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

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Message-ID: <930b5f6a-27d9-43da-bf9f-1478c8de1af8@redhat.com>
Date:   Thu, 16 Nov 2023 12:01:48 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Ryan Roberts <ryan.roberts@....com>,
        kernel test robot <lkp@...el.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Ard Biesheuvel <ardb@...nel.org>,
        Marc Zyngier <maz@...nel.org>,
        Oliver Upton <oliver.upton@...ux.dev>,
        James Morse <james.morse@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Zenghui Yu <yuzenghui@...wei.com>,
        Andrey Ryabinin <ryabinin.a.a@...il.com>,
        Alexander Potapenko <glider@...gle.com>,
        Andrey Konovalov <andreyknvl@...il.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Matthew Wilcox <willy@...radead.org>,
        Yu Zhao <yuzhao@...gle.com>,
        Mark Rutland <mark.rutland@....com>,
        Kefeng Wang <wangkefeng.wang@...wei.com>,
        John Hubbard <jhubbard@...dia.com>, Zi Yan <ziy@...dia.com>
Cc:     oe-kbuild-all@...ts.linux.dev,
        Linux Memory Management List <linux-mm@...ck.org>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 01/14] mm: Batch-copy PTE ranges during fork()

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@...el.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.

-- 
Cheers,

David / dhildenb