Re: [PATCH V2 1/3] mm/debug: Add tests validating arch page table helpers for core features

[Zi Yan <ziy@nvidia.com>]

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On 24 Mar 2020, at 1:22, Anshuman Khandual wrote:

> This adds new tests validating arch page table helpers for these following
> core memory features. These tests create and test specific mapping types at
> various page table levels.
>
> 1. SPECIAL mapping
> 2. PROTNONE mapping
> 3. DEVMAP mapping
> 4. SOFTDIRTY mapping
> 5. SWAP mapping
> 6. MIGRATION mapping
> 7. HUGETLB mapping
> 8. THP mapping
>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Mike Rapoport <rppt@linux.ibm.com>
> Cc: Vineet Gupta <vgupta@synopsys.com>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Will Deacon <will@kernel.org>
> Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
> Cc: Paul Mackerras <paulus@samba.org>
> Cc: Michael Ellerman <mpe@ellerman.id.au>
> Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
> Cc: Vasily Gorbik <gor@linux.ibm.com>
> Cc: Christian Borntraeger <borntraeger@de.ibm.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Borislav Petkov <bp@alien8.de>
> Cc: "H. Peter Anvin" <hpa@zytor.com>
> Cc: Kirill A. Shutemov <kirill@shutemov.name>
> Cc: Paul Walmsley <paul.walmsley@sifive.com>
> Cc: Palmer Dabbelt <palmer@dabbelt.com>
> Cc: linux-snps-arc@lists.infradead.org
> Cc: linux-arm-kernel@lists.infradead.org
> Cc: linuxppc-dev@lists.ozlabs.org
> Cc: linux-s390@vger.kernel.org
> Cc: linux-riscv@lists.infradead.org
> Cc: x86@kernel.org
> Cc: linux-arch@vger.kernel.org
> Cc: linux-kernel@vger.kernel.org
> Suggested-by: Catalin Marinas <catalin.marinas@arm.com>
> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
> ---
>  mm/debug_vm_pgtable.c | 291 +++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 290 insertions(+), 1 deletion(-)
>
> diff --git a/mm/debug_vm_pgtable.c b/mm/debug_vm_pgtable.c
> index 98990a515268..15055a8f6478 100644
> --- a/mm/debug_vm_pgtable.c
> +++ b/mm/debug_vm_pgtable.c
> @@ -289,6 +289,267 @@ static void __init pmd_populate_tests(struct mm_struct *mm, pmd_t *pmdp,
>  	WARN_ON(pmd_bad(pmd));
>  }
>
> +static void __init pte_special_tests(unsigned long pfn, pgprot_t prot)
> +{
> +	pte_t pte = pfn_pte(pfn, prot);
> +
> +	if (!IS_ENABLED(CONFIG_ARCH_HAS_PTE_SPECIAL))
> +		return;
> +
> +	WARN_ON(!pte_special(pte_mkspecial(pte)));
> +}
> +
> +static void __init pte_protnone_tests(unsigned long pfn, pgprot_t prot)
> +{
> +	pte_t pte = pfn_pte(pfn, prot);
> +
> +	if (!IS_ENABLED(CONFIG_NUMA_BALANCING))
> +		return;
> +
> +	WARN_ON(!pte_protnone(pte));
> +	WARN_ON(!pte_present(pte));
> +}
> +
> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> +static void __init pmd_protnone_tests(unsigned long pfn, pgprot_t prot)
> +{
> +	pmd_t pmd = pfn_pmd(pfn, prot);
> +
> +	if (!IS_ENABLED(CONFIG_NUMA_BALANCING))
> +		return;
> +
> +	WARN_ON(!pmd_protnone(pmd));
> +	WARN_ON(!pmd_present(pmd));
> +}
> +#else
> +static void __init pmd_protnone_tests(unsigned long pfn, pgprot_t prot) { }
> +#endif
> +
> +#ifdef CONFIG_ARCH_HAS_PTE_DEVMAP
> +static void __init pte_devmap_tests(unsigned long pfn, pgprot_t prot)
> +{
> +	pte_t pte = pfn_pte(pfn, prot);
> +
> +	WARN_ON(!pte_devmap(pte_mkdevmap(pte)));
> +}
> +
> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> +static void __init pmd_devmap_tests(unsigned long pfn, pgprot_t prot)
> +{
> +	pmd_t pmd = pfn_pmd(pfn, prot);
> +
> +	WARN_ON(!pmd_devmap(pmd_mkdevmap(pmd)));
> +}
> +
> +#ifdef CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD
> +static void __init pud_devmap_tests(unsigned long pfn, pgprot_t prot)
> +{
> +	pud_t pud = pfn_pud(pfn, prot);
> +
> +	WARN_ON(!pud_devmap(pud_mkdevmap(pud)));
> +}
> +#else
> +static void __init pud_devmap_tests(unsigned long pfn, pgprot_t prot) { }
> +#endif
> +#else
> +static void __init pmd_devmap_tests(unsigned long pfn, pgprot_t prot) { }
> +static void __init pud_devmap_tests(unsigned long pfn, pgprot_t prot) { }
> +#endif
> +#else
> +static void __init pte_devmap_tests(unsigned long pfn, pgprot_t prot) { }
> +static void __init pmd_devmap_tests(unsigned long pfn, pgprot_t prot) { }
> +static void __init pud_devmap_tests(unsigned long pfn, pgprot_t prot) { }
> +#endif
> +
> +static void __init pte_soft_dirty_tests(unsigned long pfn, pgprot_t prot)
> +{
> +	pte_t pte = pfn_pte(pfn, prot);
> +
> +	if (!IS_ENABLED(CONFIG_HAVE_ARCH_SOFT_DIRTY))
> +		return;
> +
> +	WARN_ON(!pte_soft_dirty(pte_mksoft_dirty(pte)));
> +	WARN_ON(pte_soft_dirty(pte_clear_soft_dirty(pte)));
> +}
> +
> +static void __init pte_swap_soft_dirty_tests(unsigned long pfn, pgprot_t prot)
> +{
> +	pte_t pte = pfn_pte(pfn, prot);
> +
> +	if (!IS_ENABLED(CONFIG_HAVE_ARCH_SOFT_DIRTY))
> +		return;
> +
> +	WARN_ON(!pte_swp_soft_dirty(pte_swp_mksoft_dirty(pte)));
> +	WARN_ON(pte_swp_soft_dirty(pte_swp_clear_soft_dirty(pte)));
> +}
> +
> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> +static void __init pmd_soft_dirty_tests(unsigned long pfn, pgprot_t prot)
> +{
> +	pmd_t pmd = pfn_pmd(pfn, prot);
> +
> +	if (!IS_ENABLED(CONFIG_HAVE_ARCH_SOFT_DIRTY))
> +		return;
> +
> +	WARN_ON(!pmd_soft_dirty(pmd_mksoft_dirty(pmd)));
> +	WARN_ON(pmd_soft_dirty(pmd_clear_soft_dirty(pmd)));
> +}
> +
> +static void __init pmd_swap_soft_dirty_tests(unsigned long pfn, pgprot_t prot)
> +{
> +	pmd_t pmd = pfn_pmd(pfn, prot);
> +
> +	if (!IS_ENABLED(CONFIG_HAVE_ARCH_SOFT_DIRTY) ||
> +		!IS_ENABLED(CONFIG_ARCH_ENABLE_THP_MIGRATION))
> +		return;
> +
> +	WARN_ON(!pmd_swp_soft_dirty(pmd_swp_mksoft_dirty(pmd)));
> +	WARN_ON(pmd_swp_soft_dirty(pmd_swp_clear_soft_dirty(pmd)));
> +}
> +#else
> +static void __init pmd_soft_dirty_tests(unsigned long pfn, pgprot_t prot) { }
> +static void __init pmd_swap_soft_dirty_tests(unsigned long pfn, pgprot_t prot)
> +{
> +}
> +#endif
> +
> +static void __init pte_swap_tests(unsigned long pfn, pgprot_t prot)
> +{
> +	swp_entry_t swp;
> +	pte_t pte;
> +
> +	pte = pfn_pte(pfn, prot);
> +	swp = __pte_to_swp_entry(pte);
> +	WARN_ON(!pte_same(pte, __swp_entry_to_pte(swp)));
> +}
> +
> +#ifdef CONFIG_ARCH_ENABLE_THP_MIGRATION
> +static void __init pmd_swap_tests(unsigned long pfn, pgprot_t prot)
> +{
> +	swp_entry_t swp;
> +	pmd_t pmd;
> +
> +	pmd = pfn_pmd(pfn, prot);
> +	swp = __pmd_to_swp_entry(pmd);
> +	WARN_ON(!pmd_same(pmd, __swp_entry_to_pmd(swp)));
> +}
> +#else
> +static void __init pmd_swap_tests(unsigned long pfn, pgprot_t prot) { }
> +#endif
> +
> +static void __init swap_migration_tests(void)
> +{
> +	struct page *page;
> +	swp_entry_t swp;
> +
> +	if (!IS_ENABLED(CONFIG_MIGRATION))
> +		return;
> +	/*
> +	 * swap_migration_tests() requires a dedicated page as it needs to
> +	 * be locked before creating a migration entry from it. Locking the
> +	 * page that actually maps kernel text ('start_kernel') can be real
> +	 * problematic. Lets allocate a dedicated page explicitly for this
> +	 * purpose that will be freed subsequently.
> +	 */
> +	page = alloc_page(GFP_KERNEL);
> +	if (!page) {
> +		pr_err("page allocation failed\n");
> +		return;
> +	}
> +
> +	/*
> +	 * make_migration_entry() expects given page to be
> +	 * locked, otherwise it stumbles upon a BUG_ON().
> +	 */
> +	__SetPageLocked(page);
> +	swp = make_migration_entry(page, 1);
> +	WARN_ON(!is_migration_entry(swp));
> +	WARN_ON(!is_write_migration_entry(swp));
> +
> +	make_migration_entry_read(&swp);
> +	WARN_ON(!is_migration_entry(swp));
> +	WARN_ON(is_write_migration_entry(swp));
> +
> +	swp = make_migration_entry(page, 0);
> +	WARN_ON(!is_migration_entry(swp));
> +	WARN_ON(is_write_migration_entry(swp));
> +	__ClearPageLocked(page);
> +	__free_page(page);
> +}
> +
> +#ifdef CONFIG_HUGETLB_PAGE
> +static void __init hugetlb_basic_tests(unsigned long pfn, pgprot_t prot)
> +{
> +	struct page *page;
> +	pte_t pte;
> +
> +	/*
> +	 * Accessing the page associated with the pfn is safe here,
> +	 * as it was previously derived from a real kernel symbol.
> +	 */
> +	page = pfn_to_page(pfn);
> +	pte = mk_huge_pte(page, prot);
> +
> +	WARN_ON(!huge_pte_dirty(huge_pte_mkdirty(pte)));
> +	WARN_ON(!huge_pte_write(huge_pte_mkwrite(huge_pte_wrprotect(pte))));
> +	WARN_ON(huge_pte_write(huge_pte_wrprotect(huge_pte_mkwrite(pte))));
> +
> +#ifdef CONFIG_ARCH_WANT_GENERAL_HUGETLB
> +	pte = pfn_pte(pfn, prot);
> +
> +	WARN_ON(!pte_huge(pte_mkhuge(pte)));
> +#endif
> +}
> +#else
> +static void __init hugetlb_basic_tests(unsigned long pfn, pgprot_t prot) { }
> +#endif
> +
> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> +static void __init pmd_thp_tests(unsigned long pfn, pgprot_t prot)
> +{
> +	pmd_t pmd;
> +
> +	/*
> +	 * pmd_trans_huge() and pmd_present() must return positive
> +	 * after MMU invalidation with pmd_mknotpresent().
> +	 */
> +	pmd = pfn_pmd(pfn, prot);
> +	WARN_ON(!pmd_trans_huge(pmd_mkhuge(pmd)));
> +
> +#ifndef __HAVE_ARCH_PMDP_INVALIDATE
> +	WARN_ON(!pmd_trans_huge(pmd_mknotpresent(pmd_mkhuge(pmd))));
> +	WARN_ON(!pmd_present(pmd_mknotpresent(pmd_mkhuge(pmd))));
> +#endif

I think we need a better comment here, because requiring pmd_trans_huge() and
pmd_present() returning true after pmd_mknotpresent() is not straightforward.

According to Andrea Arcangeli’s email (https://lore.kernel.org/linux-mm/20181017020930.GN30832@redhat.com/),
This behavior is an optimization for transparent huge page.
pmd_trans_huge() must be true if pmd_page() returns you a valid THP to avoid
taking the pmd_lock when others walk over non transhuge pmds (i.e. there are no
THP allocated). Especially when we split a THP, removing the present bit from
the pmd, pmd_trans_huge() still needs to return true. pmd_present() should
be true whenever pmd_trans_huge() returns true.

I think it is also worth either putting Andres’s email or the link to it
in the rst file in your 3rd patch. It is a good documentation for this special
case.

—
Best Regards,
Yan Zi

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