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[2003:cb:c707:fa00:74f2:89da:ed65:8b50]) by smtp.gmail.com with ESMTPSA id dn6-20020a05600c654600b004126ca2a1cbsm4473493wmb.48.2024.02.29.00.56.53 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 29 Feb 2024 00:56:53 -0800 (PST) Message-ID: <038e55eb-70a9-445d-89ef-4b989eaa9c66@redhat.com> Date: Thu, 29 Feb 2024 09:56:52 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3] filemap: avoid unnecessary major faults in filemap_fault() To: Peng Zhang , linux-mm@kvack.org, linux-kernel@vger.kernel.org Cc: akpm@linux-foundation.org, willy@infradead.org, ying.huang@intel.com, fengwei.yin@intel.com, aneesh.kumar@linux.ibm.com, shy828301@gmail.com, hughd@google.com, wangkefeng.wang@huawei.com, sunnanyong@huawei.com References: <20240229060907.836589-1-zhangpeng362@huawei.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: <20240229060907.836589-1-zhangpeng362@huawei.com> 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: 7bit X-Rspamd-Queue-Id: 897B11C000E X-Rspam-User: X-Rspamd-Server: rspam05 X-Stat-Signature: xpzhea9aatn8pcm8ps9taizf75otdckc X-HE-Tag: 1709197018-150876 X-HE-Meta: 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 cik5XM6L He8hceND8caaeVgO21i5cVJnVsdjRduLmZnrRRo6c3bQ1tgz9WFQcgpwZZR7IEfIrcaxY1LyfpQIfiyFvwN0NSmqxNl3fFQnoLGNnKVPZueCd/hW4DX5cp5xbE3BDzPA5vYKQSHTqj8KnZufMiBwebskXA4mh/4F8w9M3AuS7jIzuc+5ZwUhL08l1mdHEh6jPRMV06d4IM9INzOMGYKIu7z32ug== 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 29.02.24 07:09, Peng Zhang wrote: > From: ZhangPeng > > The major fault occurred when using mlockall(MCL_CURRENT | MCL_FUTURE) > in application, which leading to an unexpected issue[1]. > > This caused by temporarily cleared PTE during a read+clear/modify/write > update of the PTE, eg, do_numa_page()/change_pte_range(). > > For the data segment of the user-mode program, the global variable area > is a private mapping. After the pagecache is loaded, the private anonymous > page is generated after the COW is triggered. Mlockall can lock COW pages > (anonymous pages), but the original file pages cannot be locked and may > be reclaimed. If the global variable (private anon page) is accessed when > vmf->pte is zeroed in numa fault, a file page fault will be triggered. > At this time, the original private file page may have been reclaimed. > If the page cache is not available at this time, a major fault will be > triggered and the file will be read, causing additional overhead. > > This issue affects our traffic analysis service. The inbound traffic is > heavy. If a major fault occurs, the I/O schedule is triggered and the > original I/O is suspended. Generally, the I/O schedule is 0.7 ms. If > other applications are operating disks, the system needs to wait for > more than 10 ms. However, the inbound traffic is heavy and the NIC buffer > is small. As a result, packet loss occurs. But the traffic analysis service > can't tolerate packet loss. > > Fix this by holding PTL and rechecking the PTE in filemap_fault() before > triggering a major fault. We do this check only if vma is VM_LOCKED. In > our service test environment, the baseline is 7 major faults / 12 hours. > After the patch is applied, no major fault will be triggered. > > Testing file anonymous page read and write page fault performance in > ext4, tmpfs and ramdisk using will-it-scale[2] on a x86 physical machine. > The data is the average change compared with the mainline after the patch > is applied. The test results are indicates some performance regressions. > We do this check only if vma is VM_LOCKED, therefore, no performance > regressions is caused for most common cases. > > The test results are as follows: > processes processes_idle threads threads_idle > ext4 private file write: -0.51% 0.08% -0.03% -0.04% > ext4 shared file write: 0.135% -0.531% 2.883% -0.772% > ramdisk private file write: -0.48% 0.23% -1.08% 0.27% > ramdisk private file read: 0.07% -6.90% -5.85% -0.70% > tmpfs private file write: -0.344% -0.110% 0.200% 0.145% > tmpfs shared file write: 0.958% 0.101% 2.781% -0.337% > tmpfs private file read: -0.16% 0.00% -0.12% 0.41% > > [1] https://lore.kernel.org/linux-mm/9e62fd9a-bee0-52bf-50a7-498fa17434ee@huawei.com/ > [2] https://github.com/antonblanchard/will-it-scale/ > > Suggested-by: "Huang, Ying" > Suggested-by: David Hildenbrand > Signed-off-by: ZhangPeng > Signed-off-by: Kefeng Wang > --- > v2->v3: > - Do this check only if vma is VM_LOCKED per David Hildenbrand > - Hold PTL and recheck the PTE > - Place the recheck code in a new function filemap_fault_recheck_pte() > > v1->v2: > - Add more test results per Huang, Ying > - Add more comments before check PTE per Huang, Ying, David Hildenbrand > and Yin Fengwei > - Change pte_offset_map_nolock to pte_offset_map as the PTL won't > be used > > RFC->v1: > - Add error handling when ptep == NULL per Huang, Ying and Matthew > Wilcox > - Check the PTE without acquiring PTL in filemap_fault(), suggested by > Huang, Ying and Yin Fengwei > - Add pmd_none() check before PTE map > - Update commit message and add performance test information > > mm/filemap.c | 40 ++++++++++++++++++++++++++++++++++++++++ > 1 file changed, 40 insertions(+) > > diff --git a/mm/filemap.c b/mm/filemap.c > index b4858d89f1b1..2668bac68df7 100644 > --- a/mm/filemap.c > +++ b/mm/filemap.c > @@ -3181,6 +3181,42 @@ static struct file *do_async_mmap_readahead(struct vm_fault *vmf, > return fpin; > } > > +/* > + * filemap_fault_recheck_pte - hold PTL and recheck whether pte is none. > + * @vmf - the vm_fault for this fault. > + * > + * Recheck PTE as the PTE can be cleared temporarily during a read+clear/modify > + * /write update of the PTE, eg, do_numa_page()/change_pte_range(). This will > + * trigger an unexpected major fault, even if we use mlockall(), which may > + * increase IO and thus cause other unexpected behavior. > + * > + * Return VM_FAULT_NOPAGE if the PTE is not none or pte_offset_map_lock() > + * fails. In other cases, 0 is returned. > + */ That documentation is imprecise, as you are not explaining the mlock limitation. It's an internal helper, I'd drop all that and rather add a comment below right next to the conditions that are performing the check ... > +static vm_fault_t filemap_fault_recheck_pte(struct vm_fault *vmf) > +{ > + struct vm_area_struct *vma = vmf->vma; > + vm_fault_t ret = 0; > + pte_t *ptep; > + > + if (!(vma->vm_flags & VM_LOCKED)) > + return ret; I was wondering if we also want to do: if (!is_cow_mappinng(vma->vm_flags)) return 0; But likely it's not helpful. Then add something like: /* * We might have COW'ed a pageache folio and might now have an mlocked * anon folio mapped. The original pagecache folio is not mlocked and * might have been evicted. During a read+clear/modify/write update of * the PTE, such as done in do_numa_page()/change_pte_range(), we * temporarily clear the PTE under PT lock and might detect it here as * "none" when not holding the PT lock. * * Not rechecking the PTE under PT lock could result in an * unexpected major fault in an mlock'ed region. Recheck only for * this special scenario while holding the PT lock, to not degrade * non-mlocked scenarios. */ > + > + if (pmd_none(*vmf->pmd)) > + return ret; I'd simply return 0 in both cases, easier to read. > + > + ptep = pte_offset_map_lock(vma->vm_mm, vmf->pmd, vmf->address, > + &vmf->ptl); > + if (unlikely(!ptep)) > + return VM_FAULT_NOPAGE; > + > + if (unlikely(!pte_none(ptep_get(ptep)))) > + ret = VM_FAULT_NOPAGE; > + > + pte_unmap_unlock(ptep, vmf->ptl); > + return ret; > +} -- Cheers, David / dhildenb