linux-kernel - Re: [PATCH v16 1/5] mm: add VM_DROPPABLE for designating always lazily freeable mappings

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Message-ID: <Zmbq1dGPIYdRLw5_@tiehlicka>
Date: Mon, 10 Jun 2024 14:00:21 +0200
From: Michal Hocko <mhocko@...e.com>
To: Jann Horn <jannh@...gle.com>
Cc: "Jason A. Donenfeld" <Jason@...c4.com>, 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>,
	Christian Brauner <brauner@...nel.org>,
	David Hildenbrand <dhildenb@...hat.com>, linux-mm@...ck.org
Subject: Re: [PATCH v16 1/5] mm: add VM_DROPPABLE for designating always
 lazily freeable mappings

On Fri 07-06-24 17:50:34, Jann Horn wrote:
[...]
> Or, from a different angle: You're trying to allocate memory, and you
> can't make forward progress until that memory has been allocated
> (unless the process is killed). That's what GFP_KERNEL is for. Stuff
> like "__GFP_NOWARN | __GFP_NORETRY" is for when you have a backup plan
> that lets you make progress (perhaps in a slightly less efficient way,
> or by dropping some incoming data, or something like that), and it
> hints to the page allocator that it doesn't have to try hard to
> reclaim memory if it can't find free memory quickly.

Correct. A psedu-busy wait for allocation to succeed sounds like a very
bad idea to imprint into ABI. Is there really any design requirement to
make these mappings to never cause the OOM killer?

Making the content dropable under memory pressure because it is
inherently recoverable is something else (this is essentially an
implicit MADV_FREE semantic) but putting a requirement on the memory
allocation on the fault sounds just wrong to me.

-- 
Michal Hocko
SUSE Labs