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

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Message-ID: <20250529211423.GA1271329@cmpxchg.org>
Date: Thu, 29 May 2025 17:14:23 -0400
From: Johannes Weiner <hannes@...xchg.org>
To: Matthew Wilcox <willy@...radead.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 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.

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.