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fb9293b6b0
Reset nr_hugepages to zero before the start of the test.
If a non-zero number of hugepages is already set before the start of the
test, the following problems arise:
- The probability of the test getting OOM-killed increases. Proof:
The test wants to run on 80% of available memory to prevent OOM-killing
(see original code comments). Let the value of mem_free at the start
of the test, when nr_hugepages = 0, be x. In the other case, when
nr_hugepages > 0, let the memory consumed by hugepages be y. In the
former case, the test operates on 0.8 * x of memory. In the latter,
the test operates on 0.8 * (x - y) of memory, with y already filled,
hence, memory consumed is y + 0.8 * (x - y) = 0.8 * x + 0.2 * y > 0.8 *
x. Q.E.D
- The probability of a bogus test success increases. Proof: Let the
memory consumed by hugepages be greater than 25% of x, with x and y
defined as above. The definition of compaction_index is c_index = (x -
y)/z where z is the memory consumed by hugepages after trying to
increase them again. In check_compaction(), we set the number of
hugepages to zero, and then increase them back; the probability that
they will be set back to consume at least y amount of memory again is
very high (since there is not much delay between the two attempts of
changing nr_hugepages). Hence, z >= y > (x/4) (by the 25% assumption).
Therefore, c_index = (x - y)/z <= (x - y)/y = x/y - 1 < 4 - 1 = 3
hence, c_index can always be forced to be less than 3, thereby the test
succeeding always. Q.E.D
Link: https://lkml.kernel.org/r/20240521074358.675031-4-dev.jain@arm.com
Fixes:
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| .. | ||
| accounting | ||
| arch | ||
| bootconfig | ||
| bpf | ||
| build | ||
| certs | ||
| cgroup | ||
| counter | ||
| crypto | ||
| debugging | ||
| firewire | ||
| firmware | ||
| gpio | ||
| hv | ||
| iio | ||
| include | ||
| kvm/kvm_stat | ||
| laptop | ||
| leds | ||
| lib | ||
| memory-model | ||
| mm | ||
| net/ynl | ||
| objtool | ||
| pci | ||
| pcmcia | ||
| perf | ||
| power | ||
| rcu | ||
| scripts | ||
| sound | ||
| spi | ||
| testing | ||
| thermal | ||
| time | ||
| tracing | ||
| usb | ||
| verification | ||
| virtio | ||
| wmi | ||
| workqueue | ||
| writeback | ||
| Makefile | ||