linux-kernel - Re: [DISCUSSION] proposed mctl() API

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Message-ID: <20250604120013.GA1431@cmpxchg.org>
Date: Wed, 4 Jun 2025 08:00:13 -0400
From: Johannes Weiner <hannes@...xchg.org>
To: Barry Song <21cnbao@...il.com>
Cc: Matthew Wilcox <willy@...radead.org>,
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Shakeel Butt <shakeel.butt@...ux.dev>,
	"Liam R . Howlett" <Liam.Howlett@...cle.com>,
	David Hildenbrand <david@...hat.com>,
	Vlastimil Babka <vbabka@...e.cz>, Jann Horn <jannh@...gle.com>,
	Arnd Bergmann <arnd@...db.de>,
	Christian Brauner <brauner@...nel.org>,
	SeongJae Park <sj@...nel.org>, Usama Arif <usamaarif642@...il.com>,
	Mike Rapoport <rppt@...nel.org>, linux-mm@...ck.org,
	linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-api@...r.kernel.org, Pedro Falcato <pfalcato@...e.de>
Subject: Re: [DISCUSSION] proposed mctl() API

On Fri, May 30, 2025 at 07:52:28PM +1200, Barry Song wrote:
> On Fri, May 30, 2025 at 9:14 AM Johannes Weiner <hannes@...xchg.org> wrote:
> >
> > On Thu, May 29, 2025 at 04:28:46PM +0100, Matthew Wilcox wrote:
> > > Barry's problem is that we're all nervous about possibly regressing
> > > performance on some unknown workloads.  Just try Barry's proposal, see
> > > if anyone actually compains or if we're just afraid of our own shadows.
> >
> > I actually explained why I think this is a terrible idea. But okay, I
> > tried the patch anyway.
> >
> > This is 'git log' on a hot kernel repo after a large IO stream:
> >
> >                                      VANILLA                      BARRY
> > Real time                 49.93 (    +0.00%)         60.36 (   +20.48%)
> > User time                 32.10 (    +0.00%)         32.09 (    -0.04%)
> > System time               14.41 (    +0.00%)         14.64 (    +1.50%)
> > pgmajfault              9227.00 (    +0.00%)      18390.00 (   +99.30%)
> > workingset_refault_file  184.00 (    +0.00%)    236899.00 (+127954.05%)
> >
> > Clearly we can't generally ignore page cache hits just because the
> > mmaps() are intermittent.
> 
> Hi Johannes,
> Thanks!
> 
> Are you on v1, which lacks folio demotion[1], or v2, which includes it [2]?
> 
> [1] https://lore.kernel.org/linux-mm/20250412085852.48524-1-21cnbao@gmail.com/
> [2] https://lore.kernel.org/linux-mm/20250514070820.51793-1-21cnbao@gmail.com/

The subthread is about whether the reference dismissal / demotion
should be unconditional (v1) or opt-in (v2).

I'm arguing for v2.