[PATCH v2 0/3] Fixes for compaction_test

[Dev Jain]

The compaction_test memory selftest introduces fragmentation in memory
and then tries to allocate as many hugepages as possible. This series
addresses some problems.

On Aarch64, if nr_hugepages == 0, then the test trivially succeeds since
compaction_index becomes 0, which is less than 3, due to no division by
zero exception being raised. We fix that by checking for division by
zero.

Secondly, correctly set the number of hugepages to zero before trying
to set a large number of them.

Now, consider a situation in which, at the start of the test, a non-zero
number of hugepages have been already set (while running the entire
selftests/mm suite, or manually by the admin). The test operates on 80%
of memory to avoid OOM-killer invocation, and because some memory is
already blocked by hugepages, it would increase the chance of OOM-killing.
Also, since mem_free used in check_compaction() is the value before we
set nr_hugepages to zero, the chance that the compaction_index will
be small is very high if the preset nr_hugepages was high, leading to a
bogus test success.

This series applies on top of the stable 6.9 kernel.

Changes in v2:
- Handle an unsigned long number of hugepages
- Combine the first patch (previously standalone) with this series

Link to v1:
https://lore.kernel.org/all/20240513082842.4117782-1-dev.jain@xxxxxxx/
https://lore.kernel.org/all/20240515093633.54814-1-dev.jain@xxxxxxx/

Dev Jain (3):
  selftests/mm: compaction_test: Fix bogus test success on Aarch64
  selftests/mm: compaction_test: Fix incorrect write of zero to
    nr_hugepages
  selftests/mm: compaction_test: Fix bogus test success and reduce
    probability of OOM-killer invocation

 tools/testing/selftests/mm/compaction_test.c | 85 ++++++++++++++------
 1 file changed, 60 insertions(+), 25 deletions(-)

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2.34.1