[PATCH] selftests/mm: fix uffd move-pmd test crash on 64K base page systems

[PATCH] selftests/mm: fix uffd move-pmd test crash on 64K base page systems

[Zenghui Yu]

From: "Zenghui Yu (Huawei)" <zenghui.yu@linux.dev>

uffd_move_pmd_test()/uffd_move_test_common() splits the test area into
chunks of chunk_size and moves them one by one.  On arm64 with 64K base
pages the chunk_size (read_pmd_pagesize()) is 512MB, which is larger than
the whole 32MB (UFFD_TEST_MEM_SIZE) test area:

  step_count = nr_pages / (chunk_size / page_size) == 0

Because chunk_size > page_size, the test then aligns both areas up to
512MB, which points far outside of the 32MB mappings, and decrements
step_count, which underflows to ULONG_MAX.  The verification loop then
runs away and the first dereference of area_src faults with a SIGSEGV.

Skip the test when a single chunk cannot fit into the test area.

Fixes: a2bf6a9ca805 ("selftests/mm: add UFFDIO_MOVE ioctl test")
Signed-off-by: Zenghui Yu (Huawei) <zenghui.yu@linux.dev>
---
 tools/testing/selftests/mm/uffd-unit-tests.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/tools/testing/selftests/mm/uffd-unit-tests.c b/tools/testing/selftests/mm/uffd-unit-tests.c
index ef9b3956bdcf..2a97ba1d6feb 100644
--- a/tools/testing/selftests/mm/uffd-unit-tests.c
+++ b/tools/testing/selftests/mm/uffd-unit-tests.c
@@ -1937,6 +1937,11 @@ uffd_move_test_common(uffd_global_test_opts_t *gopts,
 
 	args.gopts = gopts;
 
+	if (chunk_size > gopts->nr_pages * gopts->page_size) {
+		uffd_test_skip("chunk size does not fit in the test area");
+		return;
+	}
+
 	/* Prevent source pages from being mapped more than once */
 	if (madvise(gopts->area_src, gopts->nr_pages * gopts->page_size, MADV_DONTFORK))
 		err("madvise(MADV_DONTFORK) failure");
-- 
2.53.0

Re: [PATCH] selftests/mm: fix uffd move-pmd test crash on 64K base page systems

[Sarthak Sharma]

Hi Zenghui!

On 10/11/26 10:05 PM, Zenghui Yu wrote:
> From: "Zenghui Yu (Huawei)" <zenghui.yu@linux.dev>
> 
> uffd_move_pmd_test()/uffd_move_test_common() splits the test area into
> chunks of chunk_size and moves them one by one.  On arm64 with 64K base
> pages the chunk_size (read_pmd_pagesize()) is 512MB, which is larger than
> the whole 32MB (UFFD_TEST_MEM_SIZE) test area:
> 
>   step_count = nr_pages / (chunk_size / page_size) == 0
> 
> Because chunk_size > page_size, the test then aligns both areas up to
> 512MB, which points far outside of the 32MB mappings, and decrements
> step_count, which underflows to ULONG_MAX.  The verification loop then
> runs away and the first dereference of area_src faults with a SIGSEGV.
> 
> Skip the test when a single chunk cannot fit into the test area.
> 
> Fixes: a2bf6a9ca805 ("selftests/mm: add UFFDIO_MOVE ioctl test")
> Signed-off-by: Zenghui Yu (Huawei) <zenghui.yu@linux.dev>
> ---

I run the test on a 64k kernel. Before the patch, a segmentation fault
occurred, which was fixed by the patch. So:

Tested-by: Sarthak Sharma <sarthak.sharma@arm.com>

Though Sashiko has a few nits [1] you might take a look at.

[1]
https://sashiko.dev/#/patchset/20261011163527.95988-1-zenghui.yu%40linux.dev

Re: [PATCH] selftests/mm: fix uffd move-pmd test crash on 64K base page systems

[Jose A. Perez de Azpillaga]

On Mon, Oct 12, 2026 at 12:35:27AM +0800, Zenghui Yu wrote:
> From: "Zenghui Yu (Huawei)" <zenghui.yu@linux.dev>
> 
> uffd_move_pmd_test()/uffd_move_test_common() splits the test area into
> chunks of chunk_size and moves them one by one.  On arm64 with 64K base
> pages the chunk_size (read_pmd_pagesize()) is 512MB, which is larger than
> the whole 32MB (UFFD_TEST_MEM_SIZE) test area:
> 
>   step_count = nr_pages / (chunk_size / page_size) == 0
> 
> Because chunk_size > page_size, the test then aligns both areas up to
> 512MB, which points far outside of the 32MB mappings, and decrements
> step_count, which underflows to ULONG_MAX.  The verification loop then
> runs away and the first dereference of area_src faults with a SIGSEGV.
> 
> Skip the test when a single chunk cannot fit into the test area.
> 
> Fixes: a2bf6a9ca805 ("selftests/mm: add UFFDIO_MOVE ioctl test")
> Signed-off-by: Zenghui Yu (Huawei) <zenghui.yu@linux.dev>
> ---
>  tools/testing/selftests/mm/uffd-unit-tests.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/tools/testing/selftests/mm/uffd-unit-tests.c b/tools/testing/selftests/mm/uffd-unit-tests.c
> index ef9b3956bdcf..2a97ba1d6feb 100644
> --- a/tools/testing/selftests/mm/uffd-unit-tests.c
> +++ b/tools/testing/selftests/mm/uffd-unit-tests.c
> @@ -1937,6 +1937,11 @@ uffd_move_test_common(uffd_global_test_opts_t *gopts,
>  
>  	args.gopts = gopts;
>  
> +	if (chunk_size > gopts->nr_pages * gopts->page_size) {
> +		uffd_test_skip("chunk size does not fit in the test area");
> +		return;
> +	}
> +
>  	/* Prevent source pages from being mapped more than once */
>  	if (madvise(gopts->area_src, gopts->nr_pages * gopts->page_size, MADV_DONTFORK))
>  		err("madvise(MADV_DONTFORK) failure");
> -- 
> 2.53.0
> 
> 

LGTM.

Reviewed-by: Jose A. Perez de Azpillaga <azpijr@gmail.com>

-- 
cheers,
jose a. p-a

Re: [PATCH] selftests/mm: fix uffd move-pmd test crash on 64K base page systems

[David Hildenbrand (Arm)]

On 10/11/26 18:35, Zenghui Yu wrote:
> From: "Zenghui Yu (Huawei)" <zenghui.yu@linux.dev>
> 
> uffd_move_pmd_test()/uffd_move_test_common() splits the test area into
> chunks of chunk_size and moves them one by one.  On arm64 with 64K base
> pages the chunk_size (read_pmd_pagesize()) is 512MB, which is larger than
> the whole 32MB (UFFD_TEST_MEM_SIZE) test area:
> 
>   step_count = nr_pages / (chunk_size / page_size) == 0
> 
> Because chunk_size > page_size, the test then aligns both areas up to
> 512MB, which points far outside of the 32MB mappings, and decrements
> step_count, which underflows to ULONG_MAX.  The verification loop then
> runs away and the first dereference of area_src faults with a SIGSEGV.
> 
> Skip the test when a single chunk cannot fit into the test area.

That leaves 64k untested, though? Why not adjust the chunk size accordingly?

-- 
Cheers,

David

Re: [PATCH] selftests/mm: fix uffd move-pmd test crash on 64K base page systems

[Mike Rapoport]

On Tue, Oct 13, 2026 at 09:39:54AM +0200, David Hildenbrand (Arm) wrote:
> On 10/11/26 18:35, Zenghui Yu wrote:
> > From: "Zenghui Yu (Huawei)" <zenghui.yu@linux.dev>
> > 
> > uffd_move_pmd_test()/uffd_move_test_common() splits the test area into
> > chunks of chunk_size and moves them one by one.  On arm64 with 64K base
> > pages the chunk_size (read_pmd_pagesize()) is 512MB, which is larger than
> > the whole 32MB (UFFD_TEST_MEM_SIZE) test area:
> > 
> >   step_count = nr_pages / (chunk_size / page_size) == 0
> > 
> > Because chunk_size > page_size, the test then aligns both areas up to
> > 512MB, which points far outside of the 32MB mappings, and decrements
> > step_count, which underflows to ULONG_MAX.  The verification loop then
> > runs away and the first dereference of area_src faults with a SIGSEGV.
> > 
> > Skip the test when a single chunk cannot fit into the test area.
> 
> That leaves 64k untested, though? Why not adjust the chunk size accordingly?

To test anything PMD on arm64 with 64k pages we need to adjust
UFFD_TEST_MEM_SIZE to have at least 4 pages, i.e 2G :)

Maybe we want UFFD_TEST_MEM_SIZE to scale with the page size?
 
> -- 
> Cheers,
> 
> David

-- 
Sincerely yours,
Mike.

Re: [PATCH] selftests/mm: fix uffd move-pmd test crash on 64K base page systems

[David Hildenbrand (Arm)]

On 10/13/26 12:33, Mike Rapoport wrote:
> On Tue, Oct 13, 2026 at 09:39:54AM +0200, David Hildenbrand (Arm) wrote:
>> On 10/11/26 18:35, Zenghui Yu wrote:
>>> From: "Zenghui Yu (Huawei)" <zenghui.yu@linux.dev>
>>>
>>> uffd_move_pmd_test()/uffd_move_test_common() splits the test area into
>>> chunks of chunk_size and moves them one by one.  On arm64 with 64K base
>>> pages the chunk_size (read_pmd_pagesize()) is 512MB, which is larger than
>>> the whole 32MB (UFFD_TEST_MEM_SIZE) test area:
>>>
>>>   step_count = nr_pages / (chunk_size / page_size) == 0
>>>
>>> Because chunk_size > page_size, the test then aligns both areas up to
>>> 512MB, which points far outside of the 32MB mappings, and decrements
>>> step_count, which underflows to ULONG_MAX.  The verification loop then
>>> runs away and the first dereference of area_src faults with a SIGSEGV.
>>>
>>> Skip the test when a single chunk cannot fit into the test area.
>>
>> That leaves 64k untested, though? Why not adjust the chunk size accordingly?
> 
> To test anything PMD on arm64 with 64k pages we need to adjust
> UFFD_TEST_MEM_SIZE to have at least 4 pages, i.e 2G :)
> 
> Maybe we want UFFD_TEST_MEM_SIZE to scale with the page size?

That was my thought. If you have 64k you better also have plenty of memory :)

-- 
Cheers,

David