On Wed, 14 Jan 2026, Krzysztof Karas <[email protected]> wrote:
> Migration testing in i915 uses current task's address space to
> allocate new userspace mapping, without registering real user
> for that address space in mm_struct.
>
> It was observed that mm->mm_users would occasionally drop to 0
> during the test, which reaped userspace mappings, further
> leading to failures upon reading from userland memory.
>
> Prevent this by artificially increasing mm_users counter for the
> duration of the test.
>
> Closes: https://gitlab.freedesktop.org/drm/i915/kernel/-/issues/14204
> Signed-off-by: Krzysztof Karas <[email protected]>
> ---
> drivers/gpu/drm/i915/gem/selftests/i915_gem_mman.c | 8 ++++++++
> 1 file changed, 8 insertions(+)
>
> diff --git a/drivers/gpu/drm/i915/gem/selftests/i915_gem_mman.c
> b/drivers/gpu/drm/i915/gem/selftests/i915_gem_mman.c
> index 032bc2f17685..2f3104c17324 100644
> --- a/drivers/gpu/drm/i915/gem/selftests/i915_gem_mman.c
> +++ b/drivers/gpu/drm/i915/gem/selftests/i915_gem_mman.c
> @@ -1193,6 +1193,13 @@ static int __igt_mmap_migrate(struct
> intel_memory_region **placements,
> if (IS_ERR(mock_file))
> return PTR_ERR(mock_file);
>
> + /*
> + * Artificially increase mm_users for current->mm to ensure the kernel
> + * does not try to clean up the userspace mappings of the current task
> + * during the test.
> + */
> + atomic_inc(¤t->mm->mm_users);
We have no business modifying the guts of mm directly, even if C doesn't
have means for enforcing such boundaries. There just aren't that many
places in the kernel that read, let alone modify, ->mm_users directly.
A pair of mmget_not_zero() and mmput() seem to be a more commonly used
alternative.
BR,
Jani.
> +
> addr = igt_mmap_offset_with_file(i915, offset, obj->base.size,
> PROT_WRITE, MAP_SHARED, mock_file);
> if (IS_ERR_VALUE(addr)) {
> @@ -1303,6 +1310,7 @@ static int __igt_mmap_migrate(struct
> intel_memory_region **placements,
>
> out_addr:
> vm_munmap(addr, obj->base.size);
> + atomic_dec(¤t->mm->mm_users);
>
> out_fput:
> fput(mock_file);
> --
> 2.43.0
--
Jani Nikula, Intel
