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

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Message-ID: <ZmyNemDma0bK_V8J@zx2c4.com>
Date: Fri, 14 Jun 2024 20:35:38 +0200
From: "Jason A. Donenfeld" <Jason@...c4.com>
To: Michal Hocko <mhocko@...e.com>
Cc: Jann Horn <jannh@...gle.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 Mon, Jun 10, 2024 at 02:00:21PM +0200, Michal Hocko wrote:
> 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.

The idea is that syscall getrandom() won't make a process be killed, so
neither should vgetrandom().

But there's an argument to be made that the NOWARN|NORETRY logic only
made sense with the now-dropped "skip instruction on fault" patch that
was so controversial before, since in that case, there wouldn't be
infinite retry, but rather skipping and then falling back to the
syscall. I think this is nicer behavior, but the implementation caused a
stir, so I'm not at the moment going that route. Given that, I think
I'll follow your advice and get rid of NOWARN|NORETRY for this too. And
then maybe we'll all revisit that later.

Jason