linux-kernel - Re: [PATCH v2 3/3] selftests/mm: fix va_high_addr_switch.sh failure on x86

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Message-ID: <22a9dd3e-0755-4f7f-a59c-a79a52871f56@redhat.com>
Date: Mon, 8 Sep 2025 15:09:24 +0200
From: David Hildenbrand <david@...hat.com>
To: Chunyu Hu <chuhu@...hat.com>, akpm@...ux-foundation.org,
 shuah@...nel.org, linux-mm@...ck.org
Cc: linux-kselftest@...r.kernel.org, linux-kernel@...r.kernel.org,
 lorenzo.stoakes@...cle.com, Liam.Howlett@...cle.com, vbabka@...e.cz,
 rppt@...nel.org, surenb@...gle.com, mhocko@...e.com
Subject: Re: [PATCH v2 3/3] selftests/mm: fix va_high_addr_switch.sh failure
 on x86_64

On 08.09.25 14:47, Chunyu Hu wrote:
> The test will fail as below on x86_64 with cpu la57 support (will skip if
> no la57 support). Note, the test requries nr_hugepages to be set first.
> 
>    # running bash ./va_high_addr_switch.sh
>    # -------------------------------------
>    # mmap(addr_switch_hint - pagesize, pagesize): 0x7f55b60fa000 - OK
>    # mmap(addr_switch_hint - pagesize, (2 * pagesize)): 0x7f55b60f9000 - OK
>    # mmap(addr_switch_hint, pagesize): 0x800000000000 - OK
>    # mmap(addr_switch_hint, 2 * pagesize, MAP_FIXED): 0x800000000000 - OK
>    # mmap(NULL): 0x7f55b60f9000 - OK
>    # mmap(low_addr): 0x40000000 - OK
>    # mmap(high_addr): 0x1000000000000 - OK
>    # mmap(high_addr) again: 0xffff55b6136000 - OK
>    # mmap(high_addr, MAP_FIXED): 0x1000000000000 - OK
>    # mmap(-1): 0xffff55b6134000 - OK
>    # mmap(-1) again: 0xffff55b6132000 - OK
>    # mmap(addr_switch_hint - pagesize, pagesize): 0x7f55b60fa000 - OK
>    # mmap(addr_switch_hint - pagesize, 2 * pagesize): 0x7f55b60f9000 - OK
>    # mmap(addr_switch_hint - pagesize/2 , 2 * pagesize): 0x7f55b60f7000 - OK
>    # mmap(addr_switch_hint, pagesize): 0x800000000000 - OK
>    # mmap(addr_switch_hint, 2 * pagesize, MAP_FIXED): 0x800000000000 - OK
>    # mmap(NULL, MAP_HUGETLB): 0x7f55b5c00000 - OK
>    # mmap(low_addr, MAP_HUGETLB): 0x40000000 - OK
>    # mmap(high_addr, MAP_HUGETLB): 0x1000000000000 - OK
>    # mmap(high_addr, MAP_HUGETLB) again: 0xffff55b5e00000 - OK
>    # mmap(high_addr, MAP_FIXED | MAP_HUGETLB): 0x1000000000000 - OK
>    # mmap(-1, MAP_HUGETLB): 0x7f55b5c00000 - OK
>    # mmap(-1, MAP_HUGETLB) again: 0x7f55b5a00000 - OK
>    # mmap(addr_switch_hint - pagesize, 2*hugepagesize, MAP_HUGETLB): 0x800000000000 - FAILED
>    # mmap(addr_switch_hint , 2*hugepagesize, MAP_FIXED | MAP_HUGETLB): 0x800000000000 - OK
>    # [FAIL]
> 
> addr_switch_hint is defined as DFEFAULT_MAP_WINDOW in the failed test (for
> x86_64, DFEFAULT_MAP_WINDOW is defined as (1UL<<47) - pagesize) in 64 bit.
> 
> Before commit cc92882ee218 ("mm: drop hugetlb_get_unmapped_area{_*}
> functions"), for x86_64 hugetlb_get_unmapped_area() is handled in arch code
> arch/x86/mm/hugetlbpage.c and addr is checked with map_address_hint_valid()
> after align with 'addr &= huge_page_mask(h)' which is a round down way, and
> it will fail the check because the addr is within the DEFAULT_MAP_WINDOW but
> (addr + len) is above the DFEFAULT_MAP_WINDOW. So it wil go through the
> hugetlb_get_unmmaped_area_top_down() to find an area within the
> DFEFAULT_MAP_WINDOW.
> 
> After commit cc92882ee218 ("mm: drop hugetlb_get_unmapped_area{_*}
> functions").  The addr hint for hugetlb_get_unmmaped_area() will be rounded
> up and aligned to hugepage size with ALIGN() for all arches.  And after the
> align, the addr will be above the default MAP_DEFAULT_WINDOW, and the
> map_addresshint_valid() check will pass because both aligned addr (addr0)
> and (addr + len) are above the DEFAULT_MAP_WINDOW, and the aligned hint
> address (0x800000000000) is returned as an suitable gap is found there,
> in arch_get_unmapped_area_topdown().
> 
> To still cover the case that addr is within the DEFAULT_MAP_WINDOW, and
> addr + len is above the DFEFAULT_MAP_WINDOW, make the addr hint one
> hugepage lower, so that after the align it's still within DEFAULT_MAP_WINDOW,
> and the addr + len (2 hugepages) will be above DEFAULT_MAP_WINDOW.
> 
> Fixes: cc92882ee218 ("mm: drop hugetlb_get_unmapped_area{_*} functions")
> Signed-off-by: Chunyu Hu <chuhu@...hat.com>
> ---
>   tools/testing/selftests/mm/va_high_addr_switch.c | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/tools/testing/selftests/mm/va_high_addr_switch.c b/tools/testing/selftests/mm/va_high_addr_switch.c
> index 896b3f73fc53..bd96dc3b5931 100644
> --- a/tools/testing/selftests/mm/va_high_addr_switch.c
> +++ b/tools/testing/selftests/mm/va_high_addr_switch.c
> @@ -230,10 +230,10 @@ void testcases_init(void)
>   			.msg = "mmap(-1, MAP_HUGETLB) again",
>   		},
>   		{
> -			.addr = (void *)(addr_switch_hint - pagesize),
> +			.addr = (void *)(addr_switch_hint - pagesize - hugepagesize),

Wouldn't it be more deterministic to do the alignment/rounding ourselves?

(void *)(ALIGN_DOWN(addr_switch_hint - pagesize), hugepagesize)

Unfortunately we don't have an ALIGN_DOWN helper available yet.

We could just move the one in pkey-helpers.h into vm_util.h


But now I realize that, likely,

	.addr = (void *)(addr_switch_hint - hugepagesize),

would just work and be aligned?

-- 
Cheers

David / dhildenb