linux-kernel - Re: [PATCH v21 1/4] mm: add VM_DROPPABLE for designating always lazily freeable mappings

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Message-ID: <Zovv4lzM38EHtnms@zx2c4.com>
Date: Mon, 8 Jul 2024 15:55:46 +0200
From: "Jason A. Donenfeld" <Jason@...c4.com>
To: David Hildenbrand <david@...hat.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, patches@...ts.linux.dev,
	tglx@...utronix.de, linux-crypto@...r.kernel.org,
	linux-api@...r.kernel.org, x86@...nel.org,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Adhemerval Zanella Netto <adhemerval.zanella@...aro.org>,
	Carlos O'Donell <carlos@...hat.com>,
	Florian Weimer <fweimer@...hat.com>, Arnd Bergmann <arnd@...db.de>,
	Jann Horn <jannh@...gle.com>,
	Christian Brauner <brauner@...nel.org>,
	David Hildenbrand <dhildenb@...hat.com>, linux-mm@...ck.org
Subject: Re: [PATCH v21 1/4] mm: add VM_DROPPABLE for designating always
 lazily freeable mappings

Hi David,

On Mon, Jul 08, 2024 at 10:11:24AM +0200, David Hildenbrand wrote:
> The semantics are much more intuitive. No need for separate mmap flags.

Agreed.
 
> Likely we'll have to adjust mlock() as well. Also, I think we should 
> just bail out with hugetlb as well.

Ack.

> Further, maybe we want to disallow madvise() clearing these flags here, 
> just to be consistent.

Good thinking.

> As a side note, I'll raise that I am not a particular fan of the 
> "droppable" terminology, at least with the "read 0s" approach.
> 
>  From a user perspective, the memory might suddenly lose its state and 
> read as 0s just like volatile memory when it loses power. "dropping 
> pages" sounds more like an implementation detail.
> 
> Something like MAP_VOLATILE might be more intuitive (similar to the 
> proposed MADV_VOLATILE).
> 
> But naming is hard, just mentioning to share my thought :)

Naming is hard, but *renaming* is annoying. I like droppable simply
because that's what I've been calling it in my head. MAP_VOLATILE is
fine with me though, and seems reasonable enough. So I'll name it that,
and then please don't change your mind about it later so I won't have to
rename everything again. :)

Jason