Re: [PATCH v2 12/14] arm64/mm: Wire up PTE_CONT for user mappings

Alistair Popple
2023-11-21T11:22:49Z

Ryan Roberts <ryan.roberts@arm.com> writes:

[...]

> +static void contpte_fold(struct mm_struct *mm, unsigned long addr,
> + pte_t *ptep, pte_t pte, bool fold)
> +{
> + struct vm_area_struct vma = TLB_FLUSH_VMA(mm, 0);
> + unsigned long start_addr;
> + pte_t *start_ptep;
> + int i;
> +
> + start_ptep = ptep = contpte_align_down(ptep);
> + start_addr = addr = ALIGN_DOWN(addr, CONT_PTE_SIZE);
> + pte = pfn_pte(ALIGN_DOWN(pte_pfn(pte), CONT_PTES), pte_pgprot(pte));
> + pte = fold ? pte_mkcont(pte) : pte_mknoncont(pte);
> +
> + for (i = 0; i < CONT_PTES; i++, ptep++, addr += PAGE_SIZE) {
> + pte_t ptent = __ptep_get_and_clear(mm, addr, ptep);
> +
> + if (pte_dirty(ptent))
> + pte = pte_mkdirty(pte);
> +
> + if (pte_young(ptent))
> + pte = pte_mkyoung(pte);
> + }
> +
> + __flush_tlb_range(&vma, start_addr, addr, PAGE_SIZE, true, 3);
> +
> + __set_ptes(mm, start_addr, start_ptep, pte, CONT_PTES);
> +}
> +
> +void __contpte_try_fold(struct mm_struct *mm, unsigned long addr,
> + pte_t *ptep, pte_t pte)
> +{
> + /*
> + * We have already checked that the virtual and pysical addresses are
> + * correctly aligned for a contpte mapping in contpte_try_fold() so the
> + * remaining checks are to ensure that the contpte range is fully
> + * covered by a single folio, and ensure that all the ptes are valid
> + * with contiguous PFNs and matching prots. We ignore the state of the
> + * access and dirty bits for the purpose of deciding if its a contiguous
> + * range; the folding process will generate a single contpte entry which
> + * has a single access and dirty bit. Those 2 bits are the logical OR of
> + * their respective bits in the constituent pte entries. In order to
> + * ensure the contpte range is covered by a single folio, we must
> + * recover the folio from the pfn, but special mappings don't have a
> + * folio backing them. Fortunately contpte_try_fold() already checked
> + * that the pte is not special - we never try to fold special mappings.
> + * Note we can't use vm_normal_page() for this since we don't have the
> + * vma.
> + */
> +
> + struct page *page = pte_page(pte);
> + struct folio *folio = page_folio(page);
> + unsigned long folio_saddr = addr - (page - &folio->page) * PAGE_SIZE;
> + unsigned long folio_eaddr = folio_saddr + folio_nr_pages(folio) * PAGE_SIZE;
> + unsigned long cont_saddr = ALIGN_DOWN(addr, CONT_PTE_SIZE);
> + unsigned long cont_eaddr = cont_saddr + CONT_PTE_SIZE;
> + unsigned long pfn;
> + pgprot_t prot;
> + pte_t subpte;
> + pte_t *orig_ptep;
> + int i;
> +
> + if (folio_saddr > cont_saddr || folio_eaddr < cont_eaddr)
> + return;
> +
> + pfn = pte_pfn(pte) - ((addr - cont_saddr) >> PAGE_SHIFT);
> + prot = pte_pgprot(pte_mkold(pte_mkclean(pte)));
> + orig_ptep = ptep;
> + ptep = contpte_align_down(ptep);
> +
> + for (i = 0; i < CONT_PTES; i++, ptep++, pfn++) {
> + subpte = __ptep_get(ptep);
> + subpte = pte_mkold(pte_mkclean(subpte));
> +
> + if (!pte_valid(subpte) ||
> + pte_pfn(subpte) != pfn ||
> + pgprot_val(pte_pgprot(subpte)) != pgprot_val(prot))
> + return;
> + }
> +
> + contpte_fold(mm, addr, orig_ptep, pte, true);
> +}
> +EXPORT_SYMBOL(__contpte_try_fold);
> +
> +void __contpte_try_unfold(struct mm_struct *mm, unsigned long addr,
> + pte_t *ptep, pte_t pte)
> +{
> + /*
> + * We have already checked that the ptes are contiguous in
> + * contpte_try_unfold(), so we can unfold unconditionally here.
> + */
> +
> + contpte_fold(mm, addr, ptep, pte, false);

I'm still working my way through the series but calling a fold during an
unfold stood out as it seemed wrong. Obviously further reading revealed
the boolean flag that changes the functions meaning but I think it would
be better to refactor that.

We could easily rename contpte_fold() to eg. set_cont_ptes() and factor
the pte calculation loop into a separate helper
(eg. calculate_contpte_dirty_young() or some hopefully better name)
called further up the stack. That has an added benefit of providing a
spot to add the nice comment for young/dirty rules you provided in the
patch description ;-)

In other words we'd have something like:

void __contpte_try_unfold() {
pte = calculate_contpte_dirty_young(mm, addr, ptep, pte);
pte = pte_mknoncont(pte);
set_cont_ptes(mm, addr, ptep, pte);
}

Which IMHO is more immediately understandable.

- Alistair

> +}
> +EXPORT_SYMBOL(__contpte_try_unfold);
> +
> +pte_t contpte_ptep_get(pte_t *ptep, pte_t orig_pte)
> +{
> + /*
> + * Gather access/dirty bits, which may be populated in any of the ptes
> + * of the contig range. We are guarranteed to be holding the PTL, so any
> + * contiguous range cannot be unfolded or otherwise modified under our
> + * feet.
> + */
> +
> + pte_t pte;
> + int i;
> +
> + ptep = contpte_align_down(ptep);
> +
> + for (i = 0; i < CONT_PTES; i++, ptep++) {
> + pte = __ptep_get(ptep);
> +
> + if (pte_dirty(pte))
> + orig_pte = pte_mkdirty(orig_pte);
> +
> + if (pte_young(pte))
> + orig_pte = pte_mkyoung(orig_pte);
> + }
> +
> + return orig_pte;
> +}
> +EXPORT_SYMBOL(contpte_ptep_get);
> +
> +pte_t contpte_ptep_get_lockless(pte_t *orig_ptep)
> +{
> + /*
> + * Gather access/dirty bits, which may be populated in any of the ptes
> + * of the contig range. We may not be holding the PTL, so any contiguous
> + * range may be unfolded/modified/refolded under our feet. Therefore we
> + * ensure we read a _consistent_ contpte range by checking that all ptes
> + * in the range are valid and have CONT_PTE set, that all pfns are
> + * contiguous and that all pgprots are the same (ignoring access/dirty).
> + * If we find a pte that is not consistent, then we must be racing with
> + * an update so start again. If the target pte does not have CONT_PTE
> + * set then that is considered consistent on its own because it is not
> + * part of a contpte range.
> + */
> +
> + pte_t orig_pte;
> + pgprot_t orig_prot;
> + pte_t *ptep;
> + unsigned long pfn;
> + pte_t pte;
> + pgprot_t prot;
> + int i;
> +
> +retry:
> + orig_pte = __ptep_get(orig_ptep);
> +
> + if (!pte_valid_cont(orig_pte))
> + return orig_pte;
> +
> + orig_prot = pte_pgprot(pte_mkold(pte_mkclean(orig_pte)));
> + ptep = contpte_align_down(orig_ptep);
> + pfn = pte_pfn(orig_pte) - (orig_ptep - ptep);
> +
> + for (i = 0; i < CONT_PTES; i++, ptep++, pfn++) {
> + pte = __ptep_get(ptep);
> + prot = pte_pgprot(pte_mkold(pte_mkclean(pte)));
> +
> + if (!pte_valid_cont(pte) ||
> + pte_pfn(pte) != pfn ||
> + pgprot_val(prot) != pgprot_val(orig_prot))
> + goto retry;
> +
> + if (pte_dirty(pte))
> + orig_pte = pte_mkdirty(orig_pte);
> +
> + if (pte_young(pte))
> + orig_pte = pte_mkyoung(orig_pte);
> + }
> +
> + return orig_pte;
> +}
> +EXPORT_SYMBOL(contpte_ptep_get_lockless);
> +
> +void contpte_set_ptes(struct mm_struct *mm, unsigned long addr,
> + pte_t *ptep, pte_t pte, unsigned int nr)
> +{
> + unsigned long next;
> + unsigned long end = addr + (nr << PAGE_SHIFT);
> + unsigned long pfn = pte_pfn(pte);
> + pgprot_t prot = pte_pgprot(pte);
> + pte_t orig_pte;
> +
> + do {
> + next = pte_cont_addr_end(addr, end);
> + nr = (next - addr) >> PAGE_SHIFT;
> + pte = pfn_pte(pfn, prot);
> +
> + if (((addr | next | (pfn << PAGE_SHIFT)) & ~CONT_PTE_MASK) == 0)
> + pte = pte_mkcont(pte);
> + else
> + pte = pte_mknoncont(pte);
> +
> + /*
> + * If operating on a partial contiguous range then we must first
> + * unfold the contiguous range if it was previously folded.
> + * Otherwise we could end up with overlapping tlb entries.
> + */
> + if (nr != CONT_PTES)
> + contpte_try_unfold(mm, addr, ptep, __ptep_get(ptep));
> +
> + /*
> + * If we are replacing ptes that were contiguous or if the new
> + * ptes are contiguous and any of the ptes being replaced are
> + * valid, we need to clear and flush the range to prevent
> + * overlapping tlb entries.
> + */
> + orig_pte = __ptep_get(ptep);
> + if (pte_valid_cont(orig_pte) ||
> + (pte_cont(pte) && ptep_any_valid(ptep, nr)))
> + ptep_clear_flush_range(mm, addr, ptep, nr);
> +
> + __set_ptes(mm, addr, ptep, pte, nr);
> +
> + addr = next;
> + ptep += nr;
> + pfn += nr;
> +
> + } while (addr != end);
> +}
> +EXPORT_SYMBOL(contpte_set_ptes);
> +
> +int contpte_ptep_test_and_clear_young(struct vm_area_struct *vma,
> + unsigned long addr, pte_t *ptep)
> +{
> + /*
> + * ptep_clear_flush_young() technically requires us to clear the access
> + * flag for a _single_ pte. However, the core-mm code actually tracks
> + * access/dirty per folio, not per page. And since we only create a
> + * contig range when the range is covered by a single folio, we can get
> + * away with clearing young for the whole contig range here, so we avoid
> + * having to unfold.
> + */
> +
> + int i;
> + int young = 0;
> +
> + ptep = contpte_align_down(ptep);
> + addr = ALIGN_DOWN(addr, CONT_PTE_SIZE);
> +
> + for (i = 0; i < CONT_PTES; i++, ptep++, addr += PAGE_SIZE)
> + young |= __ptep_test_and_clear_young(vma, addr, ptep);
> +
> + return young;
> +}
> +EXPORT_SYMBOL(contpte_ptep_test_and_clear_young);
> +
> +int contpte_ptep_clear_flush_young(struct vm_area_struct *vma,
> + unsigned long addr, pte_t *ptep)
> +{
> + int young;
> +
> + young = contpte_ptep_test_and_clear_young(vma, addr, ptep);
> +
> + if (young) {
> + /*
> + * See comment in __ptep_clear_flush_young(); same rationale for
> + * eliding the trailing DSB applies here.
> + */
> + addr = ALIGN_DOWN(addr, CONT_PTE_SIZE);
> + __flush_tlb_range_nosync(vma, addr, addr + CONT_PTE_SIZE,
> + PAGE_SIZE, true, 3);
> + }
> +
> + return young;
> +}
> +EXPORT_SYMBOL(contpte_ptep_clear_flush_young);
> +
> +int contpte_ptep_set_access_flags(struct vm_area_struct *vma,
> + unsigned long addr, pte_t *ptep,
> + pte_t entry, int dirty)
> +{
> + pte_t orig_pte;
> + int i;
> + unsigned long start_addr;
> +
> + /*
> + * Gather the access/dirty bits for the contiguous range. If nothing has
> + * changed, its a noop.
> + */
> + orig_pte = ptep_get(ptep);
> + if (pte_val(orig_pte) == pte_val(entry))
> + return 0;
> +
> + /*
> + * We can fix up access/dirty bits without having to unfold/fold the
> + * contig range. But if the write bit is changing, we need to go through
> + * the full unfold/fold cycle.
> + */
> + if (pte_write(orig_pte) == pte_write(entry)) {
> + /*
> + * For HW access management, we technically only need to update
> + * the flag on a single pte in the range. But for SW access
> + * management, we need to update all the ptes to prevent extra
> + * faults. Avoid per-page tlb flush in __ptep_set_access_flags()
> + * and instead flush the whole range at the end.
> + */
> + ptep = contpte_align_down(ptep);
> + start_addr = addr = ALIGN_DOWN(addr, CONT_PTE_SIZE);
> +
> + for (i = 0; i < CONT_PTES; i++, ptep++, addr += PAGE_SIZE)
> + __ptep_set_access_flags(vma, addr, ptep, entry, 0);
> +
> + if (dirty)
> + __flush_tlb_range(vma, start_addr, addr,
> + PAGE_SIZE, true, 3);
> + } else {
> + __contpte_try_unfold(vma->vm_mm, addr, ptep, orig_pte);
> + __ptep_set_access_flags(vma, addr, ptep, entry, dirty);
> + contpte_try_fold(vma->vm_mm, addr, ptep, entry);
> + }
> +
> + return 1;
> +}
> +EXPORT_SYMBOL(contpte_ptep_set_access_flags);