On Thu, Nov 06, 2025 at 05:32:47PM +0530, Ankit Khushwaha wrote:
> On Thu, Nov 06, 2025 at 12:18:57PM +0100, David Hildenbrand (Red Hat) wrote:
> > On 06.11.25 11:49, Ankit Khushwaha wrote:
> > > Pointer arthemitic with 'void * addr' and 'unsigned long long
> > > dest_alignment'
> > > triggers following warning:
> > >
> > > mremap_test.c:1035:31: warning: pointer comparison always evaluates to
> > > false [-Wtautological-compare]
> > > 1035 | if (addr + c.dest_alignment < addr) {
> > > | ^
> > >
> > > typecasting 'addr' to 'unsigned long long' to fix pointer comparison.
> >
> > With which compiler are you seeing this?
>
> Hi David,
>
> clang version 20.1.8 (Fedora 20.1.8-4.fc42) raised this warning.
>
> To reproduce:
> make -C tools/testing/selftests/mm CC=clang
FYI That doesn't work:
$ make -C tools/testing/selftests/mm CC=clang
make: Entering directory '/data/kerndev/kernels/mm/tools/testing/selftests/mm'
warning: the compiler differs from the one used to build the kernel
The kernel was built by: gcc (GCC) 15.2.1 20250813
You are using: clang version 20.1.8
CC [M] page_frag_test.o
clang: error: unknown argument: '-fmin-function-alignment=16'
clang: error: unknown argument: '-fconserve-stack'
clang: error: unsupported option '-mrecord-mcount' for target
'x86_64-unknown-linux-gnu'
make[4]: *** [/usr/lib/modules/6.17.2-arch1-1/build/scripts/Makefile.build:287:
page_frag_test.o] Error 1
make[3]: *** [/usr/lib/modules/6.17.2-arch1-1/build/Makefile:2011: .] Error 2
make[2]: *** [Makefile:248: __sub-make] Error 2
make[1]: *** [Makefile:15: all] Error 2
make: *** [../lib.mk:146: gen_mods_dir] Error 2
make: Leaving directory '/data/kerndev/kernels/mm/tools/testing/selftests/mm'
[649ms][2][review/loz-v2][~/kerndev/kernels/mm]$ cd tools/testing/selftests/mm
Even if I rebuild the entire kernel using clang via LLVM=1 it doesn't work.
The following KIND OF works:
$ make -C tools/testing/selftests/mm clean
$ make tools/testing/selftests/mm LLVM=1
But I still get:
warning: the compiler differs from the one used to build the kernel
The kernel was built by: gcc (GCC) 15.2.1 20250813
You are using: clang version 20.1.8
CC [M] page_frag_test.o
clang: error: unknown argument: '-fmin-function-alignment=16'
clang: error: unknown argument: '-fconserve-stack'
clang: error: unsupported option '-mrecord-mcount' for target
'x86_64-unknown-linux-gnu'
make[4]: *** [/usr/lib/modules/6.17.2-arch1-1/build/scripts/Makefile.build:287:
page_frag_test.o] Error 1
make[3]: *** [/usr/lib/modules/6.17.2-arch1-1/build/Makefile:2011: .] Error 2
make[2]: *** [Makefile:248: __sub-make] Error 2
make[1]: *** [Makefile:15: all] Error 2
make: *** [../lib.mk:146: gen_mods_dir] Error 2
make: Leaving directory '/data/kerndev/kernels/mm/tools/testing/selftests/mm'
Errors, presumably unless you build _the entire kernel_ using LLVM=1 :)
>
> Thanks,
> --
> Ankit
It may be worth looking at how to make this behave better because this seems
silly if clang will pick up additional warnings.
Cheers, Lorenzo
