Received-SPF: pass (google.com: domain of linux-kernel+bounces-140681-linux.lists.archive=gmail.com@vger.kernel.org designates 2604:1380:4601:e00::3 as permitted sender) client-ip=2604:1380:4601:e00::3;
Message-ID: <30c6934c-d7a1-4ed2-91a9-34b282a0b839@redhat.com>
Date: Thu, 11 Apr 2024 16:42:35 +0200
Precedence: bulk
MIME-Version: 1.0
User-Agent: Mozilla Thunderbird
Subject: Re: [PATCH v5 1/2] mm/madvise: optimize lazyfreeing with mTHP in
 madvise_free
To: Ryan Roberts <ryan.roberts@arm.com>, Lance Yang <ioworker0@gmail.com>
Cc: akpm@linux-foundation.org, 21cnbao@gmail.com, mhocko@suse.com,
 fengwei.yin@intel.com, zokeefe@google.com, shy828301@gmail.com,
 xiehuan09@gmail.com, wangkefeng.wang@huawei.com, songmuchun@bytedance.com,
 peterx@redhat.com, minchan@kernel.org, linux-mm@kvack.org,
 linux-kernel@vger.kernel.org
References: <20240408042437.10951-1-ioworker0@gmail.com>
 <20240408042437.10951-2-ioworker0@gmail.com>
 <38c4add8-53a2-49ca-9f1b-f62c2ee3e764@arm.com>
 <CAK1f24kh-vN3_6ZLUZWXu_g1UOPBuoVXq2jsnx_GBbGFXyXRFQ@mail.gmail.com>
 <3cda8e87-7095-4aad-beb1-6a420912df34@arm.com>
 <CAK1f24k6mhQZwws7fjvL0ynme4FtjqBM3T6ZYuFPytH0fG=v6w@mail.gmail.com>
 <8d674b15-ef74-4d96-bc27-8794f744517c@arm.com>
From: David Hildenbrand <david@redhat.com>
Content-Language: en-US
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: <8d674b15-ef74-4d96-bc27-8794f744517c@arm.com>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 8bit

On 11.04.24 16:39, Ryan Roberts wrote:
> On 11/04/2024 15:07, Lance Yang wrote:
>> On Thu, Apr 11, 2024 at 9:48 PM Ryan Roberts <ryan.roberts@arm.com> wrote:
>>>
>>> [...]
>>>
>>>>>> +
>>>>>> +             if (!folio_trylock(folio))
>>>>>> +                     continue;
>>>>>
>>>>> This is still wrong. This should all be protected by the "if
>>>>> (folio_test_swapcache(folio) || folio_test_dirty(folio))" as it was previously
>>>>> so that you only call folio_trylock() if that condition is true. You are
>>>>> unconditionally locking here, then unlocking, then relocking below if the
>>>>> condition is met. Just put everything inside the condition and lock once.
>>>>
>>>> I'm not sure if it's safe to call folio_mapcount() without holding the
>>>> folio lock.
>>>>
>>>> As mentioned earlier by David in the v2[1]
>>>>> What could work for large folios is making sure that #ptes that map the
>>>>> folio here correspond to the folio_mapcount(). And folio_mapcount()
>>>>> should be called under folio lock, to avoid racing with swapout/migration.
>>>>
>>>> [1] https://lore.kernel.org/all/5cc05529-eb80-410e-bc26-233b0ba0b21f@redhat.com/
>>>
>>> But I'm not suggesting that you should call folio_mapcount() without the lock.
>>> I'm proposing this:
>>>
>>>                  if (folio_test_swapcache(folio) || folio_test_dirty(folio)) {
>>>                          if (!folio_trylock(folio))
>>>                                  continue;
>>>                          /*
>>> -                        * If folio is shared with others, we mustn't clear
>>> -                        * the folio's dirty flag.
>>> +                        * If we have a large folio at this point, we know it is
>>> +                        * fully mapped so if its mapcount is the same as its
>>> +                        * number of pages, it must be exclusive.
>>>                           */
>>> -                       if (folio_mapcount(folio) != 1) {
>>> +                       if (folio_mapcount(folio) != folio_nr_pages(folio)) {
>>>                                  folio_unlock(folio);
>>>                                  continue;
>>>                          }
>>
>> IIUC, if the folio is clean and not in the swapcache, we still need to
>> compare the number of batched PTEs against folio_mapcount().
> 
> Why? That's not how the old code worked. In fact the comment says that the
> reason for the exclusive check is to avoid marking a dirty *folio* as clean if
> shared; that would be bad because we could throw away data that others relied
> upon. It's perfectly safe to clear the dirty flag from the *pte* even if it is
> shared; the ptes are private to the process so that won't affect sharers.
> 
> You should just follow the pattern already estabilished by the original code.
> The only difference is that because the folio is now (potentially) large, you
> have to change the way to detect exclusivity.

+1

-- 
Cheers,

David / dhildenb