected]>
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
