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

[Ryan Roberts <ryan.roberts@arm.com>]

On 16/11/2023 11:01, David Hildenbrand wrote:
> On 16.11.23 11:36, Ryan Roberts wrote:
>> On 16/11/2023 10:12, David Hildenbrand wrote:
>>> 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@intel.com/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@intel.com/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@intel.com>
>>>>> | Closes:
>>>>> https://lore.kernel.org/oe-kbuild-all/202311160516.kHhfmjvl-lkp@intel.com/
>>>>>
>>>>> 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.
>>
>> Yes, that seems much simpler! I think we might have to be careful about the uffd
>> wp bit too? I think that's it - are there any other exotic bits that might need
>> to be considered?
> 
> Good question. Mimicing what the current code already does should be sufficient.
> uffd-wp should have the PTE R/O. You can set the contpte bit independent of any
> SW bit (uffd-wp, softdirty, ...) I guess, no need to worry about that.
> 

OK thanks. I'll rework for this approach in v3.