selftests/mm: relax test to fail after 100 migration failures

selftests/mm: relax test to fail after 100 migration failures

It was recently observed at [1] that during the folio unmapping stage of
migration, when the PTEs are cleared, a racing thread faulting on that
folio may increase the refcount of the folio, sleep on the folio lock (the
migration path has the lock), and migration ultimately fails when
asserting the actual refcount against the expected.  Thereby, the
migration selftest fails on shared-anon mappings.  The above enforces the
fact that migration is a best-effort service, therefore, it is wrong to
fail the test for just a single failure; hence, fail the test after 100
consecutive failures (where 100 is still a subjective choice).  Note that,
this has no effect on the execution time of the test since that is
controlled by a timeout.

[1] https://lore.kernel.org/all/20240801081657.1386743-1-dev.jain@arm.com/

Link: https://lkml.kernel.org/r/20240830051609.4037834-1-dev.jain@arm.com
Signed-off-by: Dev Jain <dev.jain@arm.com>
Suggested-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Ryan Roberts <ryan.roberts@arm.com>
Tested-by: Ryan Roberts <ryan.roberts@arm.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Aneesh Kumar K.V <aneesh.kumar@kernel.org>
Cc: Anshuman Khandual <anshuman.khandual@arm.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Christoph Lameter <cl@gentwo.org>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Gavin Shan <gshan@redhat.com>
Cc: "Huang, Ying" <ying.huang@intel.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Lance Yang <ioworker0@gmail.com>
Cc: Mark Brown <broonie@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Will Deacon <will@kernel.org>
Cc: Yang Shi <yang@os.amperecomputing.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>