linux-kernel - Re: [PATCH 2/3] mm: filemap: only do access activations on reads

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Message-ID: <20160405175059.GA16935@cmpxchg.org>
Date:	Tue, 5 Apr 2016 13:50:59 -0400
From:	Johannes Weiner <hannes@...xchg.org>
To:	Rik van Riel <riel@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Andres Freund <andres@...razel.de>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, kernel-team@...com
Subject: Re: [PATCH 2/3] mm: filemap: only do access activations on reads

On Mon, Apr 04, 2016 at 05:39:47PM -0400, Rik van Riel wrote:
> As for hinting, I suspect it may make sense to differentiate
> between whole page and partial page writes, where partial
> page writes use FGP_ACCESSED, and whole page writes do not,
> under the assumption that if we write a partial page, there
> may be a higher chance that other parts of the page get
> accessed again for other writes (or reads).

The writeback cache should handle at least the multiple subpage writes
case.

What I find a little weird about counting accesses from partial writes
only is when a write covers a full page and then parts of the next. We
would cache only a small piece of what's likely one coherent chunk.

Or when a user writes out several pages in a loop of subpage chunks.

This will get even worse to program against when we start having page
cache transparently backed by pages of different sizes.

Because of that I think it'd be better to apply LRU aging decisions
based on type of access rather based on specific request sizes.