linux-kernel - Re: [PATCH 2/2] fs: Support compiling out sendfile

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Date:	Mon, 20 Oct 2014 15:24:22 -0700
From:	josh@...htriplett.org
To:	Pieter Smith <pieter@...sman.nl>
Cc:	Alexander Viro <viro@...iv.linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Eric Paris <eparis@...hat.com>,
	Matt Turner <mattst88@...il.com>,
	Michal Hocko <mhocko@...e.cz>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Fabian Frederick <fabf@...net.be>, Tejun Heo <tj@...nel.org>,
	蔡正龙 <zhenglong.cai@...c.com.cn>,
	"Luis R. Rodriguez" <mcgrof@...e.com>,
	Peter Foley <pefoley2@...oley.com>,
	Konstantin Khlebnikov <koct9i@...il.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	"H. Peter Anvin" <hpa@...or.com>, Oleg Nesterov <oleg@...hat.com>,
	Andy Lutomirski <luto@...capital.net>,
	David Herrmann <dh.herrmann@...il.com>,
	Kees Cook <keescook@...omium.org>,
	linux-fsdevel@...r.kernel.org,
	open list <linux-kernel@...r.kernel.org>,
	"open list:ABI/API" <linux-api@...r.kernel.org>
Subject: Re: [PATCH 2/2] fs: Support compiling out sendfile

On Mon, Oct 20, 2014 at 11:48:37PM +0200, Pieter Smith wrote:
> Many embedded systems will not need this syscall, and omitting it
> saves space.  Add a new EXPERT config option CONFIG_SENDFILE_SYSCALL
> (default y) to support compiling it out.

Nice work, thanks!

If there are no objections, and nobody has a tree they'd rather carry
this through, I'll take the series through the tiny tree when it's ready
to merge.

> bloat-o-meter:
> add/remove: 0/4 grow/shrink: 5/0 up/down: 23/-751 (-728)
> function                                     old     new   delta
> sys_pwritev                                  115     122      +7
> sys_preadv                                   115     122      +7
> fdput_pos                                     29      36      +7
> sys_pwrite64                                 115     116      +1
> sys_pread64                                  115     116      +1
> fdput                                         11       -     -11
> sys_sendfile                                 122       -    -122
> sys_sendfile64                               126       -    -126
> do_sendfile                                  492       -    -492

Interesting inlining decisions by GCC here.  Got a bloat-o-meter for the
two-patch series, by any chance?  (Also, is this with tinyconfig?  In
particular, with OPTIMIZE_INLINING and OPTIMIZE_FOR_SIZE?)  I'm
wondering if moving sendfile to a separate file made GCC put fdput
out-of-line, and compiling it out reversed that again.

> Signed-off-by: Pieter Smith <pieter@...sman.nl>

Reviewed-by: Josh Triplett <josh@...htriplett.org>

> ---
>  fs/Makefile     |  3 ++-
>  init/Kconfig    | 10 ++++++++++
>  kernel/sys_ni.c |  4 ++++
>  3 files changed, 16 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/Makefile b/fs/Makefile
> index 1e3423f..1bbfea7 100644
> --- a/fs/Makefile
> +++ b/fs/Makefile
> @@ -5,7 +5,7 @@
>  # Rewritten to use lists instead of if-statements.
>  # 
>  
> -obj-y :=	open.o read_write.o sendfile.o file_table.o super.o \
> +obj-y :=	open.o read_write.o file_table.o super.o \
>  		char_dev.o stat.o exec.o pipe.o namei.o fcntl.o \
>  		ioctl.o readdir.o select.o dcache.o inode.o \
>  		attr.o bad_inode.o file.o filesystems.o namespace.o \
> @@ -38,6 +38,7 @@ obj-$(CONFIG_COMPAT_BINFMT_ELF)	+= compat_binfmt_elf.o
>  obj-$(CONFIG_BINFMT_ELF_FDPIC)	+= binfmt_elf_fdpic.o
>  obj-$(CONFIG_BINFMT_SOM)	+= binfmt_som.o
>  obj-$(CONFIG_BINFMT_FLAT)	+= binfmt_flat.o
> +obj-$(CONFIG_SENDFILE_SYSCALL)	+= sendfile.o
>  
>  obj-$(CONFIG_FS_MBCACHE)	+= mbcache.o
>  obj-$(CONFIG_FS_POSIX_ACL)	+= posix_acl.o
> diff --git a/init/Kconfig b/init/Kconfig
> index 782a65b..df6785c 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -1547,6 +1547,16 @@ config ADVISE_SYSCALLS
>  	  applications use these syscalls, you can disable this option to save
>  	  space.
>  
> +config SENDFILE_SYSCALL
> +	bool "Enable sendfile syscall" if EXPERT
> +	default y
> +	help
> +	  This option enables the sendfile syscall, used by applications to copy
> +	  data between file descriptors. Because sendfile performs the copying
> +	  within the kernel, it is more efficient than the combination of read
> +	  and write.  If building an embedded system where no applications use
> +	  the sendfile syscall, you can disable this option to save space.
> +

I'm thinking of adding a submenu to group config FOO_SYSCALL options. :)
I'll probably push that as part of the 3.19 merge window, as a patch on
top of all of the individual tinification options.

>  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 d4709d4..b068de7 100644
> --- a/kernel/sys_ni.c
> +++ b/kernel/sys_ni.c
> @@ -159,6 +159,10 @@ cond_syscall(sys_uselib);
>  cond_syscall(sys_fadvise64);
>  cond_syscall(sys_fadvise64_64);
>  cond_syscall(sys_madvise);
> +cond_syscall(sys_sendfile);
> +cond_syscall(sys_sendfile64);
> +cond_syscall(compat_sys_sendfile);
> +cond_syscall(compat_sys_sendfile64);
>  
>  /* arch-specific weak syscall entries */
>  cond_syscall(sys_pciconfig_read);
> -- 
> 1.9.1
> 
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