From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.133.124]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 0A6C41C07F7 for ; Fri, 20 Dec 2024 10:12:02 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=170.10.133.124 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1734689524; cv=none; b=jCVgZG17fkw5hyCQQvRxcou9YQLkMGXADvjJkH2wmGBOKr47kr/3PUV5FtwraG+APizRcHcNfspQPFDo+9Bizk4LUU//Z2G+v7tCnt+TaeOdtF+MOxQoumYnmi2NfWyXxeazAX0avm4lWNymtDic5ZDZFniU6qqNjeQTR05UdKc= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1734689524; c=relaxed/simple; bh=iJosNCQuoOY1XbZm/TdCjjJShE7pwyY1F+SaiSP/By4=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=IPoHpqTBUvclnzlbunhXpe3ijUeFM3Q4e7weFDBCIpuhGkvioJV1UcEbvmXO3+L5+PnKzX8etTpMxthjUgHSngkTA6gAIqsXzOmwedqoR9xvkcfu2WWQxPgykAy8SvwRIEo/1ubHEZ2vty2HCZWXGGfo7FRwtcDYWa5BY6cPmfk= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=redhat.com; spf=pass smtp.mailfrom=redhat.com; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b=f/V81ZjF; arc=none smtp.client-ip=170.10.133.124 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=redhat.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=redhat.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="f/V81ZjF" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1734689521; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references:autocrypt:autocrypt; bh=llFO8qWbZFNp5Al/XuWBAVWaA6sl2FmbRXCkByVKBbc=; b=f/V81ZjF42Uqo5V6rrq7gUagcMA6e/OjNQMQvHUS8tNhRk11ErNanCLM0tFQuI9MoW+4ra lJXaEKW0BjvfAnqklFL64sYqjhT63n8GyInMLH6BLdzXRtjc7ydVrbutFEk7/ufyhrCSfq MvDvZrHUG++Is8Q37u273+S5H6byaI8= Received: from mail-wr1-f70.google.com (mail-wr1-f70.google.com [209.85.221.70]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-488-X5d_S_xiOt2el_w7c4tvEg-1; Fri, 20 Dec 2024 05:12:00 -0500 X-MC-Unique: X5d_S_xiOt2el_w7c4tvEg-1 X-Mimecast-MFC-AGG-ID: X5d_S_xiOt2el_w7c4tvEg Received: by mail-wr1-f70.google.com with SMTP id ffacd0b85a97d-385e2579507so807864f8f.1 for ; Fri, 20 Dec 2024 02:12:00 -0800 (PST) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1734689519; x=1735294319; h=content-transfer-encoding:in-reply-to:organization:autocrypt :content-language:from:references:cc:to:subject:user-agent :mime-version:date:message-id:x-gm-message-state:from:to:cc:subject :date:message-id:reply-to; bh=llFO8qWbZFNp5Al/XuWBAVWaA6sl2FmbRXCkByVKBbc=; b=Zvze07h0d9LQxFGsO3ueLufi2kBSGG9ZP3bAR1Jok+Retdi5zCdoplyEWRzKmj3tQz QHYG1TjGoWHS9VLiCiv6qKU8gZrqa24IybKaztD4NDqpsrvjxRp8sonCEOD9T21w2xPN kPPAPhhnSTLIXRyfmECottYm0nNUzl1JNShLaz7BRsTM6f5HIbnp9tsM8X1r5H+qX/I4 oQRAc4JzTk8IEBVXJ4+PGD2/mp7w3KfMLTOUhI4HhEXTqnWcfYX1fx7mx0m41Q0tWMqn 9+nBhJozzrUJSv8Sgsq4KgFOkK62eaXllvhruxQUQ5vW5o9BNXFrBrJCG0STX5q/yF3a lABg== X-Forwarded-Encrypted: i=1; AJvYcCXeobXpJdalJaVPCn+oOWuN468eAFOZq1UP5a6dwk5aPRsoj5QgSwCWMAVm6qSBh7z4j4stxyz6TZ0YJXs=@vger.kernel.org X-Gm-Message-State: AOJu0YxyFrzi6FwTdqlrSOcd7gqPn2mNiQXsaRNfJZVTn1wP4kVesJE6 o1dbTMd/ME5/IeDE00PTIXilvlNwRL4k/+8mFg1XxaZ+C98g2prhdGpK64der0qDYgU5XD1ripN 2AVcMBHGIc+usagH49ZYwXf/p7aNPDqwL5kR+wr8siQA5woi7fZME1Rccsyx+QA== X-Gm-Gg: ASbGncuP2XovtX40iaWkH1KqkYvCNXfG8xjkv5lfz9sIahiZkArQQdOY5/YR5efFoVz XNTtE7AkWFHBfSb87Z5VhD6nHq024IqhoqfwFmSsgWO/HSFKF9ih8EcJ6WiN+qYUksjX7ymU596 5RSPzsgT70RvPcItWGS7BtEA6M2Xx+ALiHOWZKAMvgKMfGDUYqXGf8TKPxsbU9W/IwIFO8Rwf+v hGeggu+fNC63oBwsi/fVWdQKdp7JVzucy6r6aWXhwe3et2CM4jnovcBkDH55KNwlhZ0lkkpWOXK AmLZFR3jSwut8F2vYijLmex7Cwi3p2VdaiF4NNRIG/g6ehwscCeE1/82AWWz2gdYHOK2gH/kt+B n5pyy8jjT X-Received: by 2002:a05:6000:1fa6:b0:388:da2a:653d with SMTP id ffacd0b85a97d-38a223fecd3mr1900691f8f.56.1734689519225; Fri, 20 Dec 2024 02:11:59 -0800 (PST) X-Google-Smtp-Source: AGHT+IFL4JlVNUcvKKZO6D1/wOjbHHuAHGuPJzzOR+Ws/K8gxnzCKFXlefVuXKv8K1oIGYJGrm2OyA== X-Received: by 2002:a05:6000:1fa6:b0:388:da2a:653d with SMTP id ffacd0b85a97d-38a223fecd3mr1900651f8f.56.1734689518802; Fri, 20 Dec 2024 02:11:58 -0800 (PST) Received: from ?IPV6:2003:cb:c708:9d00:edd9:835b:4bfb:2ce3? (p200300cbc7089d00edd9835b4bfb2ce3.dip0.t-ipconnect.de. [2003:cb:c708:9d00:edd9:835b:4bfb:2ce3]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-38a1c8a8d7esm3673783f8f.101.2024.12.20.02.11.56 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 20 Dec 2024 02:11:57 -0800 (PST) Message-ID: <454f7c85-a6d9-4635-94ff-7fab69060a82@redhat.com> Date: Fri, 20 Dec 2024 11:11:55 +0100 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm: migration :shared anonymous migration test is failing To: Donet Tom , Andrew Morton , linux-mm@kvack.org, linux-kernel@vger.kernel.org Cc: Ritesh Harjani , Baolin Wang , "Aneesh Kumar K . V" , Zi Yan , shuah Khan , Dev Jain References: <20241219124717.4907-1-donettom@linux.ibm.com> <9bc000ab-1982-41f0-9ca0-2a4ead9aa982@linux.ibm.com> From: David Hildenbrand 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: <9bc000ab-1982-41f0-9ca0-2a4ead9aa982@linux.ibm.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 20.12.24 03:55, Donet Tom wrote: > > On 12/19/24 18:28, David Hildenbrand wrote: >> On 19.12.24 13:47, Donet Tom wrote: >>> The migration selftest is currently failing for shared anonymous >>> mappings due to a race condition. >>> >>> During migration, the source folio's PTE is unmapped by nuking the >>> PTE, flushing the TLB,and then marking the page for migration >>> (by creating the swap entries). The issue arises when, immediately >>> after the PTE is nuked and the TLB is flushed, but before the page >>> is marked for migration, another thread accesses the page. This >>> triggers a page fault, and the page fault handler invokes >>> do_pte_missing() instead of do_swap_page(), as the page is not yet >>> marked for migration. >>> >>> In the fault handling path, do_pte_missing() calls __do_fault() >>> ->shmem_fault() -> shmem_get_folio_gfp() -> filemap_get_entry(). >>> This eventually calls folio_try_get(), incrementing the reference >>> count of the folio undergoing migration. The thread then blocks >>> on folio_lock(), as the migration path holds the lock. This >>> results in the migration failing in __migrate_folio(), which expects >>> the folio's reference count to be 2. However, the reference count is >>> incremented by the fault handler, leading to the failure. >>> >>> The issue arises because, after nuking the PTE and before marking the >>> page for migration, the page is accessed. To address this, we have >>> updated the logic to first nuke the PTE, then mark the page for >>> migration, and only then flush the TLB. With this patch, If the page is >>> accessed immediately after nuking the PTE, the TLB entry is still >>> valid, so no fault occurs. >> >> But what about if the PTE is not in the TLB yet, and you get an access >> from another CPU just after clearing the PTE (but not flushing the >> TLB)? The other CPU will still observe PTE=none, trigger a fault etc. >> > Yes, in this scenario, the migration will fail. Do you think the > migration test > failure, even after a retry, should be considered a major issue that > must be fixed? I think it is something we should definitely improve, but I think our page migration should handle this in a better way, not the unmap logic. I recall we discussed with Dev some ideas on how to improve that? I'm pretty sure one can trigger similar case using a tmpfs file and using read/write in a loop instead of memory access -> page faults. So where racing with page faults is completely out of the picture. -- Cheers, David / dhildenb