Date:   Tue, 3 Aug 2021 15:22:07 +1000
From:   Gavin Shan <gshan@...hat.com>
To:     Anshuman Khandual <anshuman.khandual@....com>, linux-mm@...ck.org
Cc:     linux-kernel@...r.kernel.org, gerald.schaefer@...ux.ibm.com,
        aneesh.kumar@...ux.ibm.com, christophe.leroy@...roup.eu,
        cai@....pw, catalin.marinas@....com, will@...nel.org,
        vgupta@...opsys.com, akpm@...ux-foundation.org, chuhu@...hat.com,
        shan.gavin@...il.com
Subject: Re: [PATCH v5 01/12] mm/debug_vm_pgtable: Introduce struct
 pgtable_debug_args

Hi Anshuman,

On 8/3/21 2:42 PM, Anshuman Khandual wrote:
> On 8/2/21 11:34 AM, Gavin Shan wrote:
>> In debug_vm_pgtable(), there are many local variables introduced to
>> track the needed information and they are passed to the functions for
>> various test cases. It'd better to introduce a struct as place holder
>> for these information. With it, what the tests functions need is the
>> struct. In this way, the code is simplified and easier to be maintained.
>>
>> Besides, set_xxx_at() could access the data on the corresponding pages
>> in the page table modifying tests. So the accessed pages in the tests
>> should have been allocated from buddy. Otherwise, we're accessing pages
>> that aren't owned by us. This causes issues like page flag corruption
>> or kernel crash on accessing unmapped page when CONFIG_DEBUG_PAGEALLOC
>> is enabled.
>>
>> This introduces "struct pgtable_debug_args". The struct is initialized
>> and destroyed, but the information in the struct isn't used yet. It will
>> be used in subsequent patches.
>>
>> Signed-off-by: Gavin Shan <gshan@...hat.com>
>> Reviewed-by: Anshuman Khandual <anshuman.khandual@....com>
>> ---
>>   mm/debug_vm_pgtable.c | 269 +++++++++++++++++++++++++++++++++++++++++-
>>   1 file changed, 268 insertions(+), 1 deletion(-)
>>
>> diff --git a/mm/debug_vm_pgtable.c b/mm/debug_vm_pgtable.c
>> index 1c922691aa61..6a825f0e7cd7 100644
>> --- a/mm/debug_vm_pgtable.c
>> +++ b/mm/debug_vm_pgtable.c
>> @@ -58,6 +58,37 @@
>>   #define RANDOM_ORVALUE (GENMASK(BITS_PER_LONG - 1, 0) & ~ARCH_SKIP_MASK)
>>   #define RANDOM_NZVALUE	GENMASK(7, 0)
>>   
>> +struct pgtable_debug_args {
>> +	struct mm_struct	*mm;
>> +	struct vm_area_struct	*vma;
>> +
>> +	pgd_t			*pgdp;
>> +	p4d_t			*p4dp;
>> +	pud_t			*pudp;
>> +	pmd_t			*pmdp;
>> +	pte_t			*ptep;
>> +
>> +	p4d_t			*start_p4dp;
>> +	pud_t			*start_pudp;
>> +	pmd_t			*start_pmdp;
>> +	pgtable_t		start_ptep;
>> +
>> +	unsigned long		vaddr;
>> +	pgprot_t		page_prot;
>> +	pgprot_t		page_prot_none;
>> +
>> +	bool			is_contiguous_page;
>> +	unsigned long		pud_pfn;
>> +	unsigned long		pmd_pfn;
>> +	unsigned long		pte_pfn;
>> +
>> +	unsigned long		fixed_pgd_pfn;
>> +	unsigned long		fixed_p4d_pfn;
>> +	unsigned long		fixed_pud_pfn;
>> +	unsigned long		fixed_pmd_pfn;
>> +	unsigned long		fixed_pte_pfn;
>> +};
>> +
>>   static void __init pte_basic_tests(unsigned long pfn, int idx)
>>   {
>>   	pgprot_t prot = protection_map[idx];
>> @@ -955,8 +986,238 @@ static unsigned long __init get_random_vaddr(void)
>>   	return random_vaddr;
>>   }
>>   
>> +static void __init destroy_args(struct pgtable_debug_args *args)
>> +{
>> +	struct page *page = NULL;
>> +
>> +	/* Free (huge) page */
>> +	if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE) &&
>> +	    IS_ENABLED(CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD) &&
>> +	    has_transparent_hugepage() &&
>> +	    args->pud_pfn != ULONG_MAX) {
>> +		if (args->is_contiguous_page) {
>> +			free_contig_range(args->pud_pfn,
>> +					  (1 << (HPAGE_PUD_SHIFT - PAGE_SHIFT)));
>> +		} else {
>> +			page = pfn_to_page(args->pud_pfn);
>> +			__free_pages(page, HPAGE_PUD_SHIFT - PAGE_SHIFT);
>> +		}
>> +
>> +		args->pud_pfn = ULONG_MAX;
>> +		args->pmd_pfn = ULONG_MAX;
>> +		args->pte_pfn = ULONG_MAX;
>> +	}
>> +
>> +	if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE) &&
>> +	    has_transparent_hugepage() &&
>> +	    args->pmd_pfn != ULONG_MAX) {
>> +		if (args->is_contiguous_page) {
>> +			free_contig_range(args->pmd_pfn, (1 << HPAGE_PMD_ORDER));
>> +		} else {
>> +			page = pfn_to_page(args->pmd_pfn);
>> +			__free_pages(page, HPAGE_PMD_ORDER);
>> +		}
>> +
>> +		args->pmd_pfn = ULONG_MAX;
>> +		args->pte_pfn = ULONG_MAX;
>> +	}
>> +
>> +	if (args->pte_pfn != ULONG_MAX) {
>> +		page = pfn_to_page(args->pte_pfn);
>> +		__free_pages(page, 0);
> 
> 		args->pte_pfn = ULONG_MAX ?
> 

I was thinking of this. It doesn't matter to set @pte_pfn to
ULONG_MAX as it's not used afterwards. However, I will add this
in v6 for consistency.

>> +	}
>> +
>> +	/* Free page table entries */
>> +	if (args->start_ptep) {
>> +		pte_free(args->mm, args->start_ptep);
>> +		mm_dec_nr_ptes(args->mm);
>> +	}
>> +
>> +	if (args->start_pmdp) {
>> +		pmd_free(args->mm, args->start_pmdp);
>> +		mm_dec_nr_pmds(args->mm);
>> +	}
>> +
>> +	if (args->start_pudp) {
>> +		pud_free(args->mm, args->start_pudp);
>> +		mm_dec_nr_puds(args->mm);
>> +	}
>> +
>> +	if (args->start_p4dp)
>> +		p4d_free(args->mm, args->start_p4dp);
>> +
>> +	/* Free vma and mm struct */
>> +	if (args->vma)
>> +		vm_area_free(args->vma);
>> +
>> +	if (args->mm)
>> +		mmdrop(args->mm);
>> +}
>> +
>> +static struct page * __init debug_vm_pgtable_alloc_huge_page(
>> +				struct pgtable_debug_args *args, int order)
> 
> Small nit, formatting like below would have been better. But dont change yet
> unless there is a respin.
> 

Yes, will fix in v6.

> static struct page * __init
> debug_vm_pgtable_alloc_huge_page(struct pgtable_debug_args *args, int order)
> 
>> +{
>> +	struct page *page = NULL;
>> +
>> +#ifdef CONFIG_CONTIG_ALLOC
>> +	if (order >= MAX_ORDER) {
>> +		page = alloc_contig_pages((1 << order), GFP_KERNEL,
>> +					  first_online_node, NULL);
>> +		if (page) {
>> +			args->is_contiguous_page = true;
>> +			return page;
>> +		}
>> +	}
>> +#endif
>> +
>> +	if (order < MAX_ORDER)
>> +		page = alloc_pages(GFP_KERNEL, order);
>> +
>> +	return page;
>> +}
>> +
>> +static int __init init_args(struct pgtable_debug_args *args)
>> +{
>> +	struct page *page = NULL;
>> +	phys_addr_t phys;
>> +	int ret = 0;
>> +
>> +	/*
>> +	 * Initialize the debugging data.
>> +	 *
>> +	 * __P000 (or even __S000) will help create page table entries with
>> +	 * PROT_NONE permission as required for pxx_protnone_tests().
>> +	 */
>> +	memset(args, 0, sizeof(*args));
>> +	args->vaddr              = get_random_vaddr();
>> +	args->page_prot          = vm_get_page_prot(VMFLAGS);
>> +	args->page_prot_none     = __P000;
>> +	args->is_contiguous_page = false;
>> +	args->pud_pfn            = ULONG_MAX;
>> +	args->pmd_pfn            = ULONG_MAX;
>> +	args->pte_pfn            = ULONG_MAX;
>> +	args->fixed_pgd_pfn      = ULONG_MAX;
>> +	args->fixed_p4d_pfn      = ULONG_MAX;
>> +	args->fixed_pud_pfn      = ULONG_MAX;
>> +	args->fixed_pmd_pfn      = ULONG_MAX;
>> +	args->fixed_pte_pfn      = ULONG_MAX;
>> +
>> +	/* Allocate mm and vma */
>> +	args->mm = mm_alloc();
>> +	if (!args->mm) {
>> +		pr_err("Failed to allocate mm struct\n");
>> +		ret = -ENOMEM;
>> +		goto error;
>> +	}
>> +
>> +	args->vma = vm_area_alloc(args->mm);
>> +	if (!args->vma) {
>> +		pr_err("Failed to allocate vma\n");
>> +		ret = -ENOMEM;
>> +		goto error;
>> +	}
>> +
>> +	/*
>> +	 * Allocate page table entries. They will be modified in the tests.
>> +	 * Lets save the page table entries so that they can be released
>> +	 * when the tests are completed.
>> +	 */
>> +	args->pgdp = pgd_offset(args->mm, args->vaddr);
>> +	args->p4dp = p4d_alloc(args->mm, args->pgdp, args->vaddr);
>> +	args->start_p4dp = p4d_offset(args->pgdp, 0UL);
>> +	WARN_ON(!args->start_p4dp);
>> +	if (!args->p4dp) {
>> +		pr_err("Failed to allocate p4d entries\n");
>> +		ret = -ENOMEM;
>> +		goto error;
>> +	}
> 
> Expected like this instead. 'args->start_p4dp' should not be evaluated
> unless 'args->p4dp' allocation succeeds. Otherwise on the error path,
> it would call p4d_free(args->mm, args->start_p4dp) freeing up a page
> table page which was never allocated !
> 
> 	args->pgdp = pgd_offset(args->mm, args->vaddr);
> 	args->p4dp = p4d_alloc(args->mm, args->pgdp, args->vaddr);
> 	if (!args->p4dp) {
> 		pr_err("Failed to allocate p4d entries\n");
> 		ret = -ENOMEM;
> 		goto error;
> 	}
> 	args->start_p4dp = p4d_offset(args->pgdp, 0UL);
> 	WARN_ON(!args->start_p4dp);
> 
> I had requested just to move these two sentences into the previous code
> block, without changing any order. This is also applicable for all other
> page table levels below.
> 

Yeah, I misunderstood your comments on v5 series. I wrongly thought
NULL is returned from p4d_offset() if p4d_alloc() fails, without
checking the implementation of p4d_offset() closely. I will fix
it in v6. However, I will hold posting v6 to see if there are
more comments on v5.

>> +
>> +	args->pudp = pud_alloc(args->mm, args->p4dp, args->vaddr);
>> +	args->start_pudp = pud_offset(args->p4dp, 0UL);
>> +	WARN_ON(!args->start_pudp);
>> +	if (!args->pudp) {
>> +		pr_err("Failed to allocate pud entries\n");
>> +		ret = -ENOMEM;
>> +		goto error;
>> +	}
>> +
>> +	args->pmdp = pmd_alloc(args->mm, args->pudp, args->vaddr);
>> +	args->start_pmdp = pmd_offset(args->pudp, 0UL);
>> +	WARN_ON(!args->start_pmdp);
>> +	if (!args->pmdp) {
>> +		pr_err("Failed to allocate pmd entries\n");
>> +		ret = -ENOMEM;
>> +		goto error;
>> +	}
>> +
>> +	args->ptep = pte_alloc_map(args->mm, args->pmdp, args->vaddr);
>> +	args->start_ptep = pmd_pgtable(READ_ONCE(*args->pmdp));
>> +	WARN_ON(!args->start_ptep);
>> +	if (!args->ptep) {
>> +		pr_err("Failed to allocate pte entries\n");
>> +		ret = -ENOMEM;
>> +		goto error;
>> +	}
>> +
>> +	/*
>> +	 * PFN for mapping at PTE level is determined from a standard kernel
>> +	 * text symbol. But pfns for higher page table levels are derived by
>> +	 * masking lower bits of this real pfn. These derived pfns might not
>> +	 * exist on the platform but that does not really matter as pfn_pxx()
>> +	 * helpers will still create appropriate entries for the test. This
>> +	 * helps avoid large memory block allocations to be used for mapping
>> +	 * at higher page table levels in some of the tests.
>> +	 */
>> +	phys = __pa_symbol(&start_kernel);
>> +	args->fixed_pgd_pfn = __phys_to_pfn(phys & PGDIR_MASK);
>> +	args->fixed_p4d_pfn = __phys_to_pfn(phys & P4D_MASK);
>> +	args->fixed_pud_pfn = __phys_to_pfn(phys & PUD_MASK);
>> +	args->fixed_pmd_pfn = __phys_to_pfn(phys & PMD_MASK);
>> +	args->fixed_pte_pfn = __phys_to_pfn(phys & PAGE_MASK);
>> +	WARN_ON(!pfn_valid(args->fixed_pte_pfn));
>> +
>> +	/*
>> +	 * Allocate (huge) pages because some of the tests need to access
>> +	 * the data in the pages. The corresponding tests will be skipped
>> +	 * if we fail to allocate (huge) pages.
>> +	 */
>> +	if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE) &&
>> +	    IS_ENABLED(CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD) &&
>> +	    has_transparent_hugepage()) {
>> +		page = debug_vm_pgtable_alloc_huge_page(args,
>> +				HPAGE_PUD_SHIFT - PAGE_SHIFT);
>> +		if (page) {
>> +			args->pud_pfn = page_to_pfn(page);
>> +			args->pmd_pfn = args->pud_pfn;
>> +			args->pte_pfn = args->pud_pfn;
>> +			return 0;
>> +		}
>> +	}
>> +
>> +	if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE) &&
>> +	    has_transparent_hugepage()) {
>> +		page = debug_vm_pgtable_alloc_huge_page(args, HPAGE_PMD_ORDER);
>> +		if (page) {
>> +			args->pmd_pfn = page_to_pfn(page);
>> +			args->pte_pfn = args->pmd_pfn;
>> +			return 0;
>> +		}
>> +	}
>> +
>> +	page = alloc_pages(GFP_KERNEL, 0);
>> +	if (page)
>> +		args->pte_pfn = page_to_pfn(page);
>> +
>> +	return 0;
>> +
>> +error:
>> +	destroy_args(args);
>> +	return ret;
>> +}
>> +
>>   static int __init debug_vm_pgtable(void)
>>   {
>> +	struct pgtable_debug_args args;
>>   	struct vm_area_struct *vma;
>>   	struct mm_struct *mm;
>>   	pgd_t *pgdp;
>> @@ -970,9 +1231,13 @@ static int __init debug_vm_pgtable(void)
>>   	unsigned long vaddr, pte_aligned, pmd_aligned;
>>   	unsigned long pud_aligned, p4d_aligned, pgd_aligned;
>>   	spinlock_t *ptl = NULL;
>> -	int idx;
>> +	int idx, ret;
>>   
>>   	pr_info("Validating architecture page table helpers\n");
>> +	ret = init_args(&args);
>> +	if (ret)
>> +		return ret;
>> +
>>   	prot = vm_get_page_prot(VMFLAGS);
>>   	vaddr = get_random_vaddr();
>>   	mm = mm_alloc();
>> @@ -1127,6 +1392,8 @@ static int __init debug_vm_pgtable(void)
>>   	mm_dec_nr_pmds(mm);
>>   	mm_dec_nr_ptes(mm);
>>   	mmdrop(mm);
>> +
>> +	destroy_args(&args);
>>   	return 0;
>>   }
>>   late_initcall(debug_vm_pgtable);
>>

Thanks,
Gavin

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