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[2003:cb:c707:5c00:e650:bcd7:e2a0:54fe]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4282f382abcsm39296005e9.1.2024.08.01.06.13.27 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 01 Aug 2024 06:13:32 -0700 (PDT) Message-ID: <19902a48-c59b-4e3b-afc5-e792506c2fd6@redhat.com> Date: Thu, 1 Aug 2024 15:13:26 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: Race condition observed between page migration and page fault handling on arm64 machines To: Dev Jain , akpm@linux-foundation.org, willy@infradead.org Cc: ryan.roberts@arm.com, anshuman.khandual@arm.com, catalin.marinas@arm.com, cl@gentwo.org, vbabka@suse.cz, mhocko@suse.com, apopple@nvidia.com, osalvador@suse.de, baolin.wang@linux.alibaba.com, dave.hansen@linux.intel.com, will@kernel.org, baohua@kernel.org, ioworker0@gmail.com, gshan@redhat.com, mark.rutland@arm.com, kirill.shutemov@linux.intel.com, hughd@google.com, aneesh.kumar@kernel.org, yang@os.amperecomputing.com, peterx@redhat.com, broonie@kernel.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org References: <20240801081657.1386743-1-dev.jain@arm.com> <3b82e195-5871-4880-9ce5-d01bb751f471@redhat.com> <92df0ee1-d3c9-41e2-834c-284127ae2c4c@arm.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: 3D82240019 X-Rspam-User: X-Rspamd-Server: rspam05 X-Stat-Signature: ec5c6jx8iocygboc5rs7jexxbhueam4s X-HE-Tag: 1722518019-185990 X-HE-Meta: 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 MsyR6nmV 3+yi+yQ4KKWNavf9POcvkCdaHA361vm/oLoTI6LrJX4+IpGUfE+x0RlXKqgdi7on3sroOU4O6XGfqNPt9E2y0eKcx3Myr/oSvKIpRnYAMSaJ1HxfeqiHyChATBbNB1cr+LaQsQMDupw9tccrjHGHqrUFK56TyDeyH2SGZKgEGSjgodL0AHJ211yV9LaWzpC6jRzq3e8rs3hvGifiPYGN3m4TQjY4D1lpUnxQExhyu/LmNvw7x/waWJnSCy7Fem0v4Gd/9WP5EGV92qQBMNTnVIUlBQwl2uIhMGht/Eti/V3hv+pdWfwLIBoSBlq4khvtMvR0LL3vTNSMtpNXx6KqLxE6Awn7gJiQbS9urD3K60wQcVSoYwDtGxM7X0g== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000024, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: >>> To dampen the tradeoff, we could do this in shmem_fault() instead? But >>> then, this would mean that we do this in all >>> >>> kinds of vma->vm_ops->fault, only when we discover another reference >>> count race condition :) Doing this in do_fault() >>> >>> should solve this once and for all. In fact, do_pte_missing() may call >>> do_anonymous_page() or do_fault(), and I just >>> >>> noticed that the former already checks this using vmf_pte_changed(). >> >> What I am still missing is why this is (a) arm64 only; and (b) if this >> is something we should really worry about. There are other reasons >> (e.g., speculative references) why migration could temporarily fail, >> does it happen that often that it is really something we have to worry >> about? > > > (a) See discussion at [1]; I guess it passes on x86, which is quite > strange since the race is clearly arch-independent. Yes, I think this is what we have to understand. Is the race simply less likely to trigger on x86? I would assume that it would trigger on any arch. I just ran it on a x86 VM with 2 NUMA nodes and it also seems to work here. Is this maybe related to deferred flushing? Such that the other CPU will by accident just observe the !pte_none a little less likely? But arm64 also usually defers flushes, right? At least unless ARM64_WORKAROUND_REPEAT_TLBI is around. With that we never do deferred flushes. > > (b) On my machine, on an average in under 10 iterations of move_pages(), > it fails, which seems problematic to Yes, it's a big difference compared to what I encounter. -- Cheers, David / dhildenb