linux-kernel - Re: [PATCH] fs/select: rework stack allocation hack for clang

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Message-ID: <20240219113437.y7vxeyhvjnxo7rlh@quack3>
Date: Mon, 19 Feb 2024 12:34:37 +0100
From: Jan Kara <jack@...e.cz>
To: Arnd Bergmann <arnd@...nel.org>
Cc: Alexander Viro <viro@...iv.linux.org.uk>,
	Christian Brauner <brauner@...nel.org>,
	Arnd Bergmann <arnd@...db.de>, Jan Kara <jack@...e.cz>,
	Nathan Chancellor <nathan@...nel.org>,
	Nick Desaulniers <ndesaulniers@...gle.com>,
	Bill Wendling <morbo@...gle.com>,
	Justin Stitt <justinstitt@...gle.com>,
	Kees Cook <keescook@...omium.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andi Kleen <ak@...ux.intel.com>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, llvm@...ts.linux.dev
Subject: Re: [PATCH] fs/select: rework stack allocation hack for clang

On Fri 16-02-24 21:23:34, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@...db.de>
> 
> A while ago, we changed the way that select() and poll() preallocate
> a temporary buffer just under the size of the static warning limit of
> 1024 bytes, as clang was frequently going slightly above that limit.
> 
> The warnings have recently returned and I took another look. As it turns
> out, clang is not actually inherently worse at reserving stack space,
> it just happens to inline do_select() into core_sys_select(), while gcc
> never inlines it.
> 
> Annotate do_select() to never be inlined and in turn remove the special
> case for the allocation size. This should give the same behavior for
> both clang and gcc all the time and once more avoids those warnings.
> 
> Fixes: ad312f95d41c ("fs/select: avoid clang stack usage warning")
> Signed-off-by: Arnd Bergmann <arnd@...db.de>

Looks good (if this indeed works with clang ;). Feel free to add:

Reviewed-by: Jan Kara <jack@...e.cz>

								Honza

> ---
>  fs/select.c          | 2 +-
>  include/linux/poll.h | 4 ----
>  2 files changed, 1 insertion(+), 5 deletions(-)
> 
> diff --git a/fs/select.c b/fs/select.c
> index 11a3b1312abe..9515c3fa1a03 100644
> --- a/fs/select.c
> +++ b/fs/select.c
> @@ -476,7 +476,7 @@ static inline void wait_key_set(poll_table *wait, unsigned long in,
>  		wait->_key |= POLLOUT_SET;
>  }
>  
> -static int do_select(int n, fd_set_bits *fds, struct timespec64 *end_time)
> +static noinline_for_stack int do_select(int n, fd_set_bits *fds, struct timespec64 *end_time)
>  {
>  	ktime_t expire, *to = NULL;
>  	struct poll_wqueues table;
> diff --git a/include/linux/poll.h b/include/linux/poll.h
> index a9e0e1c2d1f2..d1ea4f3714a8 100644
> --- a/include/linux/poll.h
> +++ b/include/linux/poll.h
> @@ -14,11 +14,7 @@
>  
>  /* ~832 bytes of stack space used max in sys_select/sys_poll before allocating
>     additional memory. */
> -#ifdef __clang__
> -#define MAX_STACK_ALLOC 768
> -#else
>  #define MAX_STACK_ALLOC 832
> -#endif
>  #define FRONTEND_STACK_ALLOC	256
>  #define SELECT_STACK_ALLOC	FRONTEND_STACK_ALLOC
>  #define POLL_STACK_ALLOC	FRONTEND_STACK_ALLOC
> -- 
> 2.39.2
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR