Re: [PATCH v2 00/14] Transparent Contiguous PTEs for User Mappings

Ryan Roberts
2023-11-28T11:53:02Z

On 27/11/2023 22:53, Barry Song wrote:
> On Tue, Nov 28, 2023 at 12:11 AM Ryan Roberts <ryan.roberts@arm.com> wrote:
>>
>> On 27/11/2023 10:35, Barry Song wrote:
>>> On Mon, Nov 27, 2023 at 10:15 PM Ryan Roberts <ryan.roberts@arm.com> wrote:
>>>>
>>>> On 27/11/2023 03:18, Barry Song wrote:
>>>>>> Ryan Roberts (14):
>>>>>> mm: Batch-copy PTE ranges during fork()
>>>>>> arm64/mm: set_pte(): New layer to manage contig bit
>>>>>> arm64/mm: set_ptes()/set_pte_at(): New layer to manage contig bit
>>>>>> arm64/mm: pte_clear(): New layer to manage contig bit
>>>>>> arm64/mm: ptep_get_and_clear(): New layer to manage contig bit
>>>>>> arm64/mm: ptep_test_and_clear_young(): New layer to manage contig bit
>>>>>> arm64/mm: ptep_clear_flush_young(): New layer to manage contig bit
>>>>>> arm64/mm: ptep_set_wrprotect(): New layer to manage contig bit
>>>>>> arm64/mm: ptep_set_access_flags(): New layer to manage contig bit
>>>>>> arm64/mm: ptep_get(): New layer to manage contig bit
>>>>>> arm64/mm: Split __flush_tlb_range() to elide trailing DSB
>>>>>> arm64/mm: Wire up PTE_CONT for user mappings
>>>>>> arm64/mm: Implement ptep_set_wrprotects() to optimize fork()
>>>>>> arm64/mm: Add ptep_get_and_clear_full() to optimize process teardown
>>>>>
>>>>> Hi Ryan,
>>>>> Not quite sure if I missed something, are we splitting/unfolding CONTPTES
>>>>> in the below cases
>>>>
>>>> The general idea is that the core-mm sets the individual ptes (one at a time if
>>>> it likes with set_pte_at(), or in a block with set_ptes()), modifies its
>>>> permissions (ptep_set_wrprotect(), ptep_set_access_flags()) and clears them
>>>> (ptep_clear(), etc); This is exactly the same interface as previously.
>>>>
>>>> BUT, the arm64 implementation of those interfaces will now detect when a set of
>>>> adjacent PTEs (a contpte block - so 16 naturally aligned entries when using 4K
>>>> base pages) are all appropriate for having the CONT_PTE bit set; in this case
>>>> the block is "folded". And it will detect when the first PTE in the block
>>>> changes such that the CONT_PTE bit must now be unset ("unfolded"). One of the
>>>> requirements for folding a contpte block is that all the pages must belong to
>>>> the *same* folio (that means its safe to only track access/dirty for thecontpte
>>>> block as a whole rather than for each individual pte).
>>>>
>>>> (there are a couple of optimizations that make the reality slightly more
>>>> complicated than what I've just explained, but you get the idea).
>>>>
>>>> On that basis, I believe all the specific cases you describe below are all
>>>> covered and safe - please let me know if you think there is a hole here!
>>>>
>>>>>
>>>>> 1. madvise(MADV_DONTNEED) on a part of basepages on a CONTPTE large folio
>>>>
>>>> The page will first be unmapped (e.g. ptep_clear() or ptep_get_and_clear(), or
>>>> whatever). The implementation of that will cause an unfold and the CONT_PTE bit
>>>> is removed from the whole contpte block. If there is then a subsequent
>>>> set_pte_at() to set a swap entry, the implementation will see that its not
>>>> appropriate to re-fold, so the range will remain unfolded.
>>>>
>>>>>
>>>>> 2. vma split in a large folio due to various reasons such as mprotect,
>>>>> munmap, mlock etc.
>>>>
>>>> I'm not sure if PTEs are explicitly unmapped/remapped when splitting a VMA? I
>>>> suspect not, so if the VMA is split in the middle of a currently folded contpte
>>>> block, it will remain folded. But this is safe and continues to work correctly.
>>>> The VMA arrangement is not important; it is just important that a single folio
>>>> is mapped contiguously across the whole block.
>>>
>>> I don't think it is safe to keep CONTPTE folded in a split_vma case. as
>>> otherwise, copy_ptes in your other patch might only copy a part
>>> of CONTPES.
>>> For example, if page0-page4 and page5-page15 are splitted in split_vma,
>>> in fork, while copying pte for the first VMA, we are copying page0-page4,
>>> this will immediately cause inconsistent CONTPTE. as we have to
>>> make sure all CONTPTEs are atomically mapped in a PTL.
>>
>> No that's not how it works. The CONT_PTE bit is not blindly copied from parent
>> to child. It is explicitly managed by the arch code and set when appropriate. In
>> the case above, we will end up calling set_ptes() for page0-page4 in the child.
>> set_ptes() will notice that there are only 5 contiguous pages so it will map
>> without the CONT_PTE bit.
>
> Ok. cool. alternatively, in the code I shared to you, we are doing an unfold
> immediately when split_vma happens within a large anon folio, so we disallow
> CONTPTE to cross two VMAs to avoid all kinds of complexity afterwards.
>
> https://github.com/OnePlusOSS/android_kernel_oneplus_sm8550/blob/oneplus/sm8550_u_14.0.0_oneplus11/mm/huge_memory.c
>
> #ifdef CONFIG_CONT_PTE_HUGEPAGE
> void vma_adjust_cont_pte_trans_huge(struct vm_area_struct *vma,
> unsigned long start,
> unsigned long end,
> long adjust_next)
> {
> /*
> * If the new start address isn't hpage aligned and it could
> * previously contain an hugepage: check if we need to split
> * an huge pmd.
> */
> if (start & ~HPAGE_CONT_PTE_MASK &&
> (start & HPAGE_CONT_PTE_MASK) >= vma->vm_start &&
> (start & HPAGE_CONT_PTE_MASK) + HPAGE_CONT_PTE_SIZE <= vma->vm_end)
> split_huge_cont_pte_address(vma, start, false, NULL);
>
> ....
> }
> #endif
>
> In your approach, you are still holding CONTPTE crossing two VMAs. but it seems
> ok. I can't have a case which might fail in my brain right now. only

Yes, I'm dealing with the CONT_PTE bit at the pgtable level, not at the VMA level.

> running the code on
> a large amount of real hardware will tell :-)

Indeed - is this something you might be able to help with? :)

>
>>
>>>
>>>>
>>>>>
>>>>> 3. try_to_unmap_one() to reclaim a folio, ptes are scanned one by one
>>>>> rather than being as a whole.
>>>>
>>>> Yes, as per 1; the arm64 implementation will notice when the first entry is
>>>> cleared and unfold the contpte block.
>>>>
>>>>>
>>>>> In hardware, we need to make sure CONTPTE follow the rule - always 16
>>>>> contiguous physical address with CONTPTE set. if one of them run away
>>>>> from the 16 ptes group and PTEs become unconsistent, some terrible
>>>>> errors/faults can happen in HW. for example
>>>>
>>>> Yes, the implementation obeys all these rules; see contpte_try_fold() and
>>>> contpte_try_unfold(). the fold/unfold operation is only done when all
>>>> requirements are met, and we perform it in a manner that is conformant to the
>>>> architecture requirements (see contpte_fold() - being renamed to
>>>> contpte_convert() in the next version).
>>>>
>>>> Thanks for the review!
>>>>
>>>> Thanks,
>>>> Ryan
>>>>
>>>>>
>>>>> case0:
>>>>> addr0 PTE - has no CONTPE
>>>>> addr0+4kb PTE - has CONTPTE
>>>>> ....
>>>>> addr0+60kb PTE - has CONTPTE
>>>>>
>>>>> case 1:
>>>>> addr0 PTE - has no CONTPE
>>>>> addr0+4kb PTE - has CONTPTE
>>>>> ....
>>>>> addr0+60kb PTE - has swap
>>>>>
>>>>> Unconsistent 16 PTEs will lead to crash even in the firmware based on
>>>>> our observation.
>>>>>
>>>
>
> Thanks
> Barry