Messages in this thread | | | Date | Mon, 27 Nov 2023 09:15:04 +0000 | Subject | Re: [PATCH v2 00/14] Transparent Contiguous PTEs for User Mappings | From | Ryan Roberts <> |
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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 > >
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