Re: [PATCH v3 06/13] selftest/mm: adjust hugepage-mremap test size for large huge pages

[Sayali Patil]

On 01/04/26 19:40, David Hildenbrand (Arm) wrote:
> On 3/27/26 08:16, Sayali Patil wrote:
> > The hugepage-mremap selftest uses a default size of 10MB, which is
> > sufficient for small huge page sizes. However, when the huge page size
> > is large (e.g. 1GB), 10MB is smaller than a single huge page.
> > As a result, the test does not trigger PMD sharing and the
> > corresponding unshare path in mremap(), causing the
> > test to fail (mremap succeeds where a failure is expected).
> >
> > Update run_vmtest.sh to use twice the huge page size when the huge page
> > size exceeds 10MB, while retaining the 10MB default for smaller huge
> > pages. This ensures the test exercises the intended PMD sharing and
> > unsharing paths for larger huge page sizes.
> >
> > Before patch:
> >   running ./hugepage-mremap
> >   ------------------------------
> >   TAP version 13
> >   1..1
> >    Map haddr: Returned address is 0x7eaa40000000
> >    Map daddr: Returned address is 0x7daa40000000
> >    Map vaddr: Returned address is 0x7faa40000000
> >    Address returned by mmap() = 0x7fffaa600000
> >    Mremap: Returned address is 0x7faa40000000
> >    First hex is 0
> >    First hex is 3020100
> >   Bail out! mremap: Expected failure, but call succeeded
> >   Planned tests != run tests (1 != 0)
> >   Totals: pass:0 fail:0 xfail:0 xpass:0 skip:0 error:0
> >   [FAIL]
> >   not ok 1 hugepage-mremap # exit=1
> >
> > Before patch:
> >   running ./hugepage-mremap
> >   ------------------------------
> >   TAP version 13
> >   1..1
> >    Map haddr: Returned address is 0x7eaa40000000
> >    Map daddr: Returned address is 0x7daa40000000
> >    Map vaddr: Returned address is 0x7faa40000000
> >    Address returned by mmap() = 0x7fffaa600000
> >    Mremap: Returned address is 0x7faa40000000
> >    First hex is 0
> >    First hex is 3020100
> >   Bail out! mremap: Expected failure, but call succeeded
> >   Planned tests != run tests (1 != 0)
> >   Totals: pass:0 fail:0 xfail:0 xpass:0 skip:0 error:0
> >   [FAIL]
> >   not ok 1 hugepage-mremap # exit=1
>
> Why are there two "Before patch" in here?
Thanks for pointing that out, Let me fix it in the next version.
> > After patch:
> >   running ./hugepage-mremap 2048
> >   ------------------------------
> >   TAP version 13
> >   1..1
> >    Map haddr: Returned address is 0x7eaa40000000
> >    Map daddr: Returned address is 0x7daa40000000
> >    Map vaddr: Returned address is 0x7faa40000000
> >    Address returned by mmap() = 0x7fff13000000
> >    Mremap: Returned address is 0x7faa40000000
> >    First hex is 0
> >    First hex is 3020100
> >    ok 1 Read same data
> >   Totals: pass:1 fail:0 xfail:0 xpass:0 skip:0 error:0
> >   [PASS]
> >   ok 1 hugepage-mremap 2048
> >
> > Fixes: f77a286de48c ("mm, hugepages: make memory size variable in hugepage-mremap selftest")
> > Acked-by: Zi Yan <ziy@xxxxxxxxxx>
> > Tested-by: Venkat Rao Bagalkote <venkat88@xxxxxxxxxxxxx>
> > Signed-off-by: Sayali Patil <sayalip@xxxxxxxxxxxxx>
> > ---
> >   tools/testing/selftests/mm/run_vmtests.sh | 13 ++++++++++++-
> >   1 file changed, 12 insertions(+), 1 deletion(-)
> >
> > diff --git a/tools/testing/selftests/mm/run_vmtests.sh b/tools/testing/selftests/mm/run_vmtests.sh
> > index afdcfd0d7cef..eecec0b6eb13 100755
> > --- a/tools/testing/selftests/mm/run_vmtests.sh
> > +++ b/tools/testing/selftests/mm/run_vmtests.sh
> > @@ -293,7 +293,18 @@ echo "$shmmax" > /proc/sys/kernel/shmmax
> >   echo "$shmall" > /proc/sys/kernel/shmall
> >   
> >   CATEGORY="hugetlb" run_test ./map_hugetlb
> > -CATEGORY="hugetlb" run_test ./hugepage-mremap
> > +
> > +# If the huge page size is larger than 10MB, increase the test memory size
> > +# to twice the huge page size (in MB) to ensure the test exercises PMD sharing
> > +# and the unshare path in hugepage-mremap. Otherwise, run the test with
> > +# the default 10MB memory size.
>
> PMD sharing requires, on x86, a 1 GiB area with 2 MiB hugetlb folios.
>
> How does doubling sort that out?
>
> Also, why the magic value 10mb?
Hi David,
Yes, 1GB huge pages are mapped at the PUD level and are not involved in 
PMD sharing, as huge_pte_alloc() skips sharing for sizes other than 
PMD_SIZE.

The issue here is due to an unaligned memory size on a 1GB mapping.
This leads munmap() to fail at an unaligned address, causing the 
subsequent expected-to-fail mremap() to unexpectedly succeed.
The default memory size for this test is 10MB.

Aligning the size to a multiple of 1GB avoids this failure, but it is 
not related to PMD sharing. I will update the description in v4 to 
reflect this more accurately.

I will also update the test code directly to align the memory size to 
the huge page size, rather than modifying run_vmtests.sh.

Thanks,
Sayali