Received-SPF: pass (google.com: domain of linux-kernel+bounces-7893-linux.lists.archive=gmail.com@vger.kernel.org designates 147.75.80.249 as permitted sender) client-ip=147.75.80.249;
Precedence: bulk
MIME-Version: 1.0
References: <20231220102948.1963798-1-zhaoyang.huang@unisoc.com>
 <ZYL2rbD5UTz3s8gg@casper.infradead.org> <CAGWkznFcMkkqsKJSyHJfts9ZiYsxxg_dFTccieQ4+boRDJgG4g@mail.gmail.com>
 <ZYO6bLcCRYlo290g@casper.infradead.org> <CAOUHufZ-hAAB+9iL3K-YokN4oVJoiC9dVQ+6zLu-M4Ag52TY5g@mail.gmail.com>
 <CAGWkznGUvPeWkjcy42dudx_+n19dr7SU3B3GGO6JovXHB-Vyzg@mail.gmail.com>
In-Reply-To: <CAGWkznGUvPeWkjcy42dudx_+n19dr7SU3B3GGO6JovXHB-Vyzg@mail.gmail.com>
From: Yu Zhao <yuzhao@google.com>
Date: Wed, 20 Dec 2023 23:32:35 -0700
Message-ID: <CAOUHufajdi7LjyJWNoDx7RP_i9mmN8j9dBqO7GC4A6YNPNnygw@mail.gmail.com>
Subject: Re: [RFC PATCH 1/1] mm: mark folio accessed in minor fault
To: Zhaoyang Huang <huangzhaoyang@gmail.com>
Cc: Matthew Wilcox <willy@infradead.org>, "zhaoyang.huang" <zhaoyang.huang@unisoc.com>, 
	Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org, 
	linux-kernel@vger.kernel.org, steve.kang@unisoc.com
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable

On Wed, Dec 20, 2023 at 11:28=E2=80=AFPM Zhaoyang Huang <huangzhaoyang@gmai=
l.com> wrote:
>
> On Thu, Dec 21, 2023 at 12:53=E2=80=AFPM Yu Zhao <yuzhao@google.com> wrot=
e:
> >
> > On Wed, Dec 20, 2023 at 9:09=E2=80=AFPM Matthew Wilcox <willy@infradead=
.org> wrote:
> > >
> > > On Thu, Dec 21, 2023 at 09:58:25AM +0800, Zhaoyang Huang wrote:
> > > > On Wed, Dec 20, 2023 at 10:14=E2=80=AFPM Matthew Wilcox <willy@infr=
adead.org> wrote:
> > > > >
> > > > > On Wed, Dec 20, 2023 at 06:29:48PM +0800, zhaoyang.huang wrote:
> > > > > > From: Zhaoyang Huang <zhaoyang.huang@unisoc.com>
> > > > > >
> > > > > > Inactive mapped folio will be promoted to active only when it i=
s
> > > > > > scanned in shrink_inactive_list, while the vfs folio will do th=
is
> > > > > > immidiatly when it is accessed. These will introduce two affect=
ions:
> > > > > >
> > > > > > 1. NR_ACTIVE_FILE is not accurate as expected.
> > > > > > 2. Low reclaiming efficiency caused by dummy nactive folio whic=
h should
> > > > > >    be kept as earlier as shrink_active_list.
> > > > > >
> > > > > > I would like to suggest mark the folio be accessed in minor fau=
lt to
> > > > > > solve this situation.
> > > > >
> > > > > This isn't going to be as effective as you imagine.  Almost all f=
ile
> > > > > faults are handled through filemap_map_pages().  So I must ask, w=
hat
> > > > > testing have you done with this patch?
> > > > >
> > > > > And while you're gathering data, what effect would this patch hav=
e on your
> > > > > workloads?
> > > > Thanks for heads-up, I am out of date for readahead mechanism. My g=
oal
> > >
> > > It's not a terribly new mechanism ... filemap_map_pages() was added n=
ine
> > > years ago in 2014 by commit f1820361f83d
> > >
> > > > is to have mapped file pages behave like other pages which could be
> > > > promoted immediately when they are accessed. I will update the patc=
h
> > > > and provide benchmark data in new patch set.
> > >
> > > Understood.  I don't know the history of this, so I'm not sure if the
> > > decision to not mark folios as accessed here was intentional or not.
> > > I suspect it's entirely unintentional.
> >
> > It's intentional. For the active/inactive LRU, all folios start
> > inactive. The first scan of a folio transfers the A-bit (if it's set
> > during the initial fault) to PG_referenced; the second scan of this
> > folio, if the A-bit is set again, moves it to the active list. This
> > way single-use folios, i.e., folios mapped for file streaming, can be
> > reclaimed quickly, since they are "demoted" rather than "promoted" on
> > the second scan. This RFC would regress memory streaming workloads.
> Thanks. Please correct me if I am wrong. IMO, there will be no
> minor-fault for single-use folios

Why not? What prevents a specific *access pattern* from triggering minor fa=
ults?

> which means RFC could behave the
> same as mainline does now? I think it doesn't make sense to have
> multiple-mapped pages filled in page_list to shrink_page_list since we
> can distinguish them in advance.