Re: [PATCH 2/5] mm: vmscan: kick flushers when we encounter dirty pages on the LRU

From: Johannes Weiner
Date: Thu Jan 26 2017 - 12:48:29 EST

On Thu, Jan 26, 2017 at 09:57:45AM +0000, Mel Gorman wrote:
> On Mon, Jan 23, 2017 at 01:16:38PM -0500, Johannes Weiner wrote:
> > Memory pressure can put dirty pages at the end of the LRU without
> > anybody running into dirty limits. Don't start writing individual
> > pages from kswapd while the flushers might be asleep.
> >
> > Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>
> I don't understand the motivation for checking the wb_reason name. Maybe
> it was easier to eyeball while reading ftraces. The comment about the
> flusher not doing its job could also be as simple as the writes took
> place and clean pages were reclaimed before dirty_expire was reached.
> Not impossible if there was a light writer combined with a heavy reader
> or a large number of anonymous faults.

The name change was only because try_to_free_pages() wasn't the only
function doing this flusher wakeup anymore. I associate that name with
direct reclaim rather than reclaim in general, so I figured this makes
more sense. No strong feelings either way, but I doubt this will break
anything in userspace.

The comment on dirty expiration is a good point. Let's add this to the
list of reasons why reclaim might run into dirty data. Fixlet below.

> Acked-by: Mel Gorman <mgorman@xxxxxxx>