Re: [PATCH v3 1/1] mtd: cfi_cmdset_0001: Factor out do_write_buffer_locked() to reduce stack frame

From: Lukas Wunner <lukas@wunner.de>
To: David Laight <david.laight.linux@gmail.com>
Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
	linux-mtd@lists.infradead.org, linux-kernel@vger.kernel.org,
	Miquel Raynal <miquel.raynal@bootlin.com>,
	Richard Weinberger <richard@nod.at>,
	Vignesh Raghavendra <vigneshr@ti.com>,
	Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH v3 1/1] mtd: cfi_cmdset_0001: Factor out do_write_buffer_locked() to reduce stack frame
Date: Thu, 9 Apr 2026 09:58:28 +0200	[thread overview]
Message-ID: <addcJP2erqYh8pD2@wunner.de> (raw)
In-Reply-To: <20260409082611.73fac9ab@pumpkin>

On Thu, Apr 09, 2026 at 08:26:11AM +0100, David Laight wrote:
> On Wed,  8 Apr 2026 23:11:48 +0200 Andy Shevchenko <andriy.shevchenko@linux.intel.com> wrote:
> > Compiler is not happy about used stack frame:
> > 
> > drivers/mtd/chips/cfi_cmdset_0001.c: In function 'do_write_buffer':
> > drivers/mtd/chips/cfi_cmdset_0001.c:1887:1: error: the frame size of 1296 bytes is larger than 1280 bytes [-Werror=frame-larger-than=]
> > 
> > Fix this by factoring out do_write_buffer_locked().
> 
> Does this just split the large stack frame between two nested functions?
> I'd also expect the compiler to inline do_write_buffer_locked() so it
> makes little difference.
> OTOH I can't immediately see where the large stack frame comes from.

The error occurs for an allmodconfig build on arm, which implies
CONFIG_KASAN_STACK=y and thus increases stack usage vis-à-vis a
"regular" build.

Stack usage is high here because of the three "map_word" types,
which can each be up to 256 unsigned longs (32 * 8), see the
definitions of MAX_MAP_LONGS, MAX_MAP_BANKWIDTH, map_word in
include/linux/mtd/map.h.

Possible solutions:

- Disable KASAN entirely for this file:
  https://lore.kernel.org/all/adX3SHYgazijahbG@wunner.de/

  Not always a good option, particularly for stuff like lib/maple_tree.c
  where the same issue exists in mas_wr_spanning_store() and KASAN would
  certainly be good to have for that one.

- Use heap instead of stack.

- Split function in smaller chunks and mark them "noinline".

Thanks,

Lukas

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