Re: [PATCH v2 01/14] mm: Batch-copy PTE ranges during fork()

[David Hildenbrand]

On 16.11.23 11:07, Ryan Roberts wrote:
> Hi All,
>
> Hoping for some guidance below!
>
>
> On 15/11/2023 21:26, kernel test robot wrote:
> > Hi Ryan,
> >
> > kernel test robot noticed the following build errors:
> >
> > [auto build test ERROR on akpm-mm/mm-everything]
> > [also build test ERROR on linus/master v6.7-rc1 next-20231115]
> > [cannot apply to arm64/for-next/core efi/next]
> > [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/Ryan-Roberts/mm-Batch-copy-PTE-ranges-during-fork/20231116-010123
> > base:   https://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm.git mm-everything
> > patch link:    https://lore.kernel.org/r/20231115163018.1303287-2-ryan.roberts%40arm.com
> > patch subject: [PATCH v2 01/14] mm: Batch-copy PTE ranges during fork()
> > config: arm-randconfig-002-20231116 (https://download.01.org/0day-ci/archive/20231116/202311160516.kHhfmjvl-lkp@xxxxxxxxx/config)
> > compiler: arm-linux-gnueabi-gcc (GCC) 13.2.0
> > reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20231116/202311160516.kHhfmjvl-lkp@xxxxxxxxx/reproduce)
> >
> > If you fix the issue in a separate patch/commit (i.e. not just a new version of
> > the same patch/commit), kindly add following tags
> > | Reported-by: kernel test robot <lkp@xxxxxxxxx>
> > | Closes: https://lore.kernel.org/oe-kbuild-all/202311160516.kHhfmjvl-lkp@xxxxxxxxx/
> >
> > All errors (new ones prefixed by >>):
> >
> >     mm/memory.c: In function 'folio_nr_pages_cont_mapped':
> > > > mm/memory.c:969:16: error: implicit declaration of function 'pte_pgprot'; did you mean 'ptep_get'? [-Werror=implicit-function-declaration]
> >       969 |         prot = pte_pgprot(pte_mkold(pte_mkclean(ptent)));
> >           |                ^~~~~~~~~~
> >           |                ptep_get
> >     cc1: some warnings being treated as errors
>
> It turns out that pte_pgprot() is not universal; its only implemented by
> architectures that select CONFIG_HAVE_IOREMAP_PROT (currently arc, arm64,
> loongarch, mips, powerpc, s390, sh, x86).
>
> I'm using it in core-mm to help calculate the number of "contiguously mapped"
> pages within a folio (note that's not the same as arm64's notion of
> contpte-mapped. I just want to know that there are N physically contiguous pages
> mapped virtually contiguously with the same permissions). And I'm using
> pte_pgprot() to extract the permissions for each pte to compare. It's important
> that we compare the permissions because just because the pages belongs to the
> same folio doesn't imply they are mapped with the same permissions; think
> mprotect()ing a sub-range.
>
> I don't have a great idea for how to fix this - does anyone have any thoughts?

KIS :) fork() operates on individual VMAs if I am not daydreaming.

Just check for the obvious pte_write()/dirty/ and you'll be fine.

If your code tries to optimize "between VMAs", you really shouldn't be 
doing that at this point.

If someone did an mprotect(), there are separate VMAs, and you shouldn't 
be looking at the PTEs belonging to a different VMA.

--
Cheers,

David / dhildenb