Subject: Re: [PATCH v4 3/7] fs/splice: support compiling out splice-family syscalls

From: Josh Triplett <[email-masked]>
Date: Mon, 24 Nov 2014 16:49:45 -0800

On Tue, Nov 25, 2014 at 12:01:02AM +0100, Pieter Smith wrote:
> Many embedded systems will not need the splice-family syscalls (splice,
> vmsplice, tee and sendfile). Omitting them saves space.  This adds a new EXPERT
> config option CONFIG_SYSCALL_SPLICE (default y) to support compiling them out.
> 
> The goal is to completely compile out fs/splice along with the syscalls. To
> achieve this, the remaining patch-set will deal with fs/splice exports. As far
> as possible, the impact on other device drivers will be minimized so as to
> reduce the overal maintenance burden of CONFIG_SYSCALL_SPLICE.
> 
> The use of exported functions will be solved by transparently mocking them out
> with static inlines. Uses of the exported pipe_buf_operations struct however
> require direct modification in fs/fuse and net/core. The next two patches will
> deal with this. A macro is defined that will assist with NULL'ing out callbacks
> when CONFIG_SYSCALL_SPLICE is undefined: __splice_p().

This message needs updating, since the patch series doesn't introduce or
use __splice_p anymore.

> Once all exports are solved, fs/splice can be compiled out.
> 
> The bloat benefit of this patch given a tinyconfig is:
> 
> add/remove: 0/16 grow/shrink: 2/5 up/down: 114/-3693 (-3579)
> function                                     old     new   delta
> splice_direct_to_actor                       348     416     +68
> splice_to_pipe                               371     417     +46
> splice_from_pipe_next                        107     106      -1
> fdput                                         11       -     -11
> signal_pending                                39      26     -13
> fdget                                         56      42     -14
> user_page_pipe_buf_ops                        20       -     -20
> user_page_pipe_buf_steal                      25       -     -25
> file_end_write                                58      29     -29
> file_start_write                              68      34     -34
> pipe_to_user                                  43       -     -43
> wakeup_pipe_readers                           54       -     -54
> do_splice_to                                  87       -     -87
> ipipe_prep.part                               92       -     -92
> opipe_prep.part                              119       -    -119
> sys_sendfile                                 122       -    -122
> sys_sendfile64                               126       -    -126
> sys_vmsplice                                 137       -    -137
> vmsplice_to_user                             205       -    -205
> sys_tee                                      491       -    -491
> do_sendfile                                  492       -    -492
> vmsplice_to_pipe                             558       -    -558
> sys_splice                                  1020       -   -1020
> 
> Signed-off-by: Pieter Smith <[email-masked]>
> ---
>  fs/splice.c     |  2 ++
>  init/Kconfig    | 10 ++++++++++
>  kernel/sys_ni.c |  8 ++++++++
>  3 files changed, 20 insertions(+)
> 
> diff --git a/fs/splice.c b/fs/splice.c
> index 44b201b..7c4c695 100644
> --- a/fs/splice.c
> +++ b/fs/splice.c
> @@ -1316,6 +1316,7 @@ long do_splice_direct(struct file *in, loff_t *ppos, struct file *out,
>  	return ret;
>  }
>  
> +#ifdef CONFIG_SYSCALL_SPLICE
>  static int splice_pipe_to_pipe(struct pipe_inode_info *ipipe,
>  			       struct pipe_inode_info *opipe,
>  			       size_t len, unsigned int flags);
> @@ -2200,4 +2201,5 @@ COMPAT_SYSCALL_DEFINE4(sendfile64, int, out_fd, int, in_fd,
>  	return do_sendfile(out_fd, in_fd, NULL, count, 0);
>  }
>  #endif
> +#endif
>  
> diff --git a/init/Kconfig b/init/Kconfig
> index d811d5f..dec9819 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -1571,6 +1571,16 @@ config NTP
>  	  system clock to an NTP server, you can disable this option to save
>  	  space.
>  
> +config SYSCALL_SPLICE
> +	bool "Enable splice/vmsplice/tee/sendfile syscalls" if EXPERT
> +	default y
> +	help
> +	  This option enables the splice, vmsplice, tee and sendfile syscalls. These
> +	  are used by applications to: move data between buffers and arbitrary file
> +	  descriptors; "copy" data between buffers; or copy data from userspace into
> +	  buffers. If building an embedded system where no applications use these
> +	  syscalls, you can disable this option to save space.
> +
>  config PCI_QUIRKS
>  	default y
>  	bool "Enable PCI quirk workarounds" if EXPERT
> diff --git a/kernel/sys_ni.c b/kernel/sys_ni.c
> index d2f5b00..25d5551 100644
> --- a/kernel/sys_ni.c
> +++ b/kernel/sys_ni.c
> @@ -170,6 +170,14 @@ cond_syscall(sys_fstat);
>  cond_syscall(sys_stat);
>  cond_syscall(sys_uname);
>  cond_syscall(sys_olduname);
> +cond_syscall(sys_vmsplice);
> +cond_syscall(sys_splice);
> +cond_syscall(sys_tee);
> +cond_syscall(sys_sendfile);
> +cond_syscall(sys_sendfile64);
> +cond_syscall(compat_sys_vmsplice);
> +cond_syscall(compat_sys_sendfile);
> +cond_syscall(compat_sys_sendfile64);
>  
>  /* arch-specific weak syscall entries */
>  cond_syscall(sys_pciconfig_read);
> -- 
> 2.1.0
>