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[2003:cb:c744:e000:ec38:8ec6:b1c4:c844]) by smtp.gmail.com with ESMTPSA id l21-20020a05600c4f1500b004101543e843sm23622234wmq.10.2024.03.06.02.24.29 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 06 Mar 2024 02:24:30 -0800 (PST) Message-ID: <5691dff3-a944-42f2-9848-ea7287a89192@redhat.com> Date: Wed, 6 Mar 2024 11:24:29 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v4] 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: <20240306083809.1236634-1-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: <20240306083809.1236634-1-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: 9755D80010 X-Rspam-User: X-Rspamd-Server: rspam04 X-Stat-Signature: sbmb9r8iqk9ybxrhqhgsf8atfedcjwod X-HE-Tag: 1709720674-107273 X-HE-Meta: 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 BIMdvRSI 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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 06.03.24 09:38, 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 to > reduce the performance impact in common scenarios. > > In our product environment, there were 7 major faults every 12 hours. > After the patch is applied, no major fault have been triggered. > > Testing file page read and write page fault performance in ext4 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 within the range of fluctuation. 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: 0.22% 0.26% 1.21% -0.15% > ext4 private file read: 0.03% 1.00% 1.39% 0.34% > ext4 shared file write: -0.50% -0.02% -0.14% -0.02% > ramdisk private file write: 0.07% 0.02% 0.53% 0.04% > ramdisk private file read: 0.01% 1.60% -0.32% -0.02% > > [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 > --- > v3->v4: > - Update the performance data and commit message > - Check PTE without lock firstly per Huang, Ying > - Update comments for recheck function per David Hildenbrand > - Simply return 0 to make it easier to read per David Hildenbrand > - Check !FAULT_FLAG_ORIG_PTE_VALID instead of pmd_none() > > 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 | 46 ++++++++++++++++++++++++++++++++++++++++++++++ > 1 file changed, 46 insertions(+) > > diff --git a/mm/filemap.c b/mm/filemap.c > index b4858d89f1b1..31ab455c4537 100644 > --- a/mm/filemap.c > +++ b/mm/filemap.c > @@ -3181,6 +3181,48 @@ static struct file *do_async_mmap_readahead(struct vm_fault *vmf, > return fpin; > } > > +static vm_fault_t filemap_fault_recheck_pte_none(struct vm_fault *vmf) > +{ > + struct vm_area_struct *vma = vmf->vma; > + vm_fault_t ret = 0; > + pte_t *ptep; > + > + /* > + * We might have COW'ed a pagecache 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. Recheck the PTE without PT lock firstly, thereby reducing > + * the number of times we hold PT lock. > + */ > + if (!(vma->vm_flags & VM_LOCKED)) > + return 0; > + > + if (!(vmf->flags & FAULT_FLAG_ORIG_PTE_VALID)) > + return 0; > + > + ptep = pte_offset_map(vmf->pmd, vmf->address); > + if (unlikely(!ptep)) > + return VM_FAULT_NOPAGE; > + > + if (unlikely(!pte_none(ptep_get_lockless(ptep)))) { > + ret = VM_FAULT_NOPAGE; > + } else { > + spin_lock(vmf->ptl); I assume the assumption is that vmf->ptl is still set from handle_pte_fault(). But in the meantime, we did a pte_unmap(vmf->pte). Looking at other users of vmf->ptl, this here sticks out. I would suggest replacing the pte_offset_map() above by a pte_offset_map_nolock() that gives us the ptl. With that Reviewed-by: David Hildenbrand -- Cheers, David / dhildenb