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

Barry Song
2023-11-27T22:54:06Z

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

>
> >
> >>
> >>>
> >>> 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