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

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Message-ID: <20200915070523.GA26629@gondor.apana.org.au>
Date:   Tue, 15 Sep 2020 17:05:23 +1000
From:   Herbert Xu <herbert@...dor.apana.org.au>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ard Biesheuvel <ardb@...nel.org>,
        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 Mon, Sep 14, 2020 at 11:55:53PM -0700, Linus Torvalds wrote:
>
> 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..

I'm more worried about existing users of kmap_atomic relying on
the preemption disabling semantics.  Short of someone checking
on every single instance (and that would include derived cases
such as all users of sg miter), I think the safer option is to
create something brand new and then migrate the existing users
to it.  Something like

static inline void *kmap_atomic_ifhigh(struct page *page)
{
	if (PageHighMem(page))
		return kmap_atomic(page);
	return page_address(page);
}

static inline void kunmap_atomic_ifhigh(struct page *page, void *addr)
{
	if (PageHighMem(page))
		kunmap_atomic(addr);
}

Cheers,
-- 
Email: Herbert Xu <herbert@...dor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt