On 11/25/20 3:15 PM, Mohamed Alzayat wrote:
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
AFAIK, not. The tracking is done by write-protecting the pages to cause a page fault, so it should be quite synchronous update of page table entries, and reading pagemap is a page table walk of those very entries.
But as you have the test, it should be possible to git bisect it? Just do enough trials to be sure enough that no fail means indeed a "good" kernel.
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
