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

Barry Song
2023-11-27T10:35:50Z

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.

>
> >
> > 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