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

[Ryan Roberts]

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.

> 
> 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
> 
>