Memory management issue in 4.18.15

[Spock]

Hello,

I have a workload, which creates lots of cache pages. Before 4.18.15,
the behavior was very stable: pagecache is constantly growing until it
consumes all the free memory, and then kswapd is balancing it around
low watermark. After 4.18.15, once in a while khugepaged is waking up
and reclaims almost all the pages from pagecache, so there is always
around 2G of 8G unused. THP is enabled only for madvise case and are
not used.

The exact change that leads to current behavior is
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?h=linux-4.18.y&id=62aad93f09c1952ede86405894df1b22012fd5ab