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From: Zhaoyang Huang <huangzhaoyang@gmail.com>
Date: Thu, 21 Dec 2023 14:28:15 +0800
Message-ID: <CAGWkznGUvPeWkjcy42dudx_+n19dr7SU3B3GGO6JovXHB-Vyzg@mail.gmail.com>
Subject: Re: [RFC PATCH 1/1] mm: mark folio accessed in minor fault
To: Yu Zhao <yuzhao@google.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
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On Thu, Dec 21, 2023 at 12:53=E2=80=AFPM Yu Zhao <yuzhao@google.com> wrote:
>
> On Wed, Dec 20, 2023 at 9:09=E2=80=AFPM Matthew Wilcox <willy@infradead.o=
rg> 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@infrad=
ead.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 is
> > > > > scanned in shrink_inactive_list, while the vfs folio will do this
> > > > > immidiatly when it is accessed. These will introduce two affectio=
ns:
> > > > >
> > > > > 1. NR_ACTIVE_FILE is not accurate as expected.
> > > > > 2. Low reclaiming efficiency caused by dummy nactive folio which =
should
> > > > >    be kept as earlier as shrink_active_list.
> > > > >
> > > > > I would like to suggest mark the folio be accessed in minor fault=
 to
> > > > > solve this situation.
> > > >
> > > > This isn't going to be as effective as you imagine.  Almost all fil=
e
> > > > faults are handled through filemap_map_pages().  So I must ask, wha=
t
> > > > testing have you done with this patch?
> > > >
> > > > And while you're gathering data, what effect would this patch have =
on your
> > > > workloads?
> > > Thanks for heads-up, I am out of date for readahead mechanism. My goa=
l
> >
> > It's not a terribly new mechanism ... filemap_map_pages() was added nin=
e
> > 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 patch
> > > 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 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.