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

[Anshuman Khandual]

On 02/27/2020 04:59 PM, Christophe Leroy wrote:
> 
> 
> Le 27/02/2020 Ã 11:33, Anshuman Khandual a ÃcritÂ:
>> 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.
>>
>> * SPECIAL mapping
>> * PROTNONE mapping
>> * DEVMAP mapping
>> * SOFTDIRTY mapping
>> * SWAP mapping
>> * MIGRATION mapping
>> * HUGETLB mapping
>> * THP mapping
>>
>> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
>> Cc: Mike Rapoport <rppt@xxxxxxxxxxxxx>
>> Cc: Vineet Gupta <vgupta@xxxxxxxxxxxx>
>> Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
>> Cc: Will Deacon <will@xxxxxxxxxx>
>> Cc: Benjamin Herrenschmidt <benh@xxxxxxxxxxxxxxxxxxx>
>> Cc: Paul Mackerras <paulus@xxxxxxxxx>
>> Cc: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
>> Cc: Heiko Carstens <heiko.carstens@xxxxxxxxxx>
>> Cc: Vasily Gorbik <gor@xxxxxxxxxxxxx>
>> Cc: Christian Borntraeger <borntraeger@xxxxxxxxxx>
>> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
>> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
>> Cc: Borislav Petkov <bp@xxxxxxxxx>
>> Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
>> Cc: Kirill A. Shutemov <kirill@xxxxxxxxxxxxx>
>> Cc: Paul Walmsley <paul.walmsley@xxxxxxxxxx>
>> Cc: Palmer Dabbelt <palmer@xxxxxxxxxxx>
>> Cc: linux-snps-arc@xxxxxxxxxxxxxxxxxxx
>> Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
>> Cc: linuxppc-dev@xxxxxxxxxxxxxxxx
>> Cc: linux-s390@xxxxxxxxxxxxxxx
>> Cc: linux-riscv@xxxxxxxxxxxxxxxxxxx
>> Cc: x86@xxxxxxxxxx
>> Cc: linux-arch@xxxxxxxxxxxxxxx
>> Cc: linux-kernel@xxxxxxxxxxxxxxx
>> Suggested-by: Catalin Marinas <catalin.marinas@xxxxxxx>
>> Signed-off-by: Anshuman Khandual <anshuman.khandual@xxxxxxx>
>> ---
>> Tested on arm64 and x86 platforms without any test failures. But this has
>> only been built tested on several other platforms. Individual tests need
>> to be verified on all current enabling platforms for the test i.e s390,
>> ppc32, arc etc.
>>
>> This patch must be applied on v5.6-rc3 after these patches
>>
>> 1. https://patchwork.kernel.org/patch/11385057/
>> 2. https://patchwork.kernel.org/patch/11407715/
>>
>> OR
>>
>> This patch must be applied on linux-next (next-20200227) after this patch
>>
>> 2. https://patchwork.kernel.org/patch/11407715/
>>
>> Â mm/debug_vm_pgtable.c | 310 +++++++++++++++++++++++++++++++++++++++++-
>> Â 1 file changed, 309 insertions(+), 1 deletion(-)
>>
>> diff --git a/mm/debug_vm_pgtable.c b/mm/debug_vm_pgtable.c
>> index 96dd7d574cef..3fb90d5b604e 100644
>> --- a/mm/debug_vm_pgtable.c
>> +++ b/mm/debug_vm_pgtable.c
>> @@ -41,6 +41,44 @@
>> ÂÂ * wrprotect(entry)ÂÂÂÂÂÂÂ = A write protected and not a write entry
>> ÂÂ * pxx_bad(entry)ÂÂÂÂÂÂÂ = A mapped and non-table entry
>> ÂÂ * pxx_same(entry1, entry2)ÂÂÂ = Both entries hold the exact same value
>> + *
>> + * Specific feature operations
>> + *
>> + * pte_mkspecial(entry)ÂÂÂÂÂÂÂ = Creates a special entry at PTE level
>> + * pte_special(entry)ÂÂÂÂÂÂÂ = Tests a special entry at PTE level
>> + *
>> + * pte_protnone(entry)ÂÂÂÂÂÂÂ = Tests a no access entry at PTE level
>> + * pmd_protnone(entry)ÂÂÂÂÂÂÂ = Tests a no access entry at PMD level
>> + *
>> + * pte_mkdevmap(entry)ÂÂÂÂÂÂÂ = Creates a device entry at PTE level
>> + * pmd_mkdevmap(entry)ÂÂÂÂÂÂÂ = Creates a device entry at PMD level
>> + * pud_mkdevmap(entry)ÂÂÂÂÂÂÂ = Creates a device entry at PUD level
>> + * pte_devmap(entry)ÂÂÂÂÂÂÂ = Tests a device entry at PTE level
>> + * pmd_devmap(entry)ÂÂÂÂÂÂÂ = Tests a device entry at PMD level
>> + * pud_devmap(entry)ÂÂÂÂÂÂÂ = Tests a device entry at PUD level
>> + *
>> + * pte_mksoft_dirty(entry)ÂÂÂ = Creates a soft dirty entry at PTE level
>> + * pmd_mksoft_dirty(entry)ÂÂÂ = Creates a soft dirty entry at PMD level
>> + * pte_swp_mksoft_dirty(entry)ÂÂÂ = Creates a soft dirty swap entry at PTE level
>> + * pmd_swp_mksoft_dirty(entry)ÂÂÂ = Creates a soft dirty swap entry at PMD level
>> + * pte_soft_dirty(entry)ÂÂÂ = Tests a soft dirty entry at PTE level
>> + * pmd_soft_dirty(entry)ÂÂÂ = Tests a soft dirty entry at PMD level
>> + * pte_swp_soft_dirty(entry)ÂÂÂ = Tests a soft dirty swap entry at PTE level
>> + * pmd_swp_soft_dirty(entry)ÂÂÂ = Tests a soft dirty swap entry at PMD level
>> + * pte_clear_soft_dirty(entry)ÂÂÂÂÂÂ = Clears a soft dirty entry at PTE level
>> + * pmd_clear_soft_dirty(entry)ÂÂÂÂÂÂ = Clears a soft dirty entry at PMD level
>> + * pte_swp_clear_soft_dirty(entry) = Clears a soft dirty swap entry at PTE level
>> + * pmd_swp_clear_soft_dirty(entry) = Clears a soft dirty swap entry at PMD level
>> + *
>> + * pte_mkhuge(entry)ÂÂÂÂÂÂÂ = Creates a HugeTLB entry at given level
>> + * pte_huge(entry)ÂÂÂÂÂÂÂ = Tests a HugeTLB entry at given level
>> + *
>> + * pmd_trans_huge(entry)ÂÂÂ = Tests a trans huge page at PMD level
>> + * pud_trans_huge(entry)ÂÂÂ = Tests a trans huge page at PUD level
>> + * pmd_present(entry)ÂÂÂÂÂÂÂ = Tests an entry points to memory at PMD level
>> + * pud_present(entry)ÂÂÂÂÂÂÂ = Tests an entry points to memory at PUD level
>> + * pmd_mknotpresent(entry)ÂÂÂ = Invalidates an PMD entry for MMU
>> + * pud_mknotpresent(entry)ÂÂÂ = Invalidates an PUD entry for MMU
>> ÂÂ */
>> Â #define VMFLAGSÂÂÂ (VM_READ|VM_WRITE|VM_EXEC)
>> Â @@ -287,6 +325,233 @@ static void __init pmd_populate_tests(struct mm_struct *mm, pmd_t *pmdp,
>> ÂÂÂÂÂ WARN_ON(pmd_bad(pmd));
>> Â }
>> Â +#ifdef CONFIG_ARCH_HAS_PTE_SPECIAL
> 
> Can we avoid ifdefs unless necessary ?
> 
> In mm/memory.c I see things like the following, it means pte_special() always exist and a #ifdef is not necessary.

True, #ifdef here can be dropped here, done.

> 
> ÂÂÂÂif (IS_ENABLED(CONFIG_ARCH_HAS_PTE_SPECIAL)) {
> ÂÂÂÂÂÂÂ if (likely(!pte_special(pte)))
> ÂÂÂÂÂÂÂÂÂÂÂ goto check_pfn;
> ÂÂÂÂÂÂÂ if (vma->vm_ops && vma->vm_ops->find_special_page)
> ÂÂÂÂÂÂÂÂÂÂÂ return vma->vm_ops->find_special_page(vma, addr);
> ÂÂÂÂÂÂÂ if (vma->vm_flags & (VM_PFNMAP | VM_MIXEDMAP))
> ÂÂÂÂÂÂÂÂÂÂÂ return NULL;
> ÂÂÂÂÂÂÂ if (is_zero_pfn(pfn))
> ÂÂÂÂÂÂÂÂÂÂÂ return NULL;
> ÂÂÂÂÂÂÂ if (pte_devmap(pte))
> ÂÂÂÂÂÂÂÂÂÂÂ return NULL;
> 
> ÂÂÂÂÂÂÂ print_bad_pte(vma, addr, pte, NULL);
> ÂÂÂÂÂÂÂ return NULL;
> ÂÂÂÂ}
> 
>> +static void __init pte_special_tests(unsigned long pfn, pgprot_t prot)
>> +{
>> +ÂÂÂ pte_t pte = pfn_pte(pfn, prot);
>> +
>> +ÂÂÂ WARN_ON(!pte_special(pte_mkspecial(pte)));
>> +}
>> +#else
>> +static void __init pte_special_tests(unsigned long pfn, pgprot_t prot) { }
>> +#endif
>> +
>> +#ifdef CONFIG_NUMA_BALANCING
> 
> Same here, this ifdef shouldn't be necessary because in /include/asm-generic/pgtable.h we have the following, so a if (IS_ENABLED()) should be enough.
> 
> #ifndef CONFIG_NUMA_BALANCING
> /*
> Â* Technically a PTE can be PROTNONE even when not doing NUMA balancing but
> Â* the only case the kernel cares is for NUMA balancing and is only ever set
> Â* when the VMA is accessible. For PROT_NONE VMAs, the PTEs are not marked
> Â* _PAGE_PROTNONE so by by default, implement the helper as "always no". It
> Â* is the responsibility of the caller to distinguish between PROT_NONE
> Â* protections and NUMA hinting fault protections.
> Â*/
> static inline int pte_protnone(pte_t pte)
> {
> ÂÂÂÂreturn 0;
> }
> 
> static inline int pmd_protnone(pmd_t pmd)
> {
> ÂÂÂÂreturn 0;
> }
> #endif /* CONFIG_NUMA_BALANCING */

True,  #ifdef here can be dropped, done. There is something I had missed
before, pfn_pmd() requires #ifdef CONFIG_TRANSPARENT_HUGEPAGE instead. We
need a pmd_t here with given prot. We cannot go via pfn_pte() followed by
pte_pmd(), as the later is platform specific and not available in general.

> 
>> +static void __init pte_protnone_tests(unsigned long pfn, pgprot_t prot)
>> +{
>> +ÂÂÂ pte_t pte = pfn_pte(pfn, prot);
>> +
>> +ÂÂÂ WARN_ON(!pte_protnone(pte));
>> +ÂÂÂ WARN_ON(!pte_present(pte));
>> +}
>> +
>> +static void __init pmd_protnone_tests(unsigned long pfn, pgprot_t prot)
>> +{
>> +ÂÂÂ pmd_t pmd = pfn_pmd(pfn, prot);
>> +
>> +ÂÂÂ WARN_ON(!pmd_protnone(pmd));
>> +ÂÂÂ WARN_ON(!pmd_present(pmd));
>> +}
>> +#else
>> +static void __init pte_protnone_tests(unsigned long pfn, pgprot_t prot) { }
>> +static void __init pmd_protnone_tests(unsigned long pfn, pgprot_t prot) { }
>> +#endif
>> +
>> +#ifdef CONFIG_ARCH_HAS_PTE_DEVMAP
> 
> Same here, in include/linux/mm.h we have:
> 
> #ifndef CONFIG_ARCH_HAS_PTE_DEVMAP
> static inline int pte_devmap(pte_t pte)
> {
> ÂÂÂÂreturn 0;
> }
> #endif
> 
> 
>> +static void __init pte_devmap_tests(unsigned long pfn, pgprot_t prot)
>> +{
>> +ÂÂÂ pte_t pte = pfn_pte(pfn, prot);
>> +
>> +ÂÂÂ WARN_ON(!pte_devmap(pte_mkdevmap(pte)));
>> +}
>> +
>> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> 
> Same. In inlude/asm-generic/pgtables.h you have:
> 
> #if !defined(CONFIG_ARCH_HAS_PTE_DEVMAP) || !defined(CONFIG_TRANSPARENT_HUGEPAGE)
> static inline int pmd_devmap(pmd_t pmd)
> {
> ÂÂÂÂreturn 0;
> }
> static inline int pud_devmap(pud_t pud)
> {
> ÂÂÂÂreturn 0;
> }
> static inline int pgd_devmap(pgd_t pgd)
> {
> ÂÂÂÂreturn 0;
> }
> #endif
> 
>> +static void __init pmd_devmap_tests(unsigned long pfn, pgprot_t prot)
>> +{
>> +ÂÂÂ pmd_t pmd = pfn_pmd(pfn, prot);
>> +
>> +ÂÂÂ WARN_ON(!pmd_devmap(pmd_mkdevmap(pmd)));
>> +}
>> +
>> +#ifdef CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD
> 
> Same, see above

Even though pxx_devmap() fallback definitions are present, pxx_mkdevmap()
ones are still missing. We will have to add them first as a pre-requisite
patch (which might not be popular without any non-debug use case) in order
to drop these #ifdefs here.

> 
>> +static void __init pud_devmap_tests(unsigned long pfn, pgprot_t prot)
>> +{
>> +ÂÂÂ pud_t pud = pfn_pud(pfn, prot);
>> +
>> +ÂÂÂ 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
>> +
>> +#ifdef CONFIG_MEM_SOFT_DIRTY
> 
> Same, they always exist, see include/asm-generic/pgtable.h

Yeah, this can be dropped. Though will have to again add TRANSPARENT_HUGEPAGE
to protect pfn_pmd() as explained before.

> 
>> +static void __init pte_soft_dirty_tests(unsigned long pfn, pgprot_t prot)
>> +{
>> +ÂÂÂ pte_t pte = pfn_pte(pfn, prot);
>> +
>> +ÂÂÂ WARN_ON(!pte_soft_dirty(pte_mksoft_dirty(pte)));
>> +ÂÂÂ WARN_ON(pte_soft_dirty(pte_clear_soft_dirty(pte)));
>> +}
>> +
>> +static void __init pte_swap_soft_dirty_tests(unsigned long pfn, pgprot_t prot)
>> +{
>> +ÂÂÂ pte_t pte = pfn_pte(pfn, prot);
>> +
>> +ÂÂÂ WARN_ON(!pte_swp_soft_dirty(pte_swp_mksoft_dirty(pte)));
>> +ÂÂÂ WARN_ON(pte_swp_soft_dirty(pte_swp_clear_soft_dirty(pte)));
>> +}
>> +
>> +#ifdef CONFIG_ARCH_ENABLE_THP_MIGRATION
> 
> Same

True, #ifdef here can be dropped, done.

> 
>> +static void __init pmd_soft_dirty_tests(unsigned long pfn, pgprot_t prot)
>> +{
>> +ÂÂÂ pmd_t pmd = pfn_pmd(pfn, prot);
>> +
>> +ÂÂÂ WARN_ON(!pmd_soft_dirty(pmd_mksoft_dirty(pmd)));
>> +ÂÂÂ WARN_ON(pmd_soft_dirty(pmd_clear_soft_dirty(pmd)));
>> +}
>> +
>> +static void __init pmd_swap_soft_dirty_tests(unsigned long pfn, pgprot_t prot)
>> +{
>> +ÂÂÂ pmd_t pmd = pfn_pmd(pfn, prot);
>> +
>> +ÂÂÂ WARN_ON(!pmd_swp_soft_dirty(pmd_swp_mksoft_dirty(pmd)));
>> +ÂÂÂ 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
>> +#else
>> +static void __init pte_soft_dirty_tests(unsigned long pfn, pgprot_t prot) { }
>> +static void __init pmd_soft_dirty_tests(unsigned long pfn, pgprot_t prot) { }
>> +static void __init pte_swap_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)
>> +{
>> +ÂÂÂ swp_entry_t swp;
>> +ÂÂÂ pte_t pte;
>> +
>> +ÂÂÂ pte = pfn_pte(pfn, prot);
>> +ÂÂÂ swp = __pte_to_swp_entry(pte);
>> +ÂÂÂ 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)
>> +{
>> +ÂÂÂ swp_entry_t swp;
>> +ÂÂÂ pmd_t pmd;
>> +
>> +ÂÂÂ pmd = pfn_pmd(pfn, prot);
>> +ÂÂÂ swp = __pmd_to_swp_entry(pmd);
>> +ÂÂÂ WARN_ON(!pmd_same(pmd, __swp_entry_to_pmd(swp)));
>> +}
>> +#else
>> +static void __init pmd_swap_tests(unsigned long pfn, pgprot_t prot) { }
>> +#endif
>> +
>> +#ifdef CONFIG_MIGRATION
> 
> Same. See include/linux/swapops.h

True, #ifdef here can be dropped, done. Though will have to again add
back TRANSPARENT_HUGEPAGE to protect pfn_pmd() as explained before.

> 
>> +static void __init swap_migration_tests(struct page *page)
>> +{
>> +ÂÂÂ swp_entry_t swp;
>> +
>> +ÂÂÂ /*
>> +ÂÂÂÂ * make_migration_entry() expects given page to be
>> +ÂÂÂÂ * locked, otherwise it stumbles upon a BUG_ON().
>> +ÂÂÂÂ */
>> +ÂÂÂ __SetPageLocked(page);
>> +ÂÂÂ swp = make_migration_entry(page, 1);
>> +ÂÂÂ WARN_ON(!is_migration_entry(swp));
>> +ÂÂÂ WARN_ON(!is_write_migration_entry(swp));
>> +
>> +ÂÂÂ make_migration_entry_read(&swp);
>> +ÂÂÂ WARN_ON(!is_migration_entry(swp));
>> +ÂÂÂ WARN_ON(is_write_migration_entry(swp));
>> +
>> +ÂÂÂ swp = make_migration_entry(page, 0);
>> +ÂÂÂ WARN_ON(!is_migration_entry(swp));
>> +ÂÂÂ WARN_ON(is_write_migration_entry(swp));
>> +ÂÂÂ __ClearPageLocked(page);
>> +}
>> +#else
>> +static void __init swap_migration_tests(struct page *page) { }
>> +#endif
>> +
>> +#ifdef CONFIG_HUGETLB_PAGE
>> +static void __init hugetlb_tests(unsigned long pfn, pgprot_t prot)
>> +{
>> +#ifdef CONFIG_ARCH_WANT_GENERAL_HUGETLB
>> +ÂÂÂ pte_t pte = pfn_pte(pfn, prot);
>> +
>> +ÂÂÂ WARN_ON(!pte_huge(pte_mkhuge(pte)));
> 
> We also need tests on hugepd stuff

Sure, but lets discuss this on the other thread.

> 
>> +#endif
>> +}
>> +#else
>> +static void __init hugetlb_tests(unsigned long pfn, pgprot_t prot) { }
>> +#endif
>> +
>> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> 
> Same, see include/asm-generic/pgtable.h

This is required to protect pxx_mknotpresent() which does not have a
fall back and pfn_pmd()/pfn_pud() helpers have similar situation as
well.

> 
>> +static void __init pmd_thp_tests(unsigned long pfn, pgprot_t prot)
>> +{
>> +ÂÂÂ pmd_t pmd;
>> +
>> +ÂÂÂ /*
>> +ÂÂÂÂ * pmd_trans_huge() and pmd_present() must return negative
>> +ÂÂÂÂ * after MMU invalidation with pmd_mknotpresent().
>> +ÂÂÂÂ */
>> +ÂÂÂ pmd = pfn_pmd(pfn, prot);
>> +ÂÂÂ WARN_ON(!pmd_trans_huge(pmd_mkhuge(pmd)));
>> +
>> +ÂÂÂ /*
>> +ÂÂÂÂ * Though platform specific test exclusions are not ideal,
>> +ÂÂÂÂ * in this case S390 does not define pmd_mknotpresent()
>> +ÂÂÂÂ * which should be tested on other platforms enabling THP.
>> +ÂÂÂÂ */
>> +#ifndef CONFIG_S390
>> +ÂÂÂ WARN_ON(pmd_trans_huge(pmd_mknotpresent(pmd)));
>> +ÂÂÂ WARN_ON(pmd_present(pmd_mknotpresent(pmd)));
>> +#endif
> 
> Can we add a stub on S390 instead ?

Actually we dont have to. pmd_mknotpresent() is required for platforms
that do not have __HAVE_ARCH_PMDP_INVALIDATE. Hence can wrap this code
with !__HAVE_ARCH_PMDP_INVALIDATE to prevent build failures on such
platforms like s390.

> 
>> +}
>> +
>> +#ifdef CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD
> 
> Same ?

The problem is, neither pud_mknotpresent() nor pfn_pud() have a generic
fallback definition. So will have to keep this #ifdef.

> 
>> +static void __init pud_thp_tests(unsigned long pfn, pgprot_t prot)
>> +{
>> +ÂÂÂ pud_t pud;
>> +
>> +ÂÂÂ /*
>> +ÂÂÂÂ * pud_trans_huge() and pud_present() must return negative
>> +ÂÂÂÂ * after MMU invalidation with pud_mknotpresent().
>> +ÂÂÂÂ */
>> +ÂÂÂ pud = pfn_pud(pfn, prot);
>> +ÂÂÂ WARN_ON(!pud_trans_huge(pud_mkhuge(pud)));
>> +ÂÂÂ WARN_ON(pud_trans_huge(pud_mknotpresent(pud)));
>> +ÂÂÂ WARN_ON(pud_present(pud_mknotpresent(pud)));
>> +}
>> +#else
>> +static void __init pud_thp_tests(unsigned long pfn, pgprot_t prot) { }
>> +#endif
>> +#else
>> +static void __init pmd_thp_tests(unsigned long pfn, pgprot_t prot) { }
>> +static void __init pud_thp_tests(unsigned long pfn, pgprot_t prot) { }
>> +#endif
>> +
>> Â static unsigned long __init get_random_vaddr(void)
>> Â {
>> ÂÂÂÂÂ unsigned long random_vaddr, random_pages, total_user_pages;
>> @@ -302,13 +567,14 @@ static unsigned long __init get_random_vaddr(void)
>> Â void __init debug_vm_pgtable(void)
>> Â {
>> ÂÂÂÂÂ struct mm_struct *mm;
>> +ÂÂÂ struct page *page;
>> ÂÂÂÂÂ pgd_t *pgdp;
>> ÂÂÂÂÂ p4d_t *p4dp, *saved_p4dp;
>> ÂÂÂÂÂ pud_t *pudp, *saved_pudp;
>> ÂÂÂÂÂ pmd_t *pmdp, *saved_pmdp, pmd;
>> ÂÂÂÂÂ pte_t *ptep;
>> ÂÂÂÂÂ pgtable_t saved_ptep;
>> -ÂÂÂ pgprot_t prot;
>> +ÂÂÂ pgprot_t prot, protnone;
>> ÂÂÂÂÂ phys_addr_t paddr;
>> ÂÂÂÂÂ unsigned long vaddr, pte_aligned, pmd_aligned;
>> ÂÂÂÂÂ unsigned long pud_aligned, p4d_aligned, pgd_aligned;
>> @@ -322,6 +588,25 @@ void __init debug_vm_pgtable(void)
>> ÂÂÂÂÂÂÂÂÂ return;
>> ÂÂÂÂÂ }
>> Â +ÂÂÂ /*
>> +ÂÂÂÂ * swap_migration_tests() requires a dedicated page as it needs to
>> +ÂÂÂÂ * be locked before creating a migration entry from it. Locking the
>> +ÂÂÂÂ * page that actually maps kernel text ('start_kernel') can be real
>> +ÂÂÂÂ * problematic. Lets allocate a dedicated page explicitly for this
>> +ÂÂÂÂ * purpose that will be freed later.
>> +ÂÂÂÂ */
>> +ÂÂÂ page = alloc_page(GFP_KERNEL);
> 
> Can we do the page allocation and freeing in swap_migration_tests() instead ?

Although all the resources used in the helpers have been allocated in the main
function itself before being passed down and later freed. But may be just an
exception could be made for swap_migration_tests() function as the allocated
page is being exclusively used here. Later on if we need this page for some
other future tests, then will have to move it back to debug_vm_pgtable().