On Tue, Aug 18, 2020 at 11:17:56AM +0100, Chris Down wrote:

I'd ask that you understand a bit more about the tradeoffs and intentions of
the patch before rushing in to declare its failure, considering it works
just fine :-)

Clamping the maximal time allows the application to take some action to
remediate the situation, while still being slowed down significantly. 2
seconds per allocation batch is still absolutely plenty for any use case
I've come across. If you have evidence it isn't, then present that instead
of vague notions of "wrongness".

There is no feedback from the freeing rate, therefore it cannot be
correct in maintaining a maximum amount of pages.

0.5 pages / sec is still non-zero, and if the free rate is 0, you'll
crawl across whatever limit was set without any bounds. This is math
101.

It's true that I haven't been paying attention to mm in a while, but I
was one of the original authors of the I/O dirty balancing, I do think I
understand how these things work.