linux-kernel - Re: [PATCH 6/7] selftests: only compile userfaultfd for x86 and powperpc

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Date:	Tue, 08 Sep 2015 19:54:13 +1000
From:	Michael Ellerman <mpe@...erman.id.au>
To:	Bamvor Zhang Jian <bamvor.zhangjian@...aro.org>
Cc:	linux-kernel@...r.kernel.org, Mark Brown <broonie@...nel.org>,
	khilman@...aro.org, tyler.baker@...aro.org,
	shuahkh@....samsung.com, Andrea Arcangeli <aarcange@...hat.com>
Subject: Re: [PATCH 6/7] selftests: only compile userfaultfd for x86 and
 powperpc

On Tue, 2015-09-08 at 17:15 +0800, Bamvor Zhang Jian wrote:
> Hi, Michael
> 
> I thought I reply to you, but ...
> 
> On 08/31/2015 11:26 AM, Michael Ellerman wrote:
> > On Fri, 2015-08-14 at 21:43 +0800, Bamvor Jian Zhang wrote:
> >> Signed-off-by: Bamvor Jian Zhang <bamvor.zhangjian@...aro.org>
> >> ---
> >>  tools/testing/selftests/vm/Makefile | 12 ++++++++++++
> >>  1 file changed, 12 insertions(+)
> >>
> >> diff --git a/tools/testing/selftests/vm/Makefile b/tools/testing/selftests/vm/Makefile
> >> index bb888c6..4dd6e4f 100644
> >> --- a/tools/testing/selftests/vm/Makefile
> >> +++ b/tools/testing/selftests/vm/Makefile
> >> @@ -1,5 +1,15 @@
> >>  # Makefile for vm selftests
> >>  
> >> +uname_M := $(shell uname -m 2>/dev/null || echo not)
> >> +ARCH ?= $(shell echo $(uname_M) | sed -e s/i.86/i386/ -e s/ppc.*/powerpc/)
> >> +
> >> +ifeq ($(ARCH),powerpc)
> >> +support_userfaultfd = yes
> >> +endif
> >> +ifeq ($(ARCH),x86)
> >> +support_userfaultfd = yes
> >> +endif
> >> +
> >>  CFLAGS = -Wall
> >>  BINARIES = compaction_test
> >>  BINARIES += hugepage-mmap
> >> @@ -9,7 +19,9 @@ BINARIES += mlock2-tests
> >>  BINARIES += on-fault-limit
> >>  BINARIES += thuge-gen
> >>  BINARIES += transhuge-stress
> >> +ifdef support_userfaultfd
> >>  BINARIES += userfaultfd
> >> +endif
> >>  
> >>  all: $(BINARIES)
> >>  %: %.c
> > 
> > 
> > This is nasty. It means when userfaultfd gets implemented for other arches
> > someone has to remember to update the logic here, which they won't.
> > 
> > Instead the C program should just do nothing when __NR_userfaultfd is not defined, eg:
> > 
> > #ifdef __NR_userfaultfd
> > 
> > int main(int argc, char **argv)
> > {
> > 	...
> > }
> > 
> > #else
> > 
> > int main(void)
> > {
> > 	printf("skip: Skipping userfaultfd test\n");
> > 	return 0;
> > }
> > #endif
> > 
> > 
> > This way when the syscall is implemented for other arches the test will just
> > start working.
> > 
> > cheers
> > 
> >
> When read the following code, It seems that sometimes __NR_userfaultfd is not
> defined but the syscall is exist. I am not sure why these piece is needed.
> cc'd c
> 
> #ifndef __NR_userfaultfd
> #ifdef __x86_64__
> #define __NR_userfaultfd 323
> #elif defined(__i386__)
> #define __NR_userfaultfd 374
> #elif defined(__powewrpc__)
> #define __NR_userfaultfd 364
> #else
> #error "missing __NR_userfaultfd definition"
> #endif
> #endif
> 
> Do you mean that we should remove the above code?

Well yes, it would need to be removed to make the logic I suggested work.

I'm not sure those #defines actually help in practice, because if the syscall
number is not defined then linux/userfaultfd.h will not exist and the whole
test will not compile anyway.

I was suggesting something like this, which has the properties of:
 - not breaking the build on arches that don't have the syscall
 - still printing a notice on arches that don't have the syscall, both at build
   time and runtime.
 - building correctly on an arch as soon as that arch implements the syscall,
   with no extra changes required.

cheers


diff --git a/tools/testing/selftests/vm/userfaultfd.c b/tools/testing/selftests/vm/userfaultfd.c
index 2bf1fc3f562b..652c9d805006 100644
--- a/tools/testing/selftests/vm/userfaultfd.c
+++ b/tools/testing/selftests/vm/userfaultfd.c
@@ -64,19 +64,10 @@
 #include <sys/syscall.h>
 #include <sys/ioctl.h>
 #include <pthread.h>
-#include <linux/userfaultfd.h>
 
-#ifndef __NR_userfaultfd
-#ifdef __x86_64__
-#define __NR_userfaultfd 323
-#elif defined(__i386__)
-#define __NR_userfaultfd 374
-#elif defined(__powewrpc__)
-#define __NR_userfaultfd 364
-#else
-#error "missing __NR_userfaultfd definition"
-#endif
-#endif
+#ifdef __NR_userfaultfd
+
+#include <linux/userfaultfd.h>
 
 static unsigned long nr_cpus, nr_pages, nr_pages_per_cpu, page_size;
 
@@ -636,3 +627,15 @@ int main(int argc, char **argv)
 	       nr_pages, nr_pages_per_cpu);
 	return userfaultfd_stress();
 }
+
+#else /* ! __NR_userfaultfd */
+
+#warning "missing __NR_userfaultfd definition"
+
+int main(void)
+{
+	printf("skip: Skipping userfaultfd test (missing __NR_userfaultfd)\n");
+	return 0;
+}
+
+#endif


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