Re: [PATCH v2] fs/select: mark do_select noinline_for_stack

[Nathan Chancellor]

On Tue, Oct 11, 2022 at 01:55:47PM -0700, Nick Desaulniers wrote:
> Effectively a revert of
> commit ad312f95d41c ("fs/select: avoid clang stack usage warning")
> 
> Various configs can still push the stack useage of core_sys_select()
> over the CONFIG_FRAME_WARN threshold (1024B on 32b targets).
> 
>   fs/select.c:619:5: error: stack frame size of 1048 bytes in function
>   'core_sys_select' [-Werror,-Wframe-larger-than=]
> 
> core_sys_select() has a large stack allocation for `stack_fds` where it
> tries to do something equivalent to "small string optimization" to
> potentially avoid a kmalloc.
> 
> core_sys_select() calls do_select() which has another potentially large
> stack allocation, `table`. Both of these values depend on
> FRONTEND_STACK_ALLOC.
> 
> Mix those two large allocation with register spills which are
> exacerbated by various configs and compiler versions and we can just
> barely exceed the 1024B limit.
> 
> Rather than keep trying to find the right value of MAX_STACK_ALLOC or
> FRONTEND_STACK_ALLOC, mark do_select() as noinline_for_stack.
> 
> The intent of FRONTEND_STACK_ALLOC is to help potentially avoid a
> dynamic memory allocation. In that spirit, restore the previous
> threshold but separate the stack frames.
> 
> Many tests of various configs for different architectures and various
> versions of GCC were performed; do_select() was never inlined into
> core_sys_select() or compat_core_sys_select(). The kernel is built with
> the GCC specific flag `-fconserve-stack` which can limit inlining
> depending on per-target thresholds of callee stack size, which helps
> avoid the issue when using GCC. Clang is being more aggressive and not
> considering the stack size when decided whether to inline or not. We may
> consider using the clang-16+ flag `-finline-max-stacksize=` in the
> future.
> 
> Link: https://lore.kernel.org/lkml/20221006222124.aabaemy7ofop7ccz@xxxxxxxxxx/
> Fixes: ad312f95d41c ("fs/select: avoid clang stack usage warning")
> Suggested-by: Arnd Bergmann <arnd@xxxxxxxx>
> Signed-off-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>

Tested-by: Nathan Chancellor <nathan@xxxxxxxxxx>

> ---
> Changes v1 -> v2:
> * Drop the 32b specific guard, since I could reproduce the no-inlining
>   w/ aarch64-linux-gnu-gcc-10 ARCH=arm64 defconfig, and per Arnd.
> * Drop references to 32b in commit message.
> * Add new paragraph in commit message at the end about -fconserve-stack
>   and -finline-max-stacksize=.
> * s/malloc/kmalloc/ in commit message.
> 
>  fs/select.c          | 1 +
>  include/linux/poll.h | 4 ----
>  2 files changed, 1 insertion(+), 4 deletions(-)
> 
> diff --git a/fs/select.c b/fs/select.c
> index 0ee55af1a55c..794e2a91b1fa 100644
> --- a/fs/select.c
> +++ b/fs/select.c
> @@ -476,6 +476,7 @@ static inline void wait_key_set(poll_table *wait, unsigned long in,
>  		wait->_key |= POLLOUT_SET;
>  }
>  
> +noinline_for_stack
>  static int do_select(int n, fd_set_bits *fds, struct timespec64 *end_time)
>  {
>  	ktime_t expire, *to = NULL;
> 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
> -- 
> 2.38.0.rc2.412.g84df46c1b4-goog
> 
>