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

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Message-ID: <aDh9LtSLCiTLjg2X@casper.infradead.org>
Date: Thu, 29 May 2025 16:28:46 +0100
From: Matthew Wilcox <willy@...radead.org>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: 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>,
	Johannes Weiner <hannes@...xchg.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 03:43:26PM +0100, Lorenzo Stoakes wrote:
> After discussions in various threads (Usama's series adding a new prctl()
> in [0], and a proposal to adapt process_madvise() to do the same -
> conception in [1] and RFC in [2]), it seems fairly clear that it would make
> sense to explore a dedicated API to explicitly allow for actions which
> affect the virtual address space as a whole.
> 
> Also, Barry is implementing a feature (currently under RFC) which could
> additionally make use of this API (see [3]).

I think the reason that you're having trouble coming up with a good
place to put these ideas is because they are all bad ideas.  Do none of
them.  Problem solved.

People should put more effort into allocating THPs automatically and
monitoring where they're helping performance and where they're hurting
performance, instead of coming up with these baroque reasons to blame
the sysadmin for not having tweaked some magic knob.

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.