Potential bug in soft-dirty bits (with test case)

[Mohamed Alzayat <alzayat@mpi-sws.org>]

Hi Everyone,

I have noticed a change in the synchrony of updating the soft-dirty
bits in recent kernel versions (5.6+). More precisely, up to kernel
v5.5, the soft-dirty bits as parsed from /proc/pid/pagemap accurately
capture the dirtied pages. Recently, I started testing on kernels v5.6
- v5.9, and I noticed that the soft-dirty bits are not immediately
updated.

I have prepared a short test that repeatedly causes at least one
memory page to be dirtied, then scans /proc/pid/pagemap counting the
soft-dirty bits. The test fails if this count is zero. In my
observation, this test fails once in every 10-20 trials. The test
defaults to 100 trials and can be found at
https://gitlab.mpi-sws.org/-/snippets/1696

Is this non-synchronous propagation of soft dirty bits intended? If
yes, is there a way to force the soft-dirty bits to be propagated to
the page map entries immediately, or is there an alternative interface
that has the synchronous behavior?

Thanks in advance,
Mohamed Alzayat