Re: [PATCH 6/7] md/raid10, LLVM: get rid of variable length array

From: Peter Zijlstra <peterz@infradead.org>
To: Michael Davidson <md@google.com>
Cc: 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>,
	Alexander Potapenko <glider@google.com>,
	Dmitry Vyukov <dvyukov@google.com>,
	Matthias Kaehlcke <mka@chromium.org>,
	x86@kernel.org, linux-kbuild@vger.kernel.org,
	linux-kernel@vger.kernel.org, linux-crypto@vger.kernel.org,
	linux-raid@vger.kernel.org
Subject: Re: [PATCH 6/7] md/raid10, LLVM: get rid of variable length array
Date: Fri, 17 Mar 2017 13:08:37 +0100	[thread overview]
Message-ID: <20170317120837.pr74cv3xuj7qpoin@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20170317001520.85223-7-md@google.com>

On Thu, Mar 16, 2017 at 05:15:19PM -0700, Michael Davidson wrote:
> Replace a variable length array in a struct by allocating
> the memory for the entire struct in a char array on the stack.
> 
> Signed-off-by: Michael Davidson <md@google.com>
> ---
>  drivers/md/raid10.c | 9 ++++-----
>  1 file changed, 4 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/md/raid10.c b/drivers/md/raid10.c
> index 063c43d83b72..158ebdff782c 100644
> --- a/drivers/md/raid10.c
> +++ b/drivers/md/raid10.c
> @@ -4654,11 +4654,10 @@ static int handle_reshape_read_error(struct mddev *mddev,
>  	/* Use sync reads to get the blocks from somewhere else */
>  	int sectors = r10_bio->sectors;
>  	struct r10conf *conf = mddev->private;
> -	struct {
> -		struct r10bio r10_bio;
> -		struct r10dev devs[conf->copies];
> -	} on_stack;
> -	struct r10bio *r10b = &on_stack.r10_bio;
> +	char on_stack_r10_bio[sizeof(struct r10bio) +
> +			      conf->copies * sizeof(struct r10dev)]
> +			      __aligned(__alignof__(struct r10bio));
> +	struct r10bio *r10b = (struct r10bio *)on_stack_r10_bio;
>  	int slot = 0;
>  	int idx = 0;
>  	struct bio_vec *bvec = r10_bio->master_bio->bi_io_vec;

That's disgusting. Why not fix LLVM to support this?