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[2003:cb:c717:a000:d4df:4b8f:37d4:59e4]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-35dd062fe96sm1763908f8f.67.2024.05.31.05.34.55 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 31 May 2024 05:34:55 -0700 (PDT) Message-ID: <821d29b9-cb06-49db-9fe8-6c054c8787fb@redhat.com> Date: Fri, 31 May 2024 14:34:54 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH] mm: swap: reuse exclusive folio directly instead of wp page faults To: Barry Song <21cnbao@gmail.com> Cc: akpm@linux-foundation.org, linux-mm@kvack.org, chrisl@kernel.org, surenb@google.com, kasong@tencent.com, minchan@kernel.org, willy@infradead.org, ryan.roberts@arm.com, linux-kernel@vger.kernel.org, Barry Song References: <20240531104819.140218-1-21cnbao@gmail.com> <87ac9610-5650-451f-aa54-e634a6310af4@redhat.com> From: David Hildenbrand 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: X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspamd-Queue-Id: 4CD76C0008 X-Rspam-User: X-Rspamd-Server: rspam12 X-Stat-Signature: dmz3knmbn58bcjtqcreegoi5bce7dd8j X-HE-Tag: 1717158900-783821 X-HE-Meta: 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 6o/m0MVJ 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 31.05.24 14:30, Barry Song wrote: > On Sat, Jun 1, 2024 at 12:20 AM Barry Song <21cnbao@gmail.com> wrote: >> >> On Sat, Jun 1, 2024 at 12:10 AM David Hildenbrand wrote: >>> >>> On 31.05.24 13:55, Barry Song wrote: >>>> On Fri, May 31, 2024 at 11:08 PM David Hildenbrand wrote: >>>>> >>>>> On 31.05.24 12:48, Barry Song wrote: >>>>>> From: Barry Song >>>>>> >>>>>> After swapping out, we perform a swap-in operation. If we first read >>>>>> and then write, we encounter a major fault in do_swap_page for reading, >>>>>> along with additional minor faults in do_wp_page for writing. However, >>>>>> the latter appears to be unnecessary and inefficient. Instead, we can >>>>>> directly reuse in do_swap_page and completely eliminate the need for >>>>>> do_wp_page. >>>>>> >>>>>> This patch achieves that optimization specifically for exclusive folios. >>>>>> The following microbenchmark demonstrates the significant reduction in >>>>>> minor faults. >>>>>> >>>>>> #define DATA_SIZE (2UL * 1024 * 1024) >>>>>> #define PAGE_SIZE (4UL * 1024) >>>>>> >>>>>> static void *read_write_data(char *addr) >>>>>> { >>>>>> char tmp; >>>>>> >>>>>> for (int i = 0; i < DATA_SIZE; i += PAGE_SIZE) { >>>>>> tmp = *(volatile char *)(addr + i); >>>>>> *(volatile char *)(addr + i) = tmp; >>>>>> } >>>>>> } >>>>>> >>>>>> int main(int argc, char **argv) >>>>>> { >>>>>> struct rusage ru; >>>>>> >>>>>> char *addr = mmap(NULL, DATA_SIZE, PROT_READ | PROT_WRITE, >>>>>> MAP_ANONYMOUS | MAP_PRIVATE, -1, 0); >>>>>> memset(addr, 0x11, DATA_SIZE); >>>>>> >>>>>> do { >>>>>> long old_ru_minflt, old_ru_majflt; >>>>>> long new_ru_minflt, new_ru_majflt; >>>>>> >>>>>> madvise(addr, DATA_SIZE, MADV_PAGEOUT); >>>>>> >>>>>> getrusage(RUSAGE_SELF, &ru); >>>>>> old_ru_minflt = ru.ru_minflt; >>>>>> old_ru_majflt = ru.ru_majflt; >>>>>> >>>>>> read_write_data(addr); >>>>>> getrusage(RUSAGE_SELF, &ru); >>>>>> new_ru_minflt = ru.ru_minflt; >>>>>> new_ru_majflt = ru.ru_majflt; >>>>>> >>>>>> printf("minor faults:%ld major faults:%ld\n", >>>>>> new_ru_minflt - old_ru_minflt, >>>>>> new_ru_majflt - old_ru_majflt); >>>>>> } while(0); >>>>>> >>>>>> return 0; >>>>>> } >>>>>> >>>>>> w/o patch, >>>>>> / # ~/a.out >>>>>> minor faults:512 major faults:512 >>>>>> >>>>>> w/ patch, >>>>>> / # ~/a.out >>>>>> minor faults:0 major faults:512 >>>>>> >>>>>> Minor faults decrease to 0! >>>>>> >>>>>> Signed-off-by: Barry Song >>>>>> --- >>>>>> mm/memory.c | 7 ++++--- >>>>>> 1 file changed, 4 insertions(+), 3 deletions(-) >>>>>> >>>>>> diff --git a/mm/memory.c b/mm/memory.c >>>>>> index eef4e482c0c2..e1d2e339958e 100644 >>>>>> --- a/mm/memory.c >>>>>> +++ b/mm/memory.c >>>>>> @@ -4325,9 +4325,10 @@ vm_fault_t do_swap_page(struct vm_fault *vmf) >>>>>> */ >>>>>> if (!folio_test_ksm(folio) && >>>>>> (exclusive || folio_ref_count(folio) == 1)) { >>>>>> - if (vmf->flags & FAULT_FLAG_WRITE) { >>>>>> - pte = maybe_mkwrite(pte_mkdirty(pte), vma); >>>>>> - vmf->flags &= ~FAULT_FLAG_WRITE; >>>>>> + if (vma->vm_flags & VM_WRITE) { >>>>>> + pte = pte_mkwrite(pte_mkdirty(pte), vma); >>>>>> + if (vmf->flags & FAULT_FLAG_WRITE) >>>>>> + vmf->flags &= ~FAULT_FLAG_WRITE; >>>>> >>>>> This implies, that even on a read fault, you would mark the pte dirty >>>>> and it would have to be written back to swap if still in the swap cache >>>>> and only read. >>>>> >>>>> That is controversial. >>>>> >>>>> What is less controversial is doing what mprotect() via >>>>> change_pte_range()/can_change_pte_writable() would do: mark the PTE >>>>> writable but not dirty. >>>>> >>>>> I suggest setting the pte only dirty if FAULT_FLAG_WRITE is set. >>>> >>>> Thanks! >>>> >>>> I assume you mean something as below? >>> >>> It raises an important point: uffd-wp must be handled accordingly. >>> >>>> >>>> diff --git a/mm/memory.c b/mm/memory.c >>>> index eef4e482c0c2..dbf1ba8ccfd6 100644 >>>> --- a/mm/memory.c >>>> +++ b/mm/memory.c >>>> @@ -4317,6 +4317,10 @@ vm_fault_t do_swap_page(struct vm_fault *vmf) >>>> add_mm_counter(vma->vm_mm, MM_SWAPENTS, -nr_pages); >>>> pte = mk_pte(page, vma->vm_page_prot); >>>> >>>> + if (pte_swp_soft_dirty(vmf->orig_pte)) >>>> + pte = pte_mksoft_dirty(pte); >>>> + if (pte_swp_uffd_wp(vmf->orig_pte)) >>>> + pte = pte_mkuffd_wp(pte); >>>> /* >>>> * Same logic as in do_wp_page(); however, optimize for pages that are >>>> * certainly not shared either because we just allocated them without >>>> @@ -4325,18 +4329,19 @@ vm_fault_t do_swap_page(struct vm_fault *vmf) >>>> */ >>>> if (!folio_test_ksm(folio) && >>>> (exclusive || folio_ref_count(folio) == 1)) { >>>> - if (vmf->flags & FAULT_FLAG_WRITE) { >>>> - pte = maybe_mkwrite(pte_mkdirty(pte), vma); >>>> - vmf->flags &= ~FAULT_FLAG_WRITE; >>>> + if (vma->vm_flags & VM_WRITE) { >>>> + if (vmf->flags & FAULT_FLAG_WRITE) { >>>> + pte = pte_mkwrite(pte_mkdirty(pte), vma); >>>> + vmf->flags &= ~FAULT_FLAG_WRITE; >>>> + } else if ((!vma_soft_dirty_enabled(vma) || >>>> pte_soft_dirty(pte)) >>>> + && !userfaultfd_pte_wp(vma, pte)) { >>>> + pte = pte_mkwrite(pte, vma); >>> >>> Even with FAULT_FLAG_WRITE we must respect uffd-wp and *not* do a >>> pte_mkwrite(pte). So we have to catch and handle that earlier (I could >>> have sworn we handle that somehow). >>> >>> Note that the existing >>> pte = pte_mkuffd_wp(pte); >>> >>> Will fix that up because it does an implicit pte_wrprotect(). >> >> This is exactly what I have missed as I am struggling with why WRITE_FAULT >> blindly does mkwrite without checking userfaultfd_pte_wp(). >> >>> >>> >>> So maybe what would work is >>> >>> >>> if ((vma->vm_flags & VM_WRITE) && !userfaultfd_pte_wp(vma, pte) && >>> !vma_soft_dirty_enabled(vma)) { >>> pte = pte_mkwrite(pte); >>> >>> /* Only set the PTE dirty on write fault. */ >>> if (vmf->flags & FAULT_FLAG_WRITE) { >>> pte = pte_mkdirty(pte); >>> vmf->flags &= ~FAULT_FLAG_WRITE; >>> } > > WRITE_FAULT has a pte_mkdirty, so it doesn't need to check > "!vma_soft_dirty_enabled(vma)"? > Maybe I thought too much, just the simple code below should work? That would likely not handle softdirty correctly in case we end up in pte_mkwrite(pte, vma); note that pte_mksoft_dirty() will not wrprotect ... (note that we shouldn't optimize for softdirty handling) > > if (!folio_test_ksm(folio) && > (exclusive || folio_ref_count(folio) == 1)) { > if (vma->vm_flags & VM_WRITE) { > if (vmf->flags & FAULT_FLAG_WRITE) { > pte = pte_mkwrite(pte_mkdirty(pte), vma); > vmf->flags &= ~FAULT_FLAG_WRITE; > } else { > pte = pte_mkwrite(pte, vma); > } > } > rmap_flags |= RMAP_EXCLUSIVE; > } > > if (pte_swp_soft_dirty(vmf->orig_pte)) > pte = pte_mksoft_dirty(pte); > if (pte_swp_uffd_wp(vmf->orig_pte)) > pte = pte_mkuffd_wp(pte); > > This still uses the implicit wrprotect of pte_mkuffd_wp. But the wrprotected->writable->wrprotected path really is confusing. I'd prefer to set these bits ahead of time instead, so we can properly rely on them -- like we do in other code. -- Cheers, David / dhildenb