Received-SPF: pass (google.com: domain of linux-kernel+bounces-82933-linux.lists.archive=gmail.com@vger.kernel.org designates 139.178.88.99 as permitted sender) client-ip=139.178.88.99;
Message-ID: <c95215b2-6ec5-4efb-a73b-7be92cda1c83@redhat.com>
Date: Tue, 27 Feb 2024 10:14:43 +0100
Precedence: bulk
MIME-Version: 1.0
User-Agent: Mozilla Thunderbird
Subject: Re: [PATCH] mm: export folio_pte_batch as a couple of modules might
 need it
Content-Language: en-US
To: Ryan Roberts <ryan.roberts@arm.com>, Barry Song <21cnbao@gmail.com>,
 akpm@linux-foundation.org, linux-mm@kvack.org
Cc: linux-kernel@vger.kernel.org, Barry Song <v-songbaohua@oppo.com>,
 Lance Yang <ioworker0@gmail.com>, Yin Fengwei <fengwei.yin@intel.com>
References: <20240227024050.244567-1-21cnbao@gmail.com>
 <61b9dfc9-5522-44fd-89a4-140833ede8af@arm.com>
From: David Hildenbrand <david@redhat.com>
Autocrypt: addr=david@redhat.com; keydata=
 xsFNBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ
 dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL
 QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp
 XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK
 Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9
 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt
 WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc
 UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv
 jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb
 B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzSREYXZpZCBIaWxk
 ZW5icmFuZCA8ZGF2aWRAcmVkaGF0LmNvbT7CwZgEEwEIAEICGwMGCwkIBwMCBhUIAgkKCwQW
 AgMBAh4BAheAAhkBFiEEG9nKrXNcTDpGDfzKTd4Q9wD/g1oFAl8Ox4kFCRKpKXgACgkQTd4Q
 9wD/g1oHcA//a6Tj7SBNjFNM1iNhWUo1lxAja0lpSodSnB2g4FCZ4R61SBR4l/psBL73xktp
 rDHrx4aSpwkRP6Epu6mLvhlfjmkRG4OynJ5HG1gfv7RJJfnUdUM1z5kdS8JBrOhMJS2c/gPf
 wv1TGRq2XdMPnfY2o0CxRqpcLkx4vBODvJGl2mQyJF/gPepdDfcT8/PY9BJ7FL6Hrq1gnAo4
 3Iv9qV0JiT2wmZciNyYQhmA1V6dyTRiQ4YAc31zOo2IM+xisPzeSHgw3ONY/XhYvfZ9r7W1l
 pNQdc2G+o4Di9NPFHQQhDw3YTRR1opJaTlRDzxYxzU6ZnUUBghxt9cwUWTpfCktkMZiPSDGd
 KgQBjnweV2jw9UOTxjb4LXqDjmSNkjDdQUOU69jGMUXgihvo4zhYcMX8F5gWdRtMR7DzW/YE
 BgVcyxNkMIXoY1aYj6npHYiNQesQlqjU6azjbH70/SXKM5tNRplgW8TNprMDuntdvV9wNkFs
 9TyM02V5aWxFfI42+aivc4KEw69SE9KXwC7FSf5wXzuTot97N9Phj/Z3+jx443jo2NR34XgF
 89cct7wJMjOF7bBefo0fPPZQuIma0Zym71cP61OP/i11ahNye6HGKfxGCOcs5wW9kRQEk8P9
 M/k2wt3mt/fCQnuP/mWutNPt95w9wSsUyATLmtNrwccz63XOwU0EVcufkQEQAOfX3n0g0fZz
 Bgm/S2zF/kxQKCEKP8ID+Vz8sy2GpDvveBq4H2Y34XWsT1zLJdvqPI4af4ZSMxuerWjXbVWb
 T6d4odQIG0fKx4F8NccDqbgHeZRNajXeeJ3R7gAzvWvQNLz4piHrO/B4tf8svmRBL0ZB5P5A
 2uhdwLU3NZuK22zpNn4is87BPWF8HhY0L5fafgDMOqnf4guJVJPYNPhUFzXUbPqOKOkL8ojk
 CXxkOFHAbjstSK5Ca3fKquY3rdX3DNo+EL7FvAiw1mUtS+5GeYE+RMnDCsVFm/C7kY8c2d0G
 NWkB9pJM5+mnIoFNxy7YBcldYATVeOHoY4LyaUWNnAvFYWp08dHWfZo9WCiJMuTfgtH9tc75
 7QanMVdPt6fDK8UUXIBLQ2TWr/sQKE9xtFuEmoQGlE1l6bGaDnnMLcYu+Asp3kDT0w4zYGsx
 5r6XQVRH4+5N6eHZiaeYtFOujp5n+pjBaQK7wUUjDilPQ5QMzIuCL4YjVoylWiBNknvQWBXS
 lQCWmavOT9sttGQXdPCC5ynI+1ymZC1ORZKANLnRAb0NH/UCzcsstw2TAkFnMEbo9Zu9w7Kv
 AxBQXWeXhJI9XQssfrf4Gusdqx8nPEpfOqCtbbwJMATbHyqLt7/oz/5deGuwxgb65pWIzufa
 N7eop7uh+6bezi+rugUI+w6DABEBAAHCwXwEGAEIACYCGwwWIQQb2cqtc1xMOkYN/MpN3hD3
 AP+DWgUCXw7HsgUJEqkpoQAKCRBN3hD3AP+DWrrpD/4qS3dyVRxDcDHIlmguXjC1Q5tZTwNB
 boaBTPHSy/Nksu0eY7x6HfQJ3xajVH32Ms6t1trDQmPx2iP5+7iDsb7OKAb5eOS8h+BEBDeq
 3ecsQDv0fFJOA9ag5O3LLNk+3x3q7e0uo06XMaY7UHS341ozXUUI7wC7iKfoUTv03iO9El5f
 XpNMx/YrIMduZ2+nd9Di7o5+KIwlb2mAB9sTNHdMrXesX8eBL6T9b+MZJk+mZuPxKNVfEQMQ
 a5SxUEADIPQTPNvBewdeI80yeOCrN+Zzwy/Mrx9EPeu59Y5vSJOx/z6OUImD/GhX7Xvkt3kq
 Er5KTrJz3++B6SH9pum9PuoE/k+nntJkNMmQpR4MCBaV/J9gIOPGodDKnjdng+mXliF3Ptu6
 3oxc2RCyGzTlxyMwuc2U5Q7KtUNTdDe8T0uE+9b8BLMVQDDfJjqY0VVqSUwImzTDLX9S4g/8
 kC4HRcclk8hpyhY2jKGluZO0awwTIMgVEzmTyBphDg/Gx7dZU1Xf8HFuE+UZ5UDHDTnwgv7E
 th6RC9+WrhDNspZ9fJjKWRbveQgUFCpe1sa77LAw+XFrKmBHXp9ZVIe90RMe2tRL06BGiRZr
 jPrnvUsUUsjRoRNJjKKA/REq+sAnhkNPPZ/NNMjaZ5b8Tovi8C0tmxiCHaQYqj7G2rgnT0kt
 WNyWQQ==
Organization: Red Hat
In-Reply-To: <61b9dfc9-5522-44fd-89a4-140833ede8af@arm.com>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 7bit

On 27.02.24 10:07, Ryan Roberts wrote:
> On 27/02/2024 02:40, Barry Song wrote:
>> From: Barry Song <v-songbaohua@oppo.com>
>>
>> madvise and some others might need folio_pte_batch to check if a range
>> of PTEs are completely mapped to a large folio with contiguous physcial
>> addresses. Let's export it for others to use.
>>
>> Cc: Lance Yang <ioworker0@gmail.com>
>> Cc: Ryan Roberts <ryan.roberts@arm.com>
>> Cc: David Hildenbrand <david@redhat.com>
>> Cc: Yin Fengwei <fengwei.yin@intel.com>
>> Signed-off-by: Barry Song <v-songbaohua@oppo.com>
>> ---
>>   -v1:
>>   at least two jobs madv_free and madv_pageout depend on it. To avoid
>>   conflicts and dependencies, after discussing with Lance, we prefer
>>   this one can land earlier.
> 
> I think this will also ultimately be useful for mprotect too, though I haven't
> looked at it properly yet.
> 

Yes, I think we briefly discussed that.

>>
>>   mm/internal.h | 13 +++++++++++++
>>   mm/memory.c   | 11 +----------
>>   2 files changed, 14 insertions(+), 10 deletions(-)
>>
>> diff --git a/mm/internal.h b/mm/internal.h
>> index 13b59d384845..8e2bc304f671 100644
>> --- a/mm/internal.h
>> +++ b/mm/internal.h
>> @@ -83,6 +83,19 @@ static inline void *folio_raw_mapping(struct folio *folio)
>>   	return (void *)(mapping & ~PAGE_MAPPING_FLAGS);
>>   }
>>   
>> +/* Flags for folio_pte_batch(). */
>> +typedef int __bitwise fpb_t;
>> +
>> +/* Compare PTEs after pte_mkclean(), ignoring the dirty bit. */
>> +#define FPB_IGNORE_DIRTY		((__force fpb_t)BIT(0))
>> +
>> +/* Compare PTEs after pte_clear_soft_dirty(), ignoring the soft-dirty bit. */
>> +#define FPB_IGNORE_SOFT_DIRTY		((__force fpb_t)BIT(1))
>> +
>> +extern int folio_pte_batch(struct folio *folio, unsigned long addr,
>> +		pte_t *start_ptep, pte_t pte, int max_nr, fpb_t flags,
>> +		bool *any_writable);
>> +
>>   void __acct_reclaim_writeback(pg_data_t *pgdat, struct folio *folio,
>>   						int nr_throttled);
>>   static inline void acct_reclaim_writeback(struct folio *folio)
>> diff --git a/mm/memory.c b/mm/memory.c
>> index 1c45b6a42a1b..319b3be05e75 100644
>> --- a/mm/memory.c
>> +++ b/mm/memory.c
>> @@ -953,15 +953,6 @@ static __always_inline void __copy_present_ptes(struct vm_area_struct *dst_vma,
>>   	set_ptes(dst_vma->vm_mm, addr, dst_pte, pte, nr);
>>   }
>>   
>> -/* Flags for folio_pte_batch(). */
>> -typedef int __bitwise fpb_t;
>> -
>> -/* Compare PTEs after pte_mkclean(), ignoring the dirty bit. */
>> -#define FPB_IGNORE_DIRTY		((__force fpb_t)BIT(0))
>> -
>> -/* Compare PTEs after pte_clear_soft_dirty(), ignoring the soft-dirty bit. */
>> -#define FPB_IGNORE_SOFT_DIRTY		((__force fpb_t)BIT(1))
>> -
>>   static inline pte_t __pte_batch_clear_ignored(pte_t pte, fpb_t flags)
>>   {
>>   	if (flags & FPB_IGNORE_DIRTY)
>> @@ -982,7 +973,7 @@ static inline pte_t __pte_batch_clear_ignored(pte_t pte, fpb_t flags)
>>    * If "any_writable" is set, it will indicate if any other PTE besides the
>>    * first (given) PTE is writable.
>>    */
> 
> David was talking in Lance's patch thread, about improving the docs for this
> function now that its exported. Might be worth syncing on that.

Here is my take:

Signed-off-by: David Hildenbrand <david@redhat.com>
---
  mm/memory.c | 22 ++++++++++++++++++----
  1 file changed, 18 insertions(+), 4 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index d0b855a1837a8..098356b8805ae 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -971,16 +971,28 @@ static inline pte_t __pte_batch_clear_ignored(pte_t pte, fpb_t flags)
  	return pte_wrprotect(pte_mkold(pte));
  }
  
-/*
+/**
+ * folio_pte_batch - detect a PTE batch for a large folio
+ * @folio: The large folio to detect a PTE batch for.
+ * @addr: The user virtual address the first page is mapped at.
+ * @start_ptep: Page table pointer for the first entry.
+ * @pte: Page table entry for the first page.
+ * @max_nr: The maximum number of table entries to consider.
+ * @flags: Flags to modify the PTE batch semantics.
+ * @any_writable: Optional pointer to indicate whether any entry except the
+ *		  first one is writable.
+ *
   * Detect a PTE batch: consecutive (present) PTEs that map consecutive
- * pages of the same folio.
+ * pages of the same large folio.
   *
   * All PTEs inside a PTE batch have the same PTE bits set, excluding the PFN,
   * the accessed bit, writable bit, dirty bit (with FPB_IGNORE_DIRTY) and
   * soft-dirty bit (with FPB_IGNORE_SOFT_DIRTY).
   *
- * If "any_writable" is set, it will indicate if any other PTE besides the
- * first (given) PTE is writable.
+ * start_ptep must map any page of the folio. max_nr must be at least one and
+ * must be limited by the caller so scanning cannot exceed a single page table.
+ *
+ * Return: the number of table entries in the batch.
   */
  static inline int folio_pte_batch(struct folio *folio, unsigned long addr,
  		pte_t *start_ptep, pte_t pte, int max_nr, fpb_t flags,
@@ -996,6 +1008,8 @@ static inline int folio_pte_batch(struct folio *folio, unsigned long addr,
  		*any_writable = false;
  
  	VM_WARN_ON_FOLIO(!pte_present(pte), folio);
+	VM_WARN_ON_FOLIO(!folio_test_large(folio) || max_nr < 1, folio);
+	VM_WARN_ON_FOLIO(page_folio(pfn_to_page(pte_pfn(pte))) != folio, folio);
  
  	nr = pte_batch_hint(start_ptep, pte);
  	expected_pte = __pte_batch_clear_ignored(pte_advance_pfn(pte, nr), flags);
-- 
2.43.2


> 
>> -static inline int folio_pte_batch(struct folio *folio, unsigned long addr,
>> +int folio_pte_batch(struct folio *folio, unsigned long addr,
> 
> fork() is very performance sensitive. Is there a risk we are regressing
> performance by making this out-of-line? Although its in the same compilation
> unit so the compiler may well inline it anyway?

Easy to verify by looking at the generated asm I guess?

> 
> Either way, perhaps we are better off making it inline in the header? That would
> avoid needing to rerun David's micro-benchmarks for fork() and munmap().

That way, the compiler can most certainly better optimize it also outside of mm/memory.c

-- 
Cheers,

David / dhildenb