[PATCH v5 4/9] arm64: hugetlb: Add break-before-make logic for contiguous entries
From: catalin.marinas@arm.com (Catalin Marinas)
Date: 2017-08-07 17:19:24
Also in:
linux-mm
On Wed, Aug 02, 2017 at 10:48:59AM +0100, Punit Agrawal wrote:
quoted hunk ↗ jump to hunk
diff --git a/arch/arm64/mm/hugetlbpage.c b/arch/arm64/mm/hugetlbpage.c index 08deed7c71f0..f2c976464f39 100644 --- a/arch/arm64/mm/hugetlbpage.c +++ b/arch/arm64/mm/hugetlbpage.c@@ -68,6 +68,47 @@ static int find_num_contig(struct mm_struct *mm, unsigned long addr, return CONT_PTES; } +/* + * Changing some bits of contiguous entries requires us to follow a + * Break-Before-Make approach, breaking the whole contiguous set + * before we can change any entries. See ARM DDI 0487A.k_iss10775, + * "Misprogramming of the Contiguous bit", page D4-1762. + * + * This helper performs the break step. + */ +static pte_t get_clear_flush(struct mm_struct *mm, + unsigned long addr, + pte_t *ptep, + unsigned long pgsize, + unsigned long ncontig) +{ + unsigned long i, saddr = addr; + struct vm_area_struct vma = { .vm_mm = mm }; + pte_t orig_pte = huge_ptep_get(ptep); + + /* + * If we already have a faulting entry then we don't need + * to break before make (there won't be a tlb entry cached). + */ + if (!pte_present(orig_pte)) + return orig_pte; + + for (i = 0; i < ncontig; i++, addr += pgsize, ptep++) { + pte_t pte = ptep_get_and_clear(mm, addr, ptep); + + /* + * If HW_AFDBM is enabled, then the HW could turn on + * the dirty bit for any page in the set, so check + * them all. All hugetlb entries are already young. + */ + if (IS_ENABLED(CONFIG_ARM64_HW_AFDBM) && pte_dirty(pte)) + orig_pte = pte_mkdirty(orig_pte); + } + + flush_tlb_range(&vma, saddr, addr); + return orig_pte; +} + void set_huge_pte_at(struct mm_struct *mm, unsigned long addr, pte_t *ptep, pte_t pte) {@@ -93,6 +134,8 @@ void set_huge_pte_at(struct mm_struct *mm, unsigned long addr, dpfn = pgsize >> PAGE_SHIFT; hugeprot = pte_pgprot(pte); + get_clear_flush(mm, addr, ptep, pgsize, ncontig); + for (i = 0; i < ncontig; i++, ptep++, addr += pgsize, pfn += dpfn) { pr_debug("%s: set pte %p to 0x%llx\n", __func__, ptep, pte_val(pfn_pte(pfn, hugeprot)));
Is there any risk of the huge pte being accessed (from user space on another CPU) in the short break-before-make window? Not that we can do much about it but just checking. BTW, it seems a bit overkill to use ptep_get_and_clear() (via get_clear_flush) when we just want to zero the entries. Probably not much overhead though.
quoted hunk ↗ jump to hunk
@@ -222,6 +256,7 @@ int huge_ptep_set_access_flags(struct vm_area_struct *vma, int ncontig, i, changed = 0; size_t pgsize = 0; unsigned long pfn = pte_pfn(pte), dpfn; + pte_t orig_pte; pgprot_t hugeprot; if (!pte_cont(pte))@@ -231,10 +266,12 @@ int huge_ptep_set_access_flags(struct vm_area_struct *vma, dpfn = pgsize >> PAGE_SHIFT; hugeprot = pte_pgprot(pte); - for (i = 0; i < ncontig; i++, ptep++, addr += pgsize, pfn += dpfn) { - changed |= ptep_set_access_flags(vma, addr, ptep, - pfn_pte(pfn, hugeprot), dirty); - } + orig_pte = get_clear_flush(vma->vm_mm, addr, ptep, pgsize, ncontig); + if (!pte_same(orig_pte, pte)) + changed = 1; + + for (i = 0; i < ncontig; i++, ptep++, addr += pgsize, pfn += dpfn) + set_pte_at(vma->vm_mm, addr, ptep, pfn_pte(pfn, hugeprot)); return changed; }
If hugeprot isn't dirty but orig_pte became dirty, it looks like we just drop such information from the new pte. Same comment here about the window. huge_ptep_set_access_flags() is called on a present (huge) pte and we briefly make it invalid. Can the mm subsystem cope with a fault on another CPU here? Same for the huge_ptep_set_wrprotect() below.
quoted hunk ↗ jump to hunk
@@ -244,6 +281,9 @@ void huge_ptep_set_wrprotect(struct mm_struct *mm, { int ncontig, i; size_t pgsize; + pte_t pte = pte_wrprotect(huge_ptep_get(ptep)), orig_pte; + unsigned long pfn = pte_pfn(pte), dpfn; + pgprot_t hugeprot; if (!pte_cont(*ptep)) { ptep_set_wrprotect(mm, addr, ptep);@@ -251,8 +291,15 @@ void huge_ptep_set_wrprotect(struct mm_struct *mm, } ncontig = find_num_contig(mm, addr, ptep, &pgsize); - for (i = 0; i < ncontig; i++, ptep++, addr += pgsize) - ptep_set_wrprotect(mm, addr, ptep); + dpfn = pgsize >> PAGE_SHIFT; + + orig_pte = get_clear_flush(mm, addr, ptep, pgsize, ncontig); + if (pte_dirty(orig_pte)) + pte = pte_mkdirty(pte); + + hugeprot = pte_pgprot(pte); + for (i = 0; i < ncontig; i++, ptep++, addr += pgsize, pfn += dpfn) + set_pte_at(mm, addr, ptep, pfn_pte(pfn, hugeprot)); } void huge_ptep_clear_flush(struct vm_area_struct *vma,
-- Catalin