linux-kernel - Re: div_u64/do_div stack size usage, was Re: [v3] block: Removed a warning while compiling with a cross compiler for parisc

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Date:   Wed, 7 Jul 2021 10:17:04 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Abd-Alrhman Masalkhi <abd.masalkhi@...il.com>
Cc:     Christoph Hellwig <hch@...radead.org>,
        Jens Axboe <axboe@...nel.dk>, bernie@...eler.com,
        Parisc List <linux-parisc@...r.kernel.org>,
        linux-block <linux-block@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Dan Carpenter <dan.carpenter@...cle.com>
Subject: Re: div_u64/do_div stack size usage, was Re: [v3] block: Removed a
 warning while compiling with a cross compiler for parisc

On Tue, Jul 6, 2021 at 10:59 PM Abd-Alrhman Masalkhi
<abd.masalkhi@...il.com> wrote:
>
> I have compiled the kernel with a cross compiler hppa-linux-gnu- (Ubuntu 9.3.0-17ubuntu1~20.04) 9.3.0 and the conficuration was the default, Build for generic-32bit "generic-32bit_defconfig"

Ok, thanks. I have reproduced this now with gcc-9.4.0 from
https://mirrors.edge.kernel.org/pub/tools/crosstool/files/bin/.

I have also tried it with the other gcc versions and shown that it
happens with every older compiler as well, but it does not happen
with gcc-10 or gcc-11, which bring the frame size down to 164 or
172 bytes respectively. gcc-10 also fixes similar warnings for
net/ipv4/tcp_input.c, net/sunrpc/stats.c and lib/xxhash.c that
fly under the radar with the default warning level.

My first thought was this was a result of -finline-functions getting
enabled by default in gcc-10, but enabling it on gcc-9 did not
help here. It's likely that the gcc behavior was fixed in the process
of enabling the more aggressive inliner by default though.

I also tried building genhd.o for every architecture that has
gcc-9.4 support and did not find the problem anywhere except
on parisc.

Using CONFIG_CC_OPTIMIZE_FOR_SIZE did solve the
problem for me (frame size down to 164 bytes), but I could not
pinpoint a specific -f option that fixes it for -O2. Maybe we can
simply change the defconfig here? 32-bit parisc systems are
probably memory limited enough that building a -Os kernel
is a good idea anyway. 64-bit parisc already builds with -Os
but does not see the warning with -O2 either.

    Arnd