Re: [GIT PULL] Modules updates for v5.12

From: Jessica Yu
Date: Wed Feb 24 2021 - 10:08:56 EST

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+++ Christoph Hellwig [24/02/21 08:52 +0100]:

On Tue, Feb 23, 2021 at 12:07:39PM -0800, Linus Torvalds wrote:

On Tue, Feb 23, 2021 at 12:03 PM Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> This is unacceptably slow. If that symbol trimming takes 30% of the
> whole kernel build time, it needs to be fixed or removed.

I think I'm going to mark TRIM_UNUSED_KSYMS as "depends on BROKEN".
There's no way I can accept that horrible overhead, and the rationale
for that config option is questionable at best,

I think it is pretty useful for embedded setups.

BROKEN seems pretty strong for something that absolutely works as
intendended. I guess to make you (and possibly others) not grumpy
we just need to ensure it doesn't get pulled in by allmodconfig.

So maybe just invert the symbol, default the KEEP_UNUSED_SYMBOL, and
add a message to the helptext explaining the slowdown?

Hm, something like this maybe? (untested)

---
From 08bc08229fc3801b1a580a07ce7ff3e806b3fe90 Mon Sep 17 00:00:00 2001
From: Jessica Yu <jeyu@xxxxxxxxxx>
Date: Wed, 24 Feb 2021 14:54:09 +0100
Subject: [PATCH] Kconfig: invert TRIM_UNUSED_SYMBOLS option and rename it to
KEEP_UNUSED_SYMBOLS

Removing CONFIG_UNUSED_SYMBOLS (commit 367948220fce "module: remove
EXPORT_UNUSED_SYMBOL*") unhid the CONFIG_TRIM_UNUSED_SYMBOLS option and
therefore it now gets automatically enabled with allyesconfig.

To prevent allyesconfig from enabling TRIM_UNUSED_SYMBOLS (which is known
to slow build times), invert the config option and name it
KEEP_UNUSED_SYMBOLS, which does the same thing as TRIM_UNUSED_SYMBOLS=n.
That way, allyesconfig will keep the previous behavior of not trimming
symbols.

Signed-off-by: Jessica Yu <jeyu@xxxxxxxxxx>
---
Makefile | 4 ++--
arch/mips/configs/generic_defconfig | 2 +-
include/asm-generic/export.h | 2 +-
include/linux/export.h | 2 +-
init/Kconfig | 21 +++++++++++----------
kernel/livepatch/Kconfig | 2 +-
scripts/Makefile.build | 2 +-
7 files changed, 18 insertions(+), 17 deletions(-)

diff --git a/Makefile b/Makefile
index b18dbc634690..23f50521e97f 100644
--- a/Makefile
+++ b/Makefile
@@ -1164,7 +1164,7 @@ vmlinux-deps := $(KBUILD_LDS) $(KBUILD_VMLINUX_OBJS) $(KBUILD_VMLINUX_LIBS)
# Recurse until adjust_autoksyms.sh is satisfied
PHONY += autoksyms_recursive
-ifdef CONFIG_TRIM_UNUSED_KSYMS
+ifndef CONFIG_KEEP_UNUSED_KSYMS
# For the kernel to actually contain only the needed exported symbols,
# we have to build modules as well to determine what those symbols are.
# (this can be evaluated only once include/config/auto.conf has been included)
@@ -1175,7 +1175,7 @@ autoksyms_recursive: descend modules.order
"$(MAKE) -f $(srctree)/Makefile vmlinux"
endif
-autoksyms_h := $(if $(CONFIG_TRIM_UNUSED_KSYMS), include/generated/autoksyms.h)
+autoksyms_h := $(if $(CONFIG_KEEP_UNUSED_KSYMS),,include/generated/autoksyms.h)
quiet_cmd_autoksyms_h = GEN $@
cmd_autoksyms_h = mkdir -p $(dir $@); \
diff --git a/arch/mips/configs/generic_defconfig b/arch/mips/configs/generic_defconfig
index 714169e411cf..d46da28ea032 100644
--- a/arch/mips/configs/generic_defconfig
+++ b/arch/mips/configs/generic_defconfig
@@ -29,7 +29,7 @@ CONFIG_MIPS_O32_FP64_SUPPORT=y
CONFIG_JUMP_LABEL=y
CONFIG_MODULES=y
CONFIG_MODULE_UNLOAD=y
-CONFIG_TRIM_UNUSED_KSYMS=y
+CONFIG_KEEP_UNUSED_KSYMS=n
CONFIG_NET=y
CONFIG_PACKET=y
CONFIG_UNIX=y
diff --git a/include/asm-generic/export.h b/include/asm-generic/export.h
index 07a36a874dca..06d401464195 100644
--- a/include/asm-generic/export.h
+++ b/include/asm-generic/export.h
@@ -57,7 +57,7 @@ __kstrtab_\name:
#endif
.endm
-#if defined(CONFIG_TRIM_UNUSED_KSYMS)
+#if !defined(CONFIG_KEEP_UNUSED_KSYMS)
#include <linux/kconfig.h>
#include <generated/autoksyms.h>
diff --git a/include/linux/export.h b/include/linux/export.h
index 6271a5d9c988..449f7d15e580 100644
--- a/include/linux/export.h
+++ b/include/linux/export.h
@@ -118,7 +118,7 @@ struct kernel_symbol {
*/
#define __EXPORT_SYMBOL(sym, sec, ns)
-#elif defined(CONFIG_TRIM_UNUSED_KSYMS)
+#elif !defined(CONFIG_KEEP_UNUSED_KSYMS)
#include <generated/autoksyms.h>
diff --git a/init/Kconfig b/init/Kconfig
index ba8bd5256980..db5d00bfc239 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -2272,29 +2272,30 @@ config MODULE_ALLOW_MISSING_NAMESPACE_IMPORTS
If unsure, say N.
-config TRIM_UNUSED_KSYMS
- bool "Trim unused exported kernel symbols"
- depends on BROKEN
+config KEEP_UNUSED_KSYMS
+ bool "Keep unused exported kernel symbols"
+ default y
help
The kernel and some modules make many symbols available for
other modules to use via EXPORT_SYMBOL() and variants. Depending
on the set of modules being selected in your kernel configuration,
many of those exported symbols might never be used.
- This option allows for unused exported symbols to be dropped from
- the build. In turn, this provides the compiler more opportunities
- (especially when using LTO) for optimizing the code and reducing
- binary size. This might have some security advantages as well.
+ This option allows for unused exported symbols to be kept in the
+ build. Say N when you want to trim unused symbols from the build,
+ which provides the compiler more opportunities (especially when using LTO)
+ for optimizing the code and reducing binary size. This might have some
+ security advantages as well.
- If unsure, or if you need to build out-of-tree modules, say N.
+ If unsure, or if you need to build out-of-tree modules, say Y.
config UNUSED_KSYMS_WHITELIST
string "Whitelist of symbols to keep in ksymtab"
- depends on TRIM_UNUSED_KSYMS
+ depends on !KEEP_UNUSED_KSYMS
default "scripts/lto-used-symbollist.txt" if LTO_CLANG
help
By default, all unused exported symbols will be un-exported from the
- build when TRIM_UNUSED_KSYMS is selected.
+ build when KEEP_UNUSED_KSYMS is not selected.
UNUSED_KSYMS_WHITELIST allows to whitelist symbols that must be kept
exported at all times, even in absence of in-tree users. The value to
diff --git a/kernel/livepatch/Kconfig b/kernel/livepatch/Kconfig
index 53d51ed619a3..df8ebb7984e1 100644
--- a/kernel/livepatch/Kconfig
+++ b/kernel/livepatch/Kconfig
@@ -11,7 +11,7 @@ config LIVEPATCH
depends on SYSFS
depends on KALLSYMS_ALL
depends on HAVE_LIVEPATCH
- depends on !TRIM_UNUSED_KSYMS
+ depends on KEEP_UNUSED_KSYMS
help
Say Y here if you want to support kernel live patching.
This option has no runtime impact until a kernel "patch"
diff --git a/scripts/Makefile.build b/scripts/Makefile.build
index 3f6bf0ea7c0e..e5e95a6948a7 100644
--- a/scripts/Makefile.build
+++ b/scripts/Makefile.build
@@ -242,7 +242,7 @@ objtool_dep = $(objtool_obj) \
$(wildcard include/config/orc/unwinder.h \
include/config/stack/validation.h)
-ifdef CONFIG_TRIM_UNUSED_KSYMS
+ifndef CONFIG_KEEP_UNUSED_KSYMS
cmd_gen_ksymdeps = \
$(CONFIG_SHELL) $(srctree)/scripts/gen_ksymdeps.sh $@ >> $(dot-target).cmd

Next message: Catalin Marinas: "Re: [PATCH v14 01/11] x86: kdump: replace the hard-coded alignment with macro CRASH_ALIGN"
Previous message: David Edmondson: "[PATCH v4 2/5] KVM: x86: dump_vmcs should not conflate EFER and PAT presence in VMCS"
In reply to: Christoph Hellwig: "Re: [GIT PULL] Modules updates for v5.12"
Next in thread: Masahiro Yamada: "Re: [GIT PULL] Modules updates for v5.12"
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