Re: [PATCH v3 3/7] mm: Add PG_arch_3 page flag

[Marc Zyngier <maz@kernel.org>]

On Mon, 05 Sep 2022 18:01:55 +0100,
Catalin Marinas <catalin.marinas@arm.com> wrote:
> 
> On Thu, Sep 01, 2022 at 06:59:23PM +0100, Catalin Marinas wrote:
> > On Thu, Aug 11, 2022 at 03:16:08PM +0800, kernel test robot wrote:
> > > Thank you for the patch! Perhaps something to improve:
> > > 
> > > [auto build test WARNING on arm64/for-next/core]
> > > [also build test WARNING on linus/master next-20220811]
> > > [cannot apply to kvmarm/next arm/for-next soc/for-next xilinx-xlnx/master v5.19]
> > > [If your patch is applied to the wrong git tree, kindly drop us a note.
> > > And when submitting patch, we suggest to use '--base' as documented in
> > > https://git-scm.com/docs/git-format-patch#_base_tree_information]
> > > 
> > > url:    https://github.com/intel-lab-lkp/linux/commits/Peter-Collingbourne/KVM-arm64-permit-MAP_SHARED-mappings-with-MTE-enabled/20220811-033310
> > > base:   https://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux.git for-next/core
> > > config: loongarch-defconfig (https://download.01.org/0day-ci/archive/20220811/202208111500.62e0Bl2l-lkp@intel.com/config)
> > > compiler: loongarch64-linux-gcc (GCC) 12.1.0
> > > reproduce (this is a W=1 build):
> > >         wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
> > >         chmod +x ~/bin/make.cross
> > >         # https://github.com/intel-lab-lkp/linux/commit/1a400517d8428df0ec9f86f8d303b2227ee9702f
> > >         git remote add linux-review https://github.com/intel-lab-lkp/linux
> > >         git fetch --no-tags linux-review Peter-Collingbourne/KVM-arm64-permit-MAP_SHARED-mappings-with-MTE-enabled/20220811-033310
> > >         git checkout 1a400517d8428df0ec9f86f8d303b2227ee9702f
> > >         # save the config file
> > >         mkdir build_dir && cp config build_dir/.config
> > >         COMPILER_INSTALL_PATH=$HOME/0day COMPILER=gcc-12.1.0 make.cross W=1 O=build_dir ARCH=loongarch SHELL=/bin/bash
> > > 
> > > If you fix the issue, kindly add following tag where applicable
> > > Reported-by: kernel test robot <lkp@intel.com>
> > > 
> > > All warnings (new ones prefixed by >>):
> > > 
> > > >> mm/memory.c:92:2: warning: #warning Unfortunate NUMA and NUMA Balancing config, growing page-frame for last_cpupid. [-Wcpp]
> > >       92 | #warning Unfortunate NUMA and NUMA Balancing config, growing page-frame for last_cpupid.
> > >          |  ^~~~~~~
> > > 
> > > 
> > > vim +92 mm/memory.c
> > > 
> > > 42b7772812d15b Jan Beulich    2008-07-23  90  
> > > af27d9403f5b80 Arnd Bergmann  2018-02-16  91  #if defined(LAST_CPUPID_NOT_IN_PAGE_FLAGS) && !defined(CONFIG_COMPILE_TEST)
> > > 90572890d20252 Peter Zijlstra 2013-10-07 @92  #warning Unfortunate NUMA and NUMA Balancing config, growing page-frame for last_cpupid.
> > > 75980e97daccfc Peter Zijlstra 2013-02-22  93  #endif
> > > 75980e97daccfc Peter Zijlstra 2013-02-22  94  
> > 
> > It looks like ith CONFIG_NUMA_BALANCING=y on loongarch we run out of
> > spare bits in page->flags to fit last_cpupid. The reason we don't see it
> > on arm64 is that we select SPARSEMEM_VMEMMAP and SECTIONS_WIDTH becomes
> > 0. On loongarch SECTIONS_WIDTH takes 19 bits (48 - 29) in page->flags.
> > 
> > I think instead of always defining PG_arch_{2,3} if CONFIG_64BIT, we
> > could add a CONFIG_ARCH_WANTS_PG_ARCH_23 option and only select it on
> > arm64 for the time being.
> 
> I pushed a patch as the first one on the arm64 devel/mte-pg-flags
> branch. Also updated the last patch on this branch following Steven's
> comments.
> 
> Peter, please let me know if you want to pick this series up together
> with your other KVM patches. Otherwise I can post it separately, it's
> worth merging it on its own as it clarifies the page flag vs tag setting
> ordering.

I'm looking at queuing this, but I'm confused by this comment. Do I
need to pick this as part of the series? Or is this an independent
thing (my hunch is that it is actually required not to break other
architectures...).

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.