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Subject: Re: [PATCH v3 01/12] mm/debug_vm_pgtable: Introduce struct
 pgtable_debug_args
To:     Gavin Shan <gshan@redhat.com>, linux-mm@kvack.org
Cc:     linux-kernel@vger.kernel.org, catalin.marinas@arm.com,
        will@kernel.org, akpm@linux-foundation.org, chuhu@redhat.com,
        shan.gavin@gmail.com
References: <20210719130613.334901-1-gshan@redhat.com>
 <20210719130613.334901-2-gshan@redhat.com>
 <ab0f9daa-0c49-e74c-e073-6e03a3cabb07@arm.com>
 <280a5740-b5dc-4b78-3a38-67e5adbb0afd@redhat.com>
 <04a4618f-9899-1518-cee1-0a48cb4df4c6@arm.com>
 <65078a0c-c35c-8e3f-d4d3-3090b0c3daaf@redhat.com>
From:   Anshuman Khandual <anshuman.khandual@arm.com>
Message-ID: <4a534102-11ff-c849-781e-ed173e46da56@arm.com>
Date:   Thu, 22 Jul 2021 12:38:46 +0530
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On 7/22/21 11:53 AM, Gavin Shan wrote:
> Hi Anshuman,
> 
> On 7/22/21 2:41 PM, Anshuman Khandual wrote:
>> On 7/21/21 3:50 PM, Gavin Shan wrote:
>>> On 7/21/21 3:44 PM, Anshuman Khandual wrote:
>>>> On 7/19/21 6:36 PM, 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 functions for various test
>>>>> cases need is the struct, to simplify the code. It also makes code
>>>>> 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.
>>>>>
>>>>> This introduces "struct pgtable_debug_args". The struct is initialized
>>>>> and destroyed, but the information in the struct isn't used yet. They
>>>>> will be used in subsequent patches.
>>>>>
>>>>> Signed-off-by: Gavin Shan <gshan@redhat.com>
>>>>> ---
>>>>>    mm/debug_vm_pgtable.c | 197 +++++++++++++++++++++++++++++++++++++++++-
>>>>>    1 file changed, 196 insertions(+), 1 deletion(-)
>>>>>
>>>
>>> I saw you've finished the review on PATCH[v3 01/12] and PATCH[v3 02/12].
>>> I will wait to integrate your comments to v4 until you finish the review
>>> on all patches in v3 series.
>>>
>>>>> diff --git a/mm/debug_vm_pgtable.c b/mm/debug_vm_pgtable.c
>>>>> index 1c922691aa61..ea153ff40d23 100644
>>>>> --- a/mm/debug_vm_pgtable.c
>>>>> +++ b/mm/debug_vm_pgtable.c
>>>>> @@ -58,6 +58,36 @@
>>>>>    #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;
>>>>> +
>>>>> +    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 +985,167 @@ 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) {
>>>>> +        page = pfn_to_page(args->pud_pfn);
>>>>> +        __free_pages(page, HPAGE_PUD_SHIFT - PAGE_SHIFT);
>>>>> +    } else if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE) &&
>>>>> +           has_transparent_hugepage() &&
>>>>> +           args->pmd_pfn != ULONG_MAX) {
>>>>> +        page = pfn_to_page(args->pmd_pfn);
>>>>> +        __free_pages(page, HPAGE_PMD_ORDER);
>>>>> +    } else if (args->pte_pfn != ULONG_MAX) {
>>>>> +        page = pfn_to_page(args->pte_pfn);
>>>>> +        __free_pages(page, 0);
>>>>> +    }
>>>>> +
>>>>> +    /* Free page table */
>>>>> +    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->p4dp);
>>>>> +
>>>>> +    /* Free vma and mm struct */
>>>>> +    if (args->vma)
>>>>> +        vm_area_free(args->vma);
>>>>> +    if (args->mm)
>>>>> +        mmdrop(args->mm);
>>>>> +}
>>>>> +
>>>>> +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 */
>>>>> +    memset(args, 0, sizeof(*args));
>>>>> +    args->page_prot      = vm_get_page_prot(VMFLAGS);
>>>>> +    args->page_prot_none = __P000;
>>>>
>>>> Please preserve the existing comments before this assignment.
>>>>
>>>>           /*
>>>>            * __P000 (or even __S000) will help create page table entries with
>>>>            * PROT_NONE permission as required for pxx_protnone_tests().
>>>>            */
>>>>
>>>
>>> Sure. I will combine the comments in v4 as below:
>>>
>>>      /*
>>>       * Initialize the debugging arguments.
>>>       *
>>>       * __P000 (or even __S000) will help create page table entries with
>>>           * PROT_NONE permission as required for pxx_protnone_tests().
>>>           */
>>>
>>>
>>>>> +    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;
>>>>> +    }
>>>>> +
>>>>> +    /* Figure out the virtual address and allocate page table entries */
>>>>> +    args->vaddr = get_random_vaddr();
>>>>
>>>> Please group args->vaddr's init with page_prot and page_prot_none above.
>>>>
>>>
>>> Yes, It will make the code tidy. I'll move this line accordingly in v4,
>>> but the related comments will be dropped as the code is self-explanatory.
>>>
>>>          /* Allocate page table entries */
>>>
>>>>> +    args->pgdp = pgd_offset(args->mm, args->vaddr);
>>>>> +    args->p4dp = p4d_alloc(args->mm, args->pgdp, args->vaddr);
>>>>> +    args->pudp = args->p4dp ?
>>>>> +             pud_alloc(args->mm, args->p4dp, args->vaddr) : NULL;
>>>>> +    args->pmdp = args->pudp ?
>>>>> +             pmd_alloc(args->mm, args->pudp, args->vaddr) : NULL;
>>>>> +    args->ptep = args->pmdp ?
>>>>> +             pte_alloc_map(args->mm, args->pmdp, args->vaddr) : NULL;
>>>>> +    if (!args->ptep) {
>>>>> +        pr_err("Failed to allocate page table\n");
>>>>> +        ret = -ENOMEM;
>>>>> +        goto error;
>>>>> +    }
>>>>
>>>> Why not just assert that all page table level pointers are allocated
>>>> successfully, otherwise bail out the test completely. Something like
>>>> this at each level.
>>>>
>>>>      if (!args->p4dp) {
>>>>          pr_err("Failed to allocate page table\n");
>>>>          ret = -ENOMEM;
>>>>          goto error;
>>>>      }
>>>>
>>>> Is there any value in proceeding with the test when some page table
>>>> pointers have not been allocated. Also individual tests do not cross
>>>> check these pointers. Also asserting successful allocations will
>>>> make the freeing path simpler, as I had mentioned earlier.
>>>>
>>>
>>> There is no tests will be carried out if we fail to allocate any level
>>> of page table entries. For other questions, please refer below response.
>>> In summary, this snippet needs to be combined with next snippet, as below.
>>>
>>>>> +
>>>>> +    /*
>>>>> +     * The above page table entries will be modified. Lets save the
>>>>> +     * page table entries so that they can be released when the tests
>>>>> +     * are completed.
>>>>> +     */
>>>>> +    args->start_p4dp = p4d_offset(args->pgdp, 0UL);
>>>>> +    args->start_pudp = pud_offset(args->p4dp, 0UL);
>>>>> +    args->start_pmdp = pmd_offset(args->pudp, 0UL);
>>>>> +    args->start_ptep = pmd_pgtable(READ_ONCE(*(args->pmdp)));
>>>>
>>>> If the above page table pointers have been validated to be allocated
>>>> successfully, we could add these here.
>>>>
>>>>      WARN_ON(!args->start_p4dp)
>>>>      WARN_ON(!args->start_pudp)
>>>>      WARN_ON(!args->start_pmdp)
>>>>      WARN_ON(!args->start_ptep)
>>>>
>>>> Afterwards all those if (args->start_pxdp) checks in the freeing path
>>>> will not be required anymore.
>>>>
>>>
>>> The check on @args->start_pxdp is still needed in destroy_args() for
>>> couple of cases: (1) destroy_args() is called on failing to allocate
>>> @args->mm or @args->vma. That time, no page table entries are allocated.
>>> (2) It's possible to fail allocating current level of page table entries
>>> even the previous levels of page table entries are allocated successfully.
>>
>> This makes sense as destroy_args() is getting called if any of these
>> allocations fails during init_args(). Did not realize that earlier.
>>
>>>
>>> So Lets change these (above) two snippets as below in v4:
>>>
>>>      /*
>>>       * Allocate page table entries. The allocated page table entries
>>>       * 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);
>>>      if (!args->p4dp) {
>>>          pr_err("Failed to allocate p4d entries\n");
>>>          ret = -ENOMEM;
>>>          goto error;
>>>      }
>>>
>>>      args->start_p4dp = p4d_offset(args->pgdp, 0UL);
>>
>> Dont bring the arg->start_pxdp assignments here. If all page table level
>> pointer allocations succeed, they all get assigned together like we have
>> right now. Although a sanity check afterwards like the following, might
>> still be better.
>>
>> WARN_ON(!args->start_p4dp)
>> WARN_ON(!args->start_pudp)
>> WARN_ON(!args->start_pmdp)
>> WARN_ON(!args->start_ptep)
>>
> 
> We have to assign arg->start_pxdp here because destroy_args() relies
> it to release the corresponding page tables in failing path. For example,
> the args->start_p4dp is going to be release if we fail to populate
> args->start_pudp.

Okay.

> 
> Ok. I will add WARN_ON() for each level of page table entries right after
> they are assigned in v4.
> 
>>>      args->pudp = pud_alloc(args->mm, args->p4dp, args->vaddr);
>>>      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);
>>>      if (!args->pmdp) {
>>>          pr_err("Failed to allocate PMD entries\n");
>>>          ret = -ENOMEM;
>>>          goto error;
>>>      }
>>>
>>>      args->start_pmdp = pmd_offset(args->pudp, 0UL);
>>>      args->ptep = pte_alloc_map(args->mm, args->pmdp, args->vaddr);
>>>      if (!args->ptep) {
>>>          pr_err("Failed to allocate page table\n");
>>>          ret = -ENOMEM;
>>>          goto error;
>>>      }
>>>
>>>      args->start_ptep = pmd_pgtable(READ_ONCE(*(args->pmdp)));
>>>
>>>>> +
>>>>> +    /*
>>>>> +     * Figure out the fixed addresses, which are all around the kernel
>>>>> +     * symbol (@start_kernel). The corresponding PFNs might be invalid,
>>>>> +     * but it's fine as the following tests won't access the pages.
>>>>> +     */
>>>>> +    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);
>>>>> +
>>>>> +    /*
>>>>> +     * 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 = alloc_pages(GFP_KERNEL | __GFP_NOWARN,
>>>>> +                   HPAGE_PUD_SHIFT - PAGE_SHIFT);
>>>>
>>>> Please drop __GFP_NOWARN and instead use something like alloc_contig_pages()
>>>> when required allocation order exceed (MAX_ORDER - 1). Else the test might
>>>> not be able to execute on platform configurations, where PUD THP is enabled.
>>>>
>>>
>>> Yes, It's correct that alloc_contig_pages() should be used here, depending
>>> on CONFIG_CONTIG_ALLOC. Otherwise, alloc_pages(...__GFP_NOWARN...) is still
>>> used as we're doing. This snippet will be changed like below in v4:
>>
>> First 'order > (MAX_ORDER - 1)' needs to be established before calling into
>> alloc_contig_pages() without __GFP_NOWARN and set a new flag indicating that
>> there is contig page allocated. But if 'order <= (MAX_ORDER - 1)', then call
>> alloc_pages(..) without  __GFP_NOWARN. There is no need to add  __GFP_NOWARN
>> in any case. In case CONFIG_CONTIG_ALLOC is not available, directly return a
>> NULL as that would have been the case with alloc_pages(...__GFP_NOWARN...) as
>> well.
>>
>> Symbol alloc_contig_pages() is not available outside CONFIG_CONTIG_ALLOC. So
>> IS_ENABLED() construct will not work, unless there is an empty stub added in
>> the header. Otherwise #ifdef CONFIG_CONTIG_ALLOC needs to be used instead.
>>
>> Regardless please do test this on a x86 platform with PUD based THP in order
>> to make sure every thing works as expected.
>>
> 
> Thanks, I will change the code accordingly in v4 and test it on x86
> before posting it.
> 
>>>
>>>      /*
>>>       * 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) &&
>>>          IS_ENABLED(CONFIG_CONTIG_ALLOC)) &&
>>>          has_transparent_hugepage()) {
>>>          page = alloc_contig_pages((1 << (HPAGE_PUD_SHIFT - PAGE_SHIFT)),
>>>                        GFP_KERNEL | __GFP_NOWARN,
>>>                        first_online_node, NULL);
>>>          if (page) {
>>>              args->is_contiguous_pud_page = true;
>>>              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) &&
>>>          IS_ENABLED(CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD) &&
>>>          has_transparent_hugepage()) {
>>>          page = alloc_pages(GFP_KERNEL | __GFP_NOWARN,
>>>                     HPAGE_PUD_SHIFT - PAGE_SHIFT);
>>>          if (page) {
>>>              args->is_contiguous_pud_page = false;
>>>              args->pud_pfn = page_to_pfn(page);
>>>              args->pmd_pfn = args->pud_pfn;
>>>              args->pte_pfn = args->pud_pfn;
>>>              return 0;
>>>          }
>>>      }
>>>
>>>      [... The logic to allocate PMD huge page or page is kept as of being]
>>
>> IIRC it is also not guaranteed that PMD_SHIFT <= (MAX_ORDER - 1). Hence
>> this same scheme should be followed for PMD level allocation as well.
>>
> 
> In theory, it's possible to have PMD_SHIFT <= (MAX_ORDER - 1) with misconfigured
> kernel. I will apply the similar logic to PMD huge page in v4.
> 
>>>      [... The code to release the PUD huge page needs changes based on @args->is_contiguous_pud_page]
>>
>> Right, a flag would be needed to call the appropriate free function.
>>
> 
> Yes. We need two falgs for PUD and PMD huge pages separately.

A single flag should be enough, the order would be dependent on
whether args->pud_pfn or args->pmd_pfn is valid.