On 27/11/2023 22:53, Barry Song wrote: > On Tue, Nov 28, 2023 at 12:11 AM Ryan Roberts <ryan.roberts@xxxxxxx> wrote: >> >> On 27/11/2023 10:35, Barry Song wrote: >>> On Mon, Nov 27, 2023 at 10:15 PM Ryan Roberts <ryan.roberts@xxxxxxx> 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