linux-kernel - Re: [PATCH v14 2/7] mm: add VM_DROPPABLE for designating always lazily freeable mappings

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Message-ID: <Y7SJ+/axonTK0Fir@zx2c4.com>
Date:   Tue, 3 Jan 2023 21:03:07 +0100
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Andy Lutomirski <luto@...nel.org>, Ingo Molnar <mingo@...nel.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>, linux-mm@...ck.org
Subject: Re: [PATCH v14 2/7] mm: add VM_DROPPABLE for designating always
 lazily freeable mappings

Hi Linus,

On Tue, Jan 03, 2023 at 11:54:35AM -0800, Linus Torvalds wrote:
> So 99% of the time, the solution really is just "getrandom()",
> generally with the usual buffering ("read more than you need, so that
> you don't do it all the time").\

That buffering cannot be done safely currently -- VM forks, reseeding
semantics, and so forth. Again, discussed in the cover letter of the
patch if you'd like to engage with those ideas.

> just using your own rng in user space entirely.

This is the thing that isn't quite safe.

> Let me guess: the people you talked to who were excited about this are
> mainly just library people?

No, actually. Mainly people deploying production network-facing things
that need a lot of randomness often. e.g. CBC nonces in TLS, or random
sequence numbers in UDP-based protocols.

> So when you say that this isn't about micro-optimizations, I really
> say "humbug". It's *clearly* about micro-optimization of an area that
> very few people care about, since the alternative is just our existing
> "getrandom()" that is not at all horribly slow.

The alternative is what people currently do, which is attempt to
implement a userspace RNG, which cannot be done safely. Again, -->
cover letter.

> Because the people who actually *use* the random numbers and are *so*
> performance-critical about them already have their own per-thread
> buffers or what-not

...which are not safe.

Anyway, if you're NACK'ing the whole getrandom() vDSO project, just
please outright say so, so I don't spend another 14 revisions in vain.

Jason