Received-SPF: pass (google.com: domain of linux-kernel+bounces-95420-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: <a07deb2c-49e1-4324-8e70-e897605faa9d@redhat.com>
Date: Thu, 7 Mar 2024 12:31:57 +0100
Precedence: bulk
MIME-Version: 1.0
User-Agent: Mozilla Thunderbird
Subject: Re: [PATCH v2 1/1] mm/madvise: enhance lazyfreeing with mTHP in
 madvise_free
Content-Language: en-US
To: Barry Song <21cnbao@gmail.com>, Ryan Roberts <ryan.roberts@arm.com>
Cc: Lance Yang <ioworker0@gmail.com>, Vishal Moola <vishal.moola@gmail.com>,
 akpm@linux-foundation.org, zokeefe@google.com, shy828301@gmail.com,
 mhocko@suse.com, fengwei.yin@intel.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: <20240307061425.21013-1-ioworker0@gmail.com>
 <CAGsJ_4xcRvZGdpPh1qcFTnTnDUbwz6WreQ=L_UO+oU2iFm9EPg@mail.gmail.com>
 <CAK1f24k2G_DSEjuqqqPyY0f7+btpYbjfoyMH7btLfP8nkasCTQ@mail.gmail.com>
 <CAGsJ_4xREM-P1mFqeM-s3-cJ9czb6PXwizb-3hOhwaF6+QM5QA@mail.gmail.com>
 <03458c20-5544-411b-9b8d-b4600a9b802f@arm.com>
 <CAGsJ_4zp1MXTjG=4gBO+J3owg7sHDgDJ8Ut51i1RBSnKnK0BfQ@mail.gmail.com>
 <501c9f77-1459-467a-8619-78e86b46d300@arm.com>
 <8f84c7d6-982a-4933-a7a7-3f640df64991@redhat.com>
 <e6bc142e-113d-4034-b92c-746b951a27ed@redhat.com>
 <d24f8553-33f2-4ae7-a06d-badaf9462d84@arm.com>
 <CAGsJ_4za-2xpg21phWi2WWLF1iPXhoc1xM__FDTwYYBBKsTPgw@mail.gmail.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: <CAGsJ_4za-2xpg21phWi2WWLF1iPXhoc1xM__FDTwYYBBKsTPgw@mail.gmail.com>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 8bit

On 07.03.24 12:26, Barry Song wrote:
> On Thu, Mar 7, 2024 at 7:13 PM Ryan Roberts <ryan.roberts@arm.com> wrote:
>>
>> On 07/03/2024 10:54, David Hildenbrand wrote:
>>> On 07.03.24 11:54, David Hildenbrand wrote:
>>>> On 07.03.24 11:50, Ryan Roberts wrote:
>>>>> On 07/03/2024 09:33, Barry Song wrote:
>>>>>> On Thu, Mar 7, 2024 at 10:07 PM Ryan Roberts <ryan.roberts@arm.com> wrote:
>>>>>>>
>>>>>>> On 07/03/2024 08:10, Barry Song wrote:
>>>>>>>> On Thu, Mar 7, 2024 at 9:00 PM Lance Yang <ioworker0@gmail.com> wrote:
>>>>>>>>>
>>>>>>>>> Hey Barry,
>>>>>>>>>
>>>>>>>>> Thanks for taking time to review!
>>>>>>>>>
>>>>>>>>> On Thu, Mar 7, 2024 at 3:00 PM Barry Song <21cnbao@gmail.com> wrote:
>>>>>>>>>>
>>>>>>>>>> On Thu, Mar 7, 2024 at 7:15 PM Lance Yang <ioworker0@gmail.com> wrote:
>>>>>>>>>>>
>>>>>>>>> [...]
>>>>>>>>>>> +static inline bool can_mark_large_folio_lazyfree(unsigned long addr,
>>>>>>>>>>> +                                                struct folio *folio,
>>>>>>>>>>> pte_t *start_pte)
>>>>>>>>>>> +{
>>>>>>>>>>> +       int nr_pages = folio_nr_pages(folio);
>>>>>>>>>>> +       fpb_t flags = FPB_IGNORE_DIRTY | FPB_IGNORE_SOFT_DIRTY;
>>>>>>>>>>> +
>>>>>>>>>>> +       for (int i = 0; i < nr_pages; i++)
>>>>>>>>>>> +               if (page_mapcount(folio_page(folio, i)) != 1)
>>>>>>>>>>> +                       return false;
>>>>>>>>>>
>>>>>>>>>> we have moved to folio_estimated_sharers though it is not precise, so
>>>>>>>>>> we don't do
>>>>>>>>>> this check with lots of loops and depending on the subpage's mapcount.
>>>>>>>>>
>>>>>>>>> If we don't check the subpage’s mapcount, and there is a cow folio
>>>>>>>>> associated
>>>>>>>>> with this folio and the cow folio has smaller size than this folio,
>>>>>>>>> should we still
>>>>>>>>> mark this folio as lazyfree?
>>>>>>>>
>>>>>>>> I agree, this is true. However, we've somehow accepted the fact that
>>>>>>>> folio_likely_mapped_shared
>>>>>>>> can result in false negatives or false positives to balance the
>>>>>>>> overhead.  So I really don't know :-)
>>>>>>>>
>>>>>>>> Maybe David and Vishal can give some comments here.
>>>>>>>>
>>>>>>>>>
>>>>>>>>>> BTW, do we need to rebase our work against David's changes[1]?
>>>>>>>>>> [1]
>>>>>>>>>> https://lore.kernel.org/linux-mm/20240227201548.857831-1-david@redhat.com/
>>>>>>>>>
>>>>>>>>> Yes, we should rebase our work against David’s changes.
>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>>> +
>>>>>>>>>>> +       return nr_pages == folio_pte_batch(folio, addr, start_pte,
>>>>>>>>>>> +                                        ptep_get(start_pte), nr_pages,
>>>>>>>>>>> flags, NULL);
>>>>>>>>>>> +}
>>>>>>>>>>> +
>>>>>>>>>>>     static int madvise_free_pte_range(pmd_t *pmd, unsigned long addr,
>>>>>>>>>>>                                    unsigned long end, struct mm_walk *walk)
>>>>>>>>>>>
>>>>>>>>>>> @@ -676,11 +690,45 @@ static int madvise_free_pte_range(pmd_t *pmd,
>>>>>>>>>>> unsigned long addr,
>>>>>>>>>>>                     */
>>>>>>>>>>>                    if (folio_test_large(folio)) {
>>>>>>>>>>>                            int err;
>>>>>>>>>>> +                       unsigned long next_addr, align;
>>>>>>>>>>>
>>>>>>>>>>> -                       if (folio_estimated_sharers(folio) != 1)
>>>>>>>>>>> -                               break;
>>>>>>>>>>> -                       if (!folio_trylock(folio))
>>>>>>>>>>> -                               break;
>>>>>>>>>>> +                       if (folio_estimated_sharers(folio) != 1 ||
>>>>>>>>>>> +                           !folio_trylock(folio))
>>>>>>>>>>> +                               goto skip_large_folio;
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> I don't think we can skip all the PTEs for nr_pages, as some of them
>>>>>>>>>> might be
>>>>>>>>>> pointing to other folios.
>>>>>>>>>>
>>>>>>>>>> for example, for a large folio with 16PTEs, you do MADV_DONTNEED(15-16),
>>>>>>>>>> and write the memory of PTE15 and PTE16, you get page faults, thus PTE15
>>>>>>>>>> and PTE16 will point to two different small folios. We can only skip
>>>>>>>>>> when we
>>>>>>>>>> are sure nr_pages == folio_pte_batch() is sure.
>>>>>>>>>
>>>>>>>>> Agreed. Thanks for pointing that out.
>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>>> +
>>>>>>>>>>> +                       align = folio_nr_pages(folio) * PAGE_SIZE;
>>>>>>>>>>> +                       next_addr = ALIGN_DOWN(addr + align, align);
>>>>>>>>>>> +
>>>>>>>>>>> +                       /*
>>>>>>>>>>> +                        * If we mark only the subpages as lazyfree, or
>>>>>>>>>>> +                        * cannot mark the entire large folio as lazyfree,
>>>>>>>>>>> +                        * then just split it.
>>>>>>>>>>> +                        */
>>>>>>>>>>> +                       if (next_addr > end || next_addr - addr !=
>>>>>>>>>>> align ||
>>>>>>>>>>> +                           !can_mark_large_folio_lazyfree(addr, folio,
>>>>>>>>>>> pte))
>>>>>>>>>>> +                               goto split_large_folio;
>>>>>>>>>>> +
>>>>>>>>>>> +                       /*
>>>>>>>>>>> +                        * Avoid unnecessary folio splitting if the large
>>>>>>>>>>> +                        * folio is entirely within the given range.
>>>>>>>>>>> +                        */
>>>>>>>>>>> +                       folio_clear_dirty(folio);
>>>>>>>>>>> +                       folio_unlock(folio);
>>>>>>>>>>> +                       for (; addr != next_addr; pte++, addr +=
>>>>>>>>>>> PAGE_SIZE) {
>>>>>>>>>>> +                               ptent = ptep_get(pte);
>>>>>>>>>>> +                               if (pte_young(ptent) ||
>>>>>>>>>>> pte_dirty(ptent)) {
>>>>>>>>>>> +                                       ptent = ptep_get_and_clear_full(
>>>>>>>>>>> +                                               mm, addr, pte,
>>>>>>>>>>> tlb->fullmm);
>>>>>>>>>>> +                                       ptent = pte_mkold(ptent);
>>>>>>>>>>> +                                       ptent = pte_mkclean(ptent);
>>>>>>>>>>> +                                       set_pte_at(mm, addr, pte, ptent);
>>>>>>>>>>> +                                       tlb_remove_tlb_entry(tlb, pte,
>>>>>>>>>>> addr);
>>>>>>>>>>> +                               }
>>>>>>>>>>
>>>>>>>>>> Can we do this in batches? for a CONT-PTE mapped large folio, you are
>>>>>>>>>> unfolding
>>>>>>>>>> and folding again. It seems quite expensive.
>>>>>>>
>>>>>>> I'm not convinced we should be doing this in batches. We want the initial
>>>>>>> folio_pte_batch() to be as loose as possible regarding permissions so that we
>>>>>>> reduce our chances of splitting folios to the min. (e.g. ignore SW bits like
>>>>>>> soft dirty, etc). I think it might be possible that some PTEs are RO and other
>>>>>>> RW too (e.g. due to cow - although with the current cow impl, probably not.
>>>>>>> But
>>>>>>> its fragile to assume that). Anyway, if we do an initial batch that ignores
>>>>>>> all
>>>>>>
>>>>>> You are correct. I believe this scenario could indeed occur. For instance,
>>>>>> if process A forks process B and then unmaps itself, leaving B as the
>>>>>> sole process owning the large folio.  The current wp_page_reuse() function
>>>>>> will reuse PTE one by one while the specific subpage is written.
>>>>>
>>>>> Hmm - I thought it would only reuse if the total mapcount for the folio was 1.
>>>>> And since it is a large folio with each page mapped once in proc B, I thought
>>>>> every subpage write would cause a copy except the last one? I haven't looked at
>>>>> the code for a while. But I had it in my head that this is an area we need to
>>>>> improve for mTHP.
> 
> So sad I am wrong again ????
> 
>>>>
>>>> wp_page_reuse() will currently reuse a PTE part of a large folio only if
>>>> a single PTE remains mapped (refcount == 0).
>>>
>>> ^ == 1
> 
> seems this needs improvement. it is a waste the last subpage can

My take that is WIP:

https://lore.kernel.org/all/20231124132626.235350-1-david@redhat.com/T/#u

> reuse the whole large folio. i was doing it in a quite different way,
> if the large folio had only one subpage left, i would do copy and
> released the large folio[1]. and if i could reuse the whole large folio
> with CONT-PTE, i would reuse the whole large folio[2]. in mainline,
> we don't have this cont-pte luxury exposed to mm, so i guess we can
> not do [2] easily, but [1] seems to be an optimization.

Yeah, I had essentially the same idea: just free up the large folio if 
most of the stuff is unmapped. But that's rather a corner-case 
optimization, so I did not proceed with that.

-- 
Cheers,

David / dhildenb