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 8E53518FDBE for ; Mon, 30 Jun 2025 08:19:25 +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=1751271567; cv=none; b=LDjqvEiSawWgWckktrKuB0mnlyqQRFgD/pbDggVfvq3TiweAeL1rn35aP5LkdGzknL3w9w4skR1A037EoWVYFYppXsBzrQ4iAuLWPq0DjS17XuKj5LZHFeMVxlZOu5ZbXpZB7BAKbMy8yhML/lAYrBaRQd29LNlaQLbyHy9TOhA= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1751271567; c=relaxed/simple; bh=MtsChutNFwNy1BrMKOU5qVnMBx9E1vhTkQh2TkQ2ZjM=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=BUmP1qJpjfmLX2uVV/ugLZgU6O742r+GSzEPxN1tGxwiHP2tD9G8Xqwr+IrIXlPAMk9vh5M+MxldxqYeiDUlIdIhGnf4uwBAnvENIU0PdAnoyRMneyuYOov/YUN1e2feH+cg/ItEF8xXaLGj9cLEilZ1xSrgcJzy01Owm5TEji8= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=quarantine 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=EXTwPACN; arc=none smtp.client-ip=170.10.133.124 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=quarantine 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="EXTwPACN" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1751271564; 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=6rjuiBdCgCZQ3wUNP8WG3kJosrNfsI+VGS+0nYYi6/s=; b=EXTwPACNd4cICFhE0IdJ8JjWrTdi9oGQt7gAv41tlaRhr+4R/4j3qLnaeaKajIilMe9kE8 P/ppg6AQ4TIaLiqh3NZtbXXhyI6pRI8Aym3/SJhPgZW7h5SdYsoU5optzCTZvFra3fexU6 rdrSMKZAAYmovfv8P5SFnadK7SgRB7Q= Received: from mail-wr1-f69.google.com (mail-wr1-f69.google.com [209.85.221.69]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-564-JqOr8wlBOKacxhvsxMPVxQ-1; Mon, 30 Jun 2025 04:19:23 -0400 X-MC-Unique: JqOr8wlBOKacxhvsxMPVxQ-1 X-Mimecast-MFC-AGG-ID: JqOr8wlBOKacxhvsxMPVxQ_1751271562 Received: by mail-wr1-f69.google.com with SMTP id ffacd0b85a97d-3a4f8fd1847so1790092f8f.1 for ; Mon, 30 Jun 2025 01:19:23 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1751271562; x=1751876362; 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=6rjuiBdCgCZQ3wUNP8WG3kJosrNfsI+VGS+0nYYi6/s=; b=uxMr8GWKi86MrT3Le+GJ6K/hyEq8hNyTWeJjuaA80WrSwILiOdoWXEqi1xn+HmImGK D+XvJzc+lttXwF6tg6+ftmgDm75VfD0SZZp50vDjzZSPoErHO4mw2QzPJUrcxtTiZN4h 7G4bzxD4jmtnS+s2RekpNKJoW1shKqcDsccuEWwo22aYkWzhWoQ5q1bcaThAAyKszCSC xnbD35Q57n2XuE6FoeOtZ5aeL+FWIYEoUzOQB8BRh57iKjDGDvGLTMdcDgis3QHe+EWz FVEdglNDHwvA21SiRhvaAF6vwcNnUeAA7zzPQMicyVxKSwXrsDKgtDbRaQNPUA3SndE8 yTrQ== X-Forwarded-Encrypted: i=1; AJvYcCWXi6AOQkVJPL0Hs5lYjQmOIgsuVI+dSKiEnUfzocnMIgHfynZ5br4Lwv60L/2QvaufKCUv+BblFFPmSC4=@vger.kernel.org X-Gm-Message-State: AOJu0YxSUzskPUKTs7OZeRbCs+4i8OIG1RlFeW8thW31OwwT4Td6i4vO vjUFqjWGylaBuv0kNpddjCRK3qpq+iNEdcSWPkMZ4yhmvijWNofhRflfbiELfWt1T81zibjR8N9 S8zwX5Q/rlOiuaD+1UsFEw/pwCPyDfRTxf6U0ppG0UbSJlZPfm5qTJLZSqFhaAc4FlA== X-Gm-Gg: ASbGncupIObTW/ByzE1AWc/BF6JMEZdlis3xrdQYwc03HeG+lsUWqkSItMEnpfAXOpx 9kTJ6rYV/6ii4jXDGZXhalORiwFet4Ba8T1rPPaaonNbj7OmMDCYjB92GvuROJUSZwHsDwjXURM y9PFR/uTdv61e/GNcI255bUxMt2VgS2MOD4OQ77fUV7e6hSGkNj8Bw0/I3rf17JFcdXsXt6F9Ps TdAaKdF2DnvQmJgK+4uUwWgHAlOEDhsm84aYfeXDGqdsWfXuFVd1XcbchNX1is+iisIkk/j7Oo4 kJSizWISnwxoai7YIr33ZCsqwBh+DfIN/cQFuDgb4oSoftwnx8pBW0YBfgzO/AbmEK8kDiDzUMX suKX64WZOxFcaakLR4bOFAYjtbxVlUnMwkzwU4EUvVzJDfrfMgg== X-Received: by 2002:a5d:55c9:0:b0:3a4:dd02:f724 with SMTP id ffacd0b85a97d-3a900575c7amr8096879f8f.43.1751271561848; Mon, 30 Jun 2025 01:19:21 -0700 (PDT) X-Google-Smtp-Source: AGHT+IGhLzuzoEbHQBeZGJMzeuEUvwVC7mTrWLoncZmfxcMXqw0Ym2aGta0QiTRTEJa8+OCCMSwQsQ== X-Received: by 2002:a5d:55c9:0:b0:3a4:dd02:f724 with SMTP id ffacd0b85a97d-3a900575c7amr8096848f8f.43.1751271561415; Mon, 30 Jun 2025 01:19:21 -0700 (PDT) Received: from ?IPV6:2003:d8:2f40:b300:53f7:d260:aff4:7256? (p200300d82f40b30053f7d260aff47256.dip0.t-ipconnect.de. [2003:d8:2f40:b300:53f7:d260:aff4:7256]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3a88c8013fesm9865476f8f.38.2025.06.30.01.19.20 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 30 Jun 2025 01:19:20 -0700 (PDT) Message-ID: Date: Mon, 30 Jun 2025 10:19:19 +0200 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] khugepaged: Reduce race probability between migration and khugepaged To: Dev Jain , Anshuman Khandual , akpm@linux-foundation.org Cc: ziy@nvidia.com, baolin.wang@linux.alibaba.com, lorenzo.stoakes@oracle.com, Liam.Howlett@oracle.com, npache@redhat.com, ryan.roberts@arm.com, baohua@kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20250630044837.4675-1-dev.jain@arm.com> <786c83e0-d69f-4fa3-a39c-94c4dfc08a20@arm.com> <28bc843d-615f-4848-988b-31a0a2921d14@arm.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: <28bc843d-615f-4848-988b-31a0a2921d14@arm.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 30.06.25 10:12, Dev Jain wrote: > > On 30/06/25 1:25 pm, Anshuman Khandual wrote: >> On 30/06/25 10:18 AM, Dev Jain wrote: >>> Suppose a folio is under migration, and khugepaged is also trying to >>> collapse it. collapse_pte_mapped_thp() will retrieve the folio from the >>> page cache via filemap_lock_folio(), thus taking a reference on the folio >>> and sleeping on the folio lock, since the lock is held by the migration >>> path. Migration will then fail in >>> __folio_migrate_mapping -> folio_ref_freeze. Reduce the probability of >>> such a race happening (leading to migration failure) by bailing out >>> if we detect a PMD is marked with a migration entry. >> Could the migration be re-attempted after such failure ? Seems like >> the migration failure here is traded for a scan failure instead. > > We already re-attempt migration. See NR_MAX_MIGRATE_PAGES_RETRY and > NR_MAX_MIGRATE_ASYNC_RETRY. Also just before freezing the refcount, > we do a suitable refcount check in folio_migrate_mapping(). So the > race happens after this and folio_ref_freeze() in __folio_migrate_mapping(), > therefore the window for the race is already very small in the migration > path, but large in the khugepaged path. > > >> >>> This fixes the migration-shared-anon-thp testcase failure on Apple M3. >> Could you please provide some more context why this test case was >> failing earlier and how does this change here fixes the problem ? > > IMHO the explanation I have given in the patch description is clear > and succinct: the testcase is failing due to the race. This patch > shortens the race window, and the test on this particular hardware > does not hit the race window again. > >> >>> Note that, this is not a "fix" since it only reduces the chance of >>> interference of khugepaged with migration, wherein both the kernel >>> functionalities are deemed "best-effort". >>>> Signed-off-by: Dev Jain >>> --- >>> >>> This patch was part of >>> https://lore.kernel.org/all/20250625055806.82645-1-dev.jain@arm.com/ >>> but I have sent it separately on suggestion of Lorenzo, and also because >>> I plan to send the first two patches after David Hildenbrand's >>> folio_pte_batch series gets merged. >>> >>> mm/khugepaged.c | 12 ++++++++++-- >>> 1 file changed, 10 insertions(+), 2 deletions(-) >>> >>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c >>> index 1aa7ca67c756..99977bb9bf6a 100644 >>> --- a/mm/khugepaged.c >>> +++ b/mm/khugepaged.c >>> @@ -31,6 +31,7 @@ enum scan_result { >>> SCAN_FAIL, >>> SCAN_SUCCEED, >>> SCAN_PMD_NULL, >>> + SCAN_PMD_MIGRATION, >>> SCAN_PMD_NONE, >>> SCAN_PMD_MAPPED, >>> SCAN_EXCEED_NONE_PTE, >>> @@ -941,6 +942,8 @@ static inline int check_pmd_state(pmd_t *pmd) >>> >>> if (pmd_none(pmde)) >>> return SCAN_PMD_NONE; >>> + if (is_pmd_migration_entry(pmde)) >>> + return SCAN_PMD_MIGRATION; >>> if (!pmd_present(pmde)) >>> return SCAN_PMD_NULL; >>> if (pmd_trans_huge(pmde)) >>> @@ -1502,9 +1505,12 @@ int collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr, >>> !range_in_vma(vma, haddr, haddr + HPAGE_PMD_SIZE)) >>> return SCAN_VMA_CHECK; >>> >>> - /* Fast check before locking page if already PMD-mapped */ >>> + /* >>> + * Fast check before locking folio if already PMD-mapped, or if the >>> + * folio is under migration >>> + */ >>> result = find_pmd_or_thp_or_none(mm, haddr, &pmd); >>> - if (result == SCAN_PMD_MAPPED) >>> + if (result == SCAN_PMD_MAPPED || result == SCAN_PMD_MIGRATION) >> Should mapped PMD and migrating PMD be treated equally while scanning ? > > SCAN_PMD_MAPPED is used as an indicator to change result to SCAN_SUCCEED > in khugepaged_scan_mm_slot: after the call to collapse_pte_mapped_thp. And, > it is also used in madvise_collapse() to do ++thps which is used to set the > return value of madvise_collapse. So I think this approach will be wrong. But if it already is PMD mapped (just temporarily through a migration entry), isn't this exactly what we want? -- Cheers, David / dhildenb