Message-ID: <aL_JD_vNvM2kXoRJ@gmail.com>
Date: Tue, 9 Sep 2025 14:28:31 +0800
From: Chunyu Hu <chuhu@...hat.com>
To: David Hildenbrand <david@...hat.com>
Cc: akpm@...ux-foundation.org, shuah@...nel.org, linux-mm@...ck.org,
	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 Mon, Sep 08, 2025 at 03:09:24PM +0200, David Hildenbrand wrote:
> 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

Thanks for the review!

This is good idea and it would be more deterministic if we provide an
aligned address directly, then the kernel change won't affect the test.

> 
> 
> But now I realize that, likely,
> 
> 	.addr = (void *)(addr_switch_hint - hugepagesize),
> 
> would just work and be aligned?

Yes, it's aligned to the hugepagesize, align down and works. I prefer
this way as it's easier and all other tests in the file do like this.
Thanks for the suggestion.

I thought we would lose some test coverage on testing if it will work when
an un-hugepagesize aligned addr is provided. Do you think it's
necessary? If not, I'll change to:

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

or we can add them both if necesasry.

> 
> -- 
> Cheers
> 
> David / dhildenb
> 

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