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

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Message-ID: <aD3nFMS9kWHp7a4F@casper.infradead.org>
Date: Mon, 2 Jun 2025 19:01:56 +0100
From: Matthew Wilcox <willy@...radead.org>
To: Johannes Weiner <hannes@...xchg.org>
Cc: 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>, Barry Song <21cnbao@...il.com>,
	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 Thu, May 29, 2025 at 05:14:23PM -0400, Johannes Weiner 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.

Sorry, I must've missed that one ;-(

> 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.
> 
> The whole point is to cache across processes and their various
> apertures into a common, long-lived filesystem space.
> 
> Barry knows something about the relationship between certain processes
> and certain files that he could exploit with MADV_COLD-on-exit
> semantics. But that's not something the kernel can safely assume. Not
> without defeating the page cache for an entire class of file accesses.

So what about distinguishing between exited-normally processes (ie git
log) vs killed-by-oom processes (ie Barry's usecase)?  Update the
referenced bit in the first case and not the second?