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

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Date:   Thu, 26 Jan 2017 18:47:45 +0000
From:   Mel Gorman <mgorman@...e.de>
To:     Johannes Weiner <hannes@...xchg.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, kernel-team@...com
Subject: Re: [PATCH 2/5] mm: vmscan: kick flushers when we encounter dirty
 pages on the LRU

On Thu, Jan 26, 2017 at 12:47:39PM -0500, Johannes Weiner wrote:
> 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@...xchg.org>
> > 
> > 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.

Ah, ok. I was thinking of it in terms of "we are trying to free pages"
and not the specific name of the direct reclaim function.

> 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.
> 

Doubtful, maybe some tracing analysis scripts but they routinely have
to adapt.

> 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.
> 

Looks good.

-- 
Mel Gorman
SUSE Labs