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From: Michael Davidson <md@google.com>
Date: Fri, 17 Mar 2017 11:52:01 -0700
Message-ID: <CA+=D-XXy9yEQc-jLxSqRUwq=4ADnFqaxnsjSzV4=8jOisym=bQ@mail.gmail.com>
Subject: Re: [PATCH 6/7] md/raid10, LLVM: get rid of variable length array
To: Peter Zijlstra <peterz@infradead.org>
Cc: Alexander Potapenko <glider@google.com>,
        Michal Marek <mmarek@suse.com>, Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>,
        Herbert Xu <herbert@gondor.apana.org.au>,
        "David S. Miller" <davem@davemloft.net>, Shaohua Li <shli@kernel.org>,
        Dmitry Vyukov <dvyukov@google.com>,
        Matthias Kaehlcke <mka@chromium.org>, x86@kernel.org,
        linux-kbuild@vger.kernel.org, LKML <linux-kernel@vger.kernel.org>,
        linux-crypto@vger.kernel.org, linux-raid@vger.kernel.org
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On Fri, Mar 17, 2017 at 5:44 AM, Peter Zijlstra <peterz@infradead.org> wrote:
>
> Be that as it may; what you construct above is disgusting. Surely the
> code can be refactored to not look like dog vomit?
>
> Also; its not immediately obvious conf->copies is 'small' and this
> doesn't blow up the stack; I feel that deserves a comment somewhere.
>

I agree that the code is horrible.

It is, in fact, exactly the same solution that was used to remove
variable length arrays in structs from several of the crypto drivers a
few years ago - see the definition of SHASH_DESC_ON_STACK() in
"crypto/hash.h" - I did not, however, hide the horrors in a macro
preferring to leave the implementation visible as a warning to whoever
might touch the code next.

I believe that the actual stack usage is exactly the same as it was previously.

I can certainly wrap this  up in a macro and add comments with
appropriately dire warnings in it if you feel that is both necessary
and sufficient.