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

Barry Song
2023-11-28T19:38:03Z

On Wed, Nov 29, 2023 at 1:08 AM Ryan Roberts <ryan.roberts@arm.com> wrote:
>
> On 28/11/2023 05:49, Barry Song wrote:
> > On Mon, Nov 27, 2023 at 5:15 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.
> >>
> >>>
> >>> 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).
> >
> > Hi Ryan,
> >
> > sorry for too many comments, I remembered another case
> >
> > 4. mremap
> >
> > a CONTPTE might be remapped to another address which might not be
> > aligned with 16*basepage. thus, in move_ptes(), we are copying CONPTEs
> > from src to dst.
> > static int move_ptes(struct vm_area_struct *vma, pmd_t *old_pmd,
> > unsigned long old_addr, unsigned long old_end,
> > struct vm_area_struct *new_vma, pmd_t *new_pmd,
> > unsigned long new_addr, bool need_rmap_locks)
> > {
> > struct mm_struct *mm = vma->vm_mm;
> > pte_t *old_pte, *new_pte, pte;
> > ...
> >
> > /*
> > * We don't have to worry about the ordering of src and dst
> > * pte locks because exclusive mmap_lock prevents deadlock.
> > */
> > old_pte = pte_offset_map_lock(mm, old_pmd, old_addr, &old_ptl);
> > if (!old_pte) {
> > err = -EAGAIN;
> > goto out;
> > }
> > new_pte = pte_offset_map_nolock(mm, new_pmd, new_addr, &new_ptl);
> > if (!new_pte) {
> > pte_unmap_unlock(old_pte, old_ptl);
> > err = -EAGAIN;
> > goto out;
> > }
> > if (new_ptl != old_ptl)
> > spin_lock_nested(new_ptl, SINGLE_DEPTH_NESTING);
> > flush_tlb_batched_pending(vma->vm_mm);
> > arch_enter_lazy_mmu_mode();
> >
> > for (; old_addr < old_end; old_pte++, old_addr += PAGE_SIZE,
> > new_pte++, new_addr += PAGE_SIZE) {
> > if (pte_none(ptep_get(old_pte)))
> > continue;
> >
> > pte = ptep_get_and_clear(mm, old_addr, old_pte);
> > ....
> > }
> >
> > This has two possibilities
> > 1. new_pte is aligned with CONT_PTES, we can still keep CONTPTE;
> > 2. new_pte is not aligned with CONT_PTES, we should drop CONTPTE
> > while copying.
> >
> > does your code also handle this properly？
>
> Yes; same mechanism - the arm64 arch code does the CONT_PTE bit management and
> folds/unfolds as neccessary.
>
> Admittedly this may be non-optimal because we are iterating a single PTE at a
> time. When we clear the first pte of a contpte block in the source, the block
> will be unfolded. When we set the last pte of the contpte block in the dest, the
> block will be folded. If we had a batching mechanism, we could just clear the
> whole source contpte block in one hit (no need to unfold first) and we could
> just set the dest contpte block in one hit (no need to fold at the end).
>
> I haven't personally seen this as a hotspot though; I don't know if you have any
> data to the contrary? I've followed this type of batching technique for the fork
> case (patch 13). We could do a similar thing in theory, but its a bit more

in my previous testing, i don't see mremap quite often, so no worries.
as long as it is bug-free,
it is fine to me though a mremap microbench will definitely lose :-)

> complex because of the ptep_get_and_clear() return value; you would need to
> return all ptes for the cleared range, or somehow collapse the actual info that
> the caller requires (presumably access/dirty info).
>

Thanks
Barry