linux-kernel - Re: [PATCH] crypto: lib/chacha20poly1305 - Set SG_MITER

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Date:   Tue, 15 Sep 2020 10:08:53 +0300
From:   Ard Biesheuvel <ardb@...nel.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Herbert Xu <herbert@...dor.apana.org.au>,
        Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux Crypto Mailing List <linux-crypto@...r.kernel.org>
Subject: Re: [PATCH] crypto: lib/chacha20poly1305 - Set SG_MITER_ATOMIC unconditionally

On Tue, 15 Sep 2020 at 09:56, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> On Mon, Sep 14, 2020 at 11:45 PM Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
> >
> > I mean, I did find one case that didn't set it (cb710-mmc.c), but
> > pattern-matching to the other mmc cases, that one looks like it
> > _should_ have set the atomic flag like everybody else did.
>
> Oh, and immediately after sending that out I notice
> nvmet_bdev_execute_rw(), which does seem to make allocations inside
> that sg_miter loop.
>
> So those non-atomic cases do clearly exist.
>
> It does make the case for why kmap_atomic() wants to have the
> debugging code. It will "just work" on 64-bit to do it wrong, because
> the address doesn't become invalid just because you sleep or get
> rescheduled. But then the code that every developer tests (since
> developers tend to run on good hardware) might be completely broken on
> bad old hardware.
>

If we want code that is optimal on recent hardware, and yet still
correct on older 32-bit hardware, kmap() is definitely a better choice
here than kmap_atomic(), since it is a no-op on !HIGHMEM, and
tolerates sleeping on 32-bit. /That/ is why I wrote the code this way.

The problem is of course that kmap() itself might sleep.

So I would argue that the semantics in the name of kmap_atomic() are
not about the fact that it starts a non-preemptible section, but that
it can be *called from* a non-preemptible section. And starting a
non-preemptible section is unnecessary on !HIGHMEM, and should be
avoided if possible.

> Maybe we could hide it behind a debug option, at least.
>
> Or, alterantively, introduce a new "debug_preempt_count" that doesn't
> actually disable preemption, but warns about actual sleeping
> operations..
>
>              Linus