Received-SPF: pass (google.com: domain of linux-kernel+bounces-177709-linux.lists.archive=gmail.com@vger.kernel.org designates 139.178.88.99 as permitted sender) client-ip=139.178.88.99;
Message-ID: <a5eecb04-c73c-41cc-b989-80bfc54b4c58@arm.com>
Date: Mon, 13 May 2024 15:52:15 +0100
Precedence: bulk
MIME-Version: 1.0
User-Agent: Mozilla Thunderbird
Subject: Re: [PATCH RESEND v2 1/9] riscv: Restore the pfn in a NAPOT pte when
 manipulated by core mm code
Content-Language: en-GB
To: Alexandre Ghiti <alexghiti@rivosinc.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>, Will Deacon <will@kernel.org>,
 Mark Rutland <mark.rutland@arm.com>, Paul Walmsley
 <paul.walmsley@sifive.com>, Palmer Dabbelt <palmer@dabbelt.com>,
 Albert Ou <aou@eecs.berkeley.edu>, Andrew Morton
 <akpm@linux-foundation.org>, linux-arm-kernel@lists.infradead.org,
 linux-kernel@vger.kernel.org, linux-riscv@lists.infradead.org,
 linux-mm@kvack.org
References: <20240508113419.18620-1-alexghiti@rivosinc.com>
 <20240508113419.18620-2-alexghiti@rivosinc.com>
 <8aa0a606-f47c-4826-96ad-121ac1f7eb96@arm.com>
 <CAHVXubjTbDd6t5NxbvsLTMtHFDd0ZL2aJFPtcjJe=jZDtWjP6A@mail.gmail.com>
From: Ryan Roberts <ryan.roberts@arm.com>
In-Reply-To: <CAHVXubjTbDd6t5NxbvsLTMtHFDd0ZL2aJFPtcjJe=jZDtWjP6A@mail.gmail.com>
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

On 13/05/2024 14:06, Alexandre Ghiti wrote:
> Hi Ryan,
> 
> On Fri, May 10, 2024 at 2:20 PM Ryan Roberts <ryan.roberts@arm.com> wrote:
>>
>> On 08/05/2024 12:34, Alexandre Ghiti wrote:
>>> The core mm code expects to be able to extract the pfn from a pte. NAPOT
>>> mappings work differently since its ptes actually point to the first pfn
>>> of the mapping, the other bits being used to encode the size of the
>>> mapping.
>>>
>>> So modify ptep_get() so that it returns a pte value that contains the
>>> *real* pfn (which is then different from what the HW expects) and right
>>> before storing the ptes to the page table, reset the pfn LSBs to the
>>> size of the mapping.
>>
>> Did you consider leaving the pte as is and instead modifying your pte_pfn()
>> implementation?
>>
>> For arm64 at least, it is beneficial to keep the pte marked as contiguous when
>> passing it up to core-mm because there are other helpers which need to parse the
>> contiguous bit (e.g. pte_leaf_size()). If we were to clear the cont bit in
>> ptep_get() that info would be lost and perf_get_pgtable_size() would always
>> conclude the leaf size is 4K even when it is actually 64K.
> 
> I don't clear the contpte bit here (ie the napot bit), I'm just
> setting the right pfn so that the core-mm code knows exactly which
> page is targeted by each pte of a contpte region (remember riscv napot
> extension uses the lsb of the pte pfn to encode the size the mapping,
> so all ptes of a contpte region will return the same pfn).
> 
> And from pte_pfn(), we have no way of knowing from the pte value alone
> which page is targeted, we need to know its position in the page table
> to "guess" the right pfn.

Ahh yes - good point!

> 
>>
>>>
>>> And make sure that all NAPOT mappings are set using set_ptes().
>>>
>>> Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
>>> ---
>>>  arch/riscv/include/asm/pgtable-64.h |  11 +++
>>>  arch/riscv/include/asm/pgtable.h    | 105 ++++++++++++++++++++++++++--
>>>  arch/riscv/mm/hugetlbpage.c         |  38 +++++-----
>>>  3 files changed, 128 insertions(+), 26 deletions(-)
>>>
>>> diff --git a/arch/riscv/include/asm/pgtable-64.h b/arch/riscv/include/asm/pgtable-64.h
>>> index 221a5c1ee287..9fe076fc503e 100644
>>> --- a/arch/riscv/include/asm/pgtable-64.h
>>> +++ b/arch/riscv/include/asm/pgtable-64.h
>>> @@ -106,6 +106,17 @@ enum napot_cont_order {
>>>  #define napot_cont_mask(order)       (~(napot_cont_size(order) - 1UL))
>>>  #define napot_pte_num(order) BIT(order)
>>>
>>> +static inline bool is_napot_order(unsigned int order)
>>> +{
>>> +     unsigned int napot_order;
>>> +
>>> +     for_each_napot_order(napot_order)
>>> +             if (order == napot_order)
>>> +                     return true;
>>> +
>>> +     return false;
>>> +}
>>> +
>>>  #ifdef CONFIG_RISCV_ISA_SVNAPOT
>>>  #define HUGE_MAX_HSTATE              (2 + (NAPOT_ORDER_MAX - NAPOT_CONT_ORDER_BASE))
>>>  #else
>>> diff --git a/arch/riscv/include/asm/pgtable.h b/arch/riscv/include/asm/pgtable.h
>>> index 9f8ea0e33eb1..268c828f5152 100644
>>> --- a/arch/riscv/include/asm/pgtable.h
>>> +++ b/arch/riscv/include/asm/pgtable.h
>>> @@ -297,6 +297,8 @@ static inline unsigned long pte_napot(pte_t pte)
>>>       return pte_val(pte) & _PAGE_NAPOT;
>>>  }
>>>
>>> +#define pte_valid_napot(pte) (pte_present(pte) && pte_napot(pte))
>>> +
>>>  static inline pte_t pte_mknapot(pte_t pte, unsigned int order)
>>>  {
>>>       int pos = order - 1 + _PAGE_PFN_SHIFT;
>>> @@ -306,6 +308,12 @@ static inline pte_t pte_mknapot(pte_t pte, unsigned int order)
>>>       return __pte((pte_val(pte) & napot_mask) | napot_bit | _PAGE_NAPOT);
>>>  }
>>>
>>> +/* pte at entry must *not* encode the mapping size in the pfn LSBs. */
>>> +static inline pte_t pte_clear_napot(pte_t pte)
>>> +{
>>> +     return __pte(pte_val(pte) & ~_PAGE_NAPOT);
>>> +}
>>> +
>>>  #else
>>>
>>>  static __always_inline bool has_svnapot(void) { return false; }
>>> @@ -315,17 +323,14 @@ static inline unsigned long pte_napot(pte_t pte)
>>>       return 0;
>>>  }
>>>
>>> +#define pte_valid_napot(pte) false
>>> +
>>>  #endif /* CONFIG_RISCV_ISA_SVNAPOT */
>>>
>>>  /* Yields the page frame number (PFN) of a page table entry */
>>>  static inline unsigned long pte_pfn(pte_t pte)
>>>  {
>>> -     unsigned long res  = __page_val_to_pfn(pte_val(pte));
>>> -
>>> -     if (has_svnapot() && pte_napot(pte))
>>> -             res = res & (res - 1UL);
>>> -
>>> -     return res;
>>> +     return __page_val_to_pfn(pte_val(pte));
>>>  }
>>>
>>>  #define pte_page(x)     pfn_to_page(pte_pfn(x))
>>> @@ -525,9 +530,91 @@ static inline void __set_pte_at(struct mm_struct *mm, pte_t *ptep, pte_t pteval)
>>>
>>>  #define PFN_PTE_SHIFT                _PAGE_PFN_SHIFT
>>>
>>> +#ifdef CONFIG_RISCV_ISA_SVNAPOT
>>> +static inline int arch_contpte_get_num_contig(pte_t *ptep, unsigned long size,
>>> +                                           size_t *pgsize)
>>> +{
>>> +     pte_t __pte;
>>> +
>>> +     /* We must read the raw value of the pte to get the size of the mapping */
>>> +     __pte = READ_ONCE(*ptep);
>>> +
>>> +     if (pgsize) {
>>> +             if (size >= PGDIR_SIZE)
>>> +                     *pgsize = PGDIR_SIZE;
>>> +             else if (size >= P4D_SIZE)
>>> +                     *pgsize = P4D_SIZE;
>>> +             else if (size >= PUD_SIZE)
>>> +                     *pgsize = PUD_SIZE;
>>> +             else if (size >= PMD_SIZE)
>>> +                     *pgsize = PMD_SIZE;
>>> +             else
>>> +                     *pgsize = PAGE_SIZE;
>>> +     }
>>> +
>>> +     /* Make sure __pte is not a swap entry */
>>> +     if (pte_valid_napot(__pte))
>>> +             return napot_pte_num(napot_cont_order(__pte));
>>> +
>>> +     return 1;
>>> +}
>>> +#endif
>>> +
>>> +static inline pte_t ptep_get(pte_t *ptep)
>>> +{
>>> +     pte_t pte = READ_ONCE(*ptep);
>>> +
>>> +#ifdef CONFIG_RISCV_ISA_SVNAPOT
>>> +     /*
>>> +      * The pte we load has the N bit set and the size of the mapping in
>>> +      * the pfn LSBs: keep the N bit and replace the mapping size with
>>> +      * the *real* pfn since the core mm code expects to find it there.
>>> +      * The mapping size will be reset just before being written to the
>>> +      * page table in set_ptes().
>>> +      */
>>> +     if (unlikely(pte_valid_napot(pte))) {
>>> +             unsigned int order = napot_cont_order(pte);
>>> +             int pos = order - 1 + _PAGE_PFN_SHIFT;
>>> +             unsigned long napot_mask = ~GENMASK(pos, _PAGE_PFN_SHIFT);
>>> +             pte_t *orig_ptep = PTR_ALIGN_DOWN(ptep, sizeof(*ptep) * napot_pte_num(order));
>>> +
>>> +             pte = __pte((pte_val(pte) & napot_mask) + ((ptep - orig_ptep) << _PAGE_PFN_SHIFT));
>>> +     }
>>> +#endif
>>> +
>>> +     return pte;
>>> +}
>>> +#define ptep_get     ptep_get
>>> +
>>>  static inline void set_ptes(struct mm_struct *mm, unsigned long addr,
>>>               pte_t *ptep, pte_t pteval, unsigned int nr)
>>>  {
>>> +#ifdef CONFIG_RISCV_ISA_SVNAPOT
>>> +     if (unlikely(pte_valid_napot(pteval))) {
>>> +             unsigned int order = ilog2(nr);
>>> +
>>> +             if (!is_napot_order(order)) {
>>> +                     /*
>>> +                      * Something's weird, we are given a NAPOT pte but the
>>> +                      * size of the mapping is not a known NAPOT mapping
>>> +                      * size, so clear the NAPOT bit and map this without
>>> +                      * NAPOT support: core mm only manipulates pte with the
>>> +                      * real pfn so we know the pte is valid without the N
>>> +                      * bit.
>>> +                      */
>>> +                     pr_err("Incorrect NAPOT mapping, resetting.\n");
>>> +                     pteval = pte_clear_napot(pteval);
>>> +             } else {
>>> +                     /*
>>> +                      * NAPOT ptes that arrive here only have the N bit set
>>> +                      * and their pfn does not contain the mapping size, so
>>> +                      * set that here.
>>> +                      */
>>> +                     pteval = pte_mknapot(pteval, order);
>>> +             }
>>> +     }
>>> +#endif
>>
>> I think all this complexity comes along due to using this function both as a
>> public interface that the core-mm uses (which never sets napot)
>> and also using
>> it as an internal interface that riscv-hugetlb uses (which does set napot)? It
>> might be more understandable if you layer it into a lower level/internal API and
>> a higher level/public API (similar to arm64)?
> 
> I think you're right here, I'll try to do that too.
> 
> Thanks for your comments,
> 
> Alex
> 
>>
>>> +
>>>       page_table_check_ptes_set(mm, ptep, pteval, nr);
>>>
>>>       for (;;) {
>>> @@ -535,6 +622,12 @@ static inline void set_ptes(struct mm_struct *mm, unsigned long addr,
>>>               if (--nr == 0)
>>>                       break;
>>>               ptep++;
>>> +
>>> +#ifdef CONFIG_RISCV_ISA_SVNAPOT
>>> +             if (unlikely(pte_valid_napot(pteval)))
>>> +                     continue;
>>> +#endif
>>> +
>>>               pte_val(pteval) += 1 << _PAGE_PFN_SHIFT;
>>>       }
>>>  }
>>> diff --git a/arch/riscv/mm/hugetlbpage.c b/arch/riscv/mm/hugetlbpage.c
>>> index 5ef2a6891158..fe8067ee71b4 100644
>>> --- a/arch/riscv/mm/hugetlbpage.c
>>> +++ b/arch/riscv/mm/hugetlbpage.c
>>> @@ -256,8 +256,7 @@ void set_huge_pte_at(struct mm_struct *mm,
>>>
>>>       clear_flush(mm, addr, ptep, pgsize, pte_num);
>>>
>>> -     for (i = 0; i < pte_num; i++, ptep++, addr += pgsize)
>>> -             set_pte_at(mm, addr, ptep, pte);
>>> +     set_ptes(mm, addr, ptep, pte, pte_num);
>>>  }
>>>
>>>  int huge_ptep_set_access_flags(struct vm_area_struct *vma,
>>> @@ -267,16 +266,16 @@ int huge_ptep_set_access_flags(struct vm_area_struct *vma,
>>>                              int dirty)
>>>  {
>>>       struct mm_struct *mm = vma->vm_mm;
>>> -     unsigned long order;
>>> +     size_t pgsize;
>>>       pte_t orig_pte;
>>> -     int i, pte_num;
>>> +     int pte_num;
>>>
>>>       if (!pte_napot(pte))
>>>               return ptep_set_access_flags(vma, addr, ptep, pte, dirty);
>>>
>>> -     order = napot_cont_order(pte);
>>> -     pte_num = napot_pte_num(order);
>>> -     ptep = huge_pte_offset(mm, addr, napot_cont_size(order));
>>> +     pte_num = arch_contpte_get_num_contig(ptep, 0, &pgsize);
>>> +     ptep = huge_pte_offset(mm, addr, pte_num * pgsize);
>>> +
>>>       orig_pte = get_clear_contig_flush(mm, addr, ptep, pte_num);
>>>
>>>       if (pte_dirty(orig_pte))
>>> @@ -285,8 +284,7 @@ int huge_ptep_set_access_flags(struct vm_area_struct *vma,
>>>       if (pte_young(orig_pte))
>>>               pte = pte_mkyoung(pte);
>>>
>>> -     for (i = 0; i < pte_num; i++, addr += PAGE_SIZE, ptep++)
>>> -             set_pte_at(mm, addr, ptep, pte);
>>> +     set_ptes(mm, addr, ptep, pte, pte_num);
>>>
>>>       return true;
>>>  }
>>> @@ -301,7 +299,7 @@ pte_t huge_ptep_get_and_clear(struct mm_struct *mm,
>>>       if (!pte_napot(orig_pte))
>>>               return ptep_get_and_clear(mm, addr, ptep);
>>>
>>> -     pte_num = napot_pte_num(napot_cont_order(orig_pte));
>>> +     pte_num = arch_contpte_get_num_contig(ptep, 0, NULL);
>>>
>>>       return get_clear_contig(mm, addr, ptep, pte_num);
>>>  }
>>> @@ -311,24 +309,23 @@ void huge_ptep_set_wrprotect(struct mm_struct *mm,
>>>                            pte_t *ptep)
>>>  {
>>>       pte_t pte = ptep_get(ptep);
>>> -     unsigned long order;
>>> +     size_t pgsize;
>>>       pte_t orig_pte;
>>> -     int i, pte_num;
>>> +     int pte_num;
>>>
>>>       if (!pte_napot(pte)) {
>>>               ptep_set_wrprotect(mm, addr, ptep);
>>>               return;
>>>       }
>>>
>>> -     order = napot_cont_order(pte);
>>> -     pte_num = napot_pte_num(order);
>>> -     ptep = huge_pte_offset(mm, addr, napot_cont_size(order));
>>> +     pte_num = arch_contpte_get_num_contig(ptep, 0, &pgsize);
>>> +     ptep = huge_pte_offset(mm, addr, pte_num * pgsize);
>>> +
>>>       orig_pte = get_clear_contig_flush(mm, addr, ptep, pte_num);
>>>
>>>       orig_pte = pte_wrprotect(orig_pte);
>>>
>>> -     for (i = 0; i < pte_num; i++, addr += PAGE_SIZE, ptep++)
>>> -             set_pte_at(mm, addr, ptep, orig_pte);
>>> +     set_ptes(mm, addr, ptep, orig_pte, pte_num);
>>>  }
>>>
>>>  pte_t huge_ptep_clear_flush(struct vm_area_struct *vma,
>>> @@ -341,7 +338,7 @@ pte_t huge_ptep_clear_flush(struct vm_area_struct *vma,
>>>       if (!pte_napot(pte))
>>>               return ptep_clear_flush(vma, addr, ptep);
>>>
>>> -     pte_num = napot_pte_num(napot_cont_order(pte));
>>> +     pte_num = arch_contpte_get_num_contig(ptep, 0, NULL);
>>>
>>>       return get_clear_contig_flush(vma->vm_mm, addr, ptep, pte_num);
>>>  }
>>> @@ -351,6 +348,7 @@ void huge_pte_clear(struct mm_struct *mm,
>>>                   pte_t *ptep,
>>>                   unsigned long sz)
>>>  {
>>> +     size_t pgsize;
>>>       pte_t pte = ptep_get(ptep);
>>>       int i, pte_num;
>>>
>>> @@ -359,8 +357,8 @@ void huge_pte_clear(struct mm_struct *mm,
>>>               return;
>>>       }
>>>
>>> -     pte_num = napot_pte_num(napot_cont_order(pte));
>>> -     for (i = 0; i < pte_num; i++, addr += PAGE_SIZE, ptep++)
>>> +     pte_num = arch_contpte_get_num_contig(ptep, 0, &pgsize);
>>> +     for (i = 0; i < pte_num; i++, addr += pgsize, ptep++)
>>>               pte_clear(mm, addr, ptep);
>>>  }
>>>
>>