selftests/mm: run_vmtests.sh: fix half_ufd_size_MB calculation

Rafael Aquinirequested to merge
raquini/centos-stream-9:RHEL-74363 into main

JIRA: https://issues.redhat.com/browse/RHEL-74363

This patch is a backport of the following upstream commit:
commit 67a2f86846f244d81601cf2e1552c4656b8556d6
Author: Rafael Aquini raquini@redhat.com
Date: Tue Feb 18 14:22:51 2025 -0500

selftests/mm: run_vmtests.sh: fix half_ufd_size_MB calculation  

We noticed that uffd-stress test was always failing to run when invoked  
for the hugetlb profiles on x86_64 systems with a processor count of 64 or  
bigger:  

  ...  
  ...  
  not ok 3 uffd-stress hugetlb 128 32 # exit=1  
  ...  

The problem boils down to how run_vmtests.sh (mis)calculates the size of  
the region it feeds to uffd-stress.  The latter expects to see an amount  
of MiB while the former is just giving out the number of free hugepages  
halved down.  This measurement discrepancy ends up violating uffd-stress'  
assertion on number of hugetlb pages allocated per CPU, causing it to bail  
out with the error above.  

This commit fixes that issue by adjusting run_vmtests.sh's  
half_ufd_size_MB calculation so it properly renders the region size in  
MiB, as expected, while maintaining all of its original constraints in  
place.  

Link: https://lkml.kernel.org/r/20250218192251.53243-1-aquini@redhat.com  
Fixes: 2e47a445d7b3 ("selftests/mm: run_vmtests.sh: fix hugetlb mem size calculation")  
Signed-off-by: Rafael Aquini <raquini@redhat.com>  
Reviewed-by: David Hildenbrand <david@redhat.com>  
Reviewed-by: Peter Xu <peterx@redhat.com>  
Cc: Shuah Khan <shuah@kernel.org>  
Cc: <stable@vger.kernel.org>  
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Signed-off-by: Rafael Aquini raquini@redhat.com

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