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From: Zi Yan <ziy@nvidia.com>
To: Anshuman Khandual <anshuman.khandual@arm.com>
CC: <linux-mm@kvack.org>, <christophe.leroy@c-s.fr>, Andrew Morton
	<akpm@linux-foundation.org>, Mike Rapoport <rppt@linux.ibm.com>, Vineet Gupta
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Subject: Re: [PATCH V2 1/3] mm/debug: Add tests validating arch page table
 helpers for core features
Date: Thu, 26 Mar 2020 13:08:34 -0400
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On 25 Mar 2020, at 22:18, Anshuman Khandual wrote:

> External email: Use caution opening links or attachments
>
>
> On 03/24/2020 06:59 PM, Zi Yan wrote:
>> On 24 Mar 2020, at 1:22, Anshuman Khandual wrote:
>>
>>> This adds new tests validating arch page table helpers for these foll=
owing
>>> core memory features. These tests create and test specific mapping ty=
pes 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 pro=
t)
>>> +{
>>> +    pte_t pte =3D 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 pr=
ot)
>>> +{
>>> +    pte_t pte =3D 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 pr=
ot)
>>> +{
>>> +    pmd_t pmd =3D 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 pr=
ot) { }
>>> +#endif
>>> +
>>> +#ifdef CONFIG_ARCH_HAS_PTE_DEVMAP
>>> +static void __init pte_devmap_tests(unsigned long pfn, pgprot_t prot=
)
>>> +{
>>> +    pte_t pte =3D 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 =3D 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 =3D 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 =3D 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, pgpr=
ot_t prot)
>>> +{
>>> +    pte_t pte =3D 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 =3D 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, pgpr=
ot_t prot)
>>> +{
>>> +    pmd_t pmd =3D 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, pgpr=
ot_t prot)
>>> +{
>>> +}
>>> +#endif
>>> +
>>> +static void __init pte_swap_tests(unsigned long pfn, pgprot_t prot)
>>> +{
>>> +    swp_entry_t swp;
>>> +    pte_t pte;
>>> +
>>> +    pte =3D pfn_pte(pfn, prot);
>>> +    swp =3D __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 =3D pfn_pmd(pfn, prot);
>>> +    swp =3D __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 r=
eal
>>> +     * problematic. Lets allocate a dedicated page explicitly for th=
is
>>> +     * purpose that will be freed subsequently.
>>> +     */
>>> +    page =3D 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 =3D 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 =3D 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 p=
rot)
>>> +{
>>> +    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 =3D pfn_to_page(pfn);
>>> +    pte =3D 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 =3D pfn_pte(pfn, prot);
>>> +
>>> +    WARN_ON(!pte_huge(pte_mkhuge(pte)));
>>> +#endif
>>> +}
>>> +#else
>>> +static void __init hugetlb_basic_tests(unsigned long pfn, pgprot_t p=
rot) { }
>>> +#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 =3D 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_hug=
e() and
>> pmd_present() returning true after pmd_mknotpresent() is not straightf=
orward.
>
> Thats right.
>
>>
>> According to Andrea Arcangeli=E2=80=99s 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. the=
re are no
>> THP allocated). Especially when we split a THP, removing the present b=
it from
>> the pmd, pmd_trans_huge() still needs to return true. pmd_present() sh=
ould
>> be true whenever pmd_trans_huge() returns true.
>
> Sure, will modify the existing comment here like this.
>
>         /*
>          * pmd_trans_huge() and pmd_present() must return positive afte=
r
>          * MMU invalidation with pmd_mknotpresent(). This behavior is a=
n
>          * optimization for transparent huge page. pmd_trans_huge() mus=
t
>          * be true if pmd_page() returns a valid THP to avoid taking th=
e
>          * pmd_lock when others walk over non transhuge pmds (i.e. ther=
e
>          * are no THP allocated). Especially when splitting a THP and
>          * removing the present bit from the pmd, pmd_trans_huge() stil=
l
>          * needs to return true. pmd_present() should be true whenever
>          * pmd_trans_huge() returns true.
>          */
>
>>
>> I think it is also worth either putting Andres=E2=80=99s email or the =
link to it
>> in the rst file in your 3rd patch. It is a good documentation for this=
 special
>> case.
>
> Makes sense. Will update Andrea's email link in the .rst file as well.
>

Look good to me. Thanks.

Reviewed-by: Zi Yan <ziy@nvidia.com>

=E2=80=94
Best Regards,
Yan Zi

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