Re: khugepaged selftest: intermittent collapse_full(madvise:anon) failure and low-memory false failures on arm64 64K

[Zhijian Han]

Hi Kiryl,

Thanks — instrumented as suggested: errno + thp_collapse_alloc_failed
printed on failure, then 30 full runs on the same setup (QEMU arm64 64K,
6G, linux-next 20261019). The failure reproduced 11/30 times and the
data does not point at allocation:

  6x EAGAIN, 5x EINVAL, 0x ENOMEM
  thp_collapse_alloc_failed=0 in every failing run

Per madvise_collapse_errno(), EAGAIN maps to transient page states
(SCAN_PAGE_LOCK/PAGE_LRU/PAGE_DIRTY_OR_WRITEBACK/...) and EINVAL to
"intrinsic" scan results — neither is the allocation-failure path,
which would have shown ENOMEM and a bump in the counter.

One thing that stands out: madvise_collapse_retry() retries EAGAIN only
once, so two consecutive EAGAINs fail the case even though the errno
table documents EAGAIN as "trying again might succeed". Could the test
retry harder on EAGAIN (loop with a short sleep), or is the single retry
deliberate to expose genuine kernel-side transients?

For the EINVAL cases I would need kernel-side detail on which scan
result is being hit; happy to run instrumented (tracefs or a debug
counter) if you point me at the right knob.

Full 30-run logs and the instrumentation diff available on request.

Understood and agreed on the low-memory guard — dropped.

Assisted-by: LLM (QEMU test harness and instrumentation)

-- 
Zhijian Han