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

[Ryan Roberts]

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@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.

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?

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

No I'm not doing that; It's one VMA at a time.

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

Yep understood, thanks.