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[2003:d8:2f0a:7f00:fad7:3bc9:69d:31f]) by smtp.gmail.com with ESMTPSA id r10sm17959868wrq.32.2021.08.23.03.24.16 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 23 Aug 2021 03:24:17 -0700 (PDT) To: =?UTF-8?Q?Philippe_Mathieu-Daud=c3=a9?= , Peter Maydell References: <20210820155211.3153137-1-philmd@redhat.com> <20a53e29-ba23-fe0d-f961-63d0b5ca9a89@redhat.com> <6165f86e-1ce7-d178-1f5c-4b3c5110f0c1@redhat.com> <1a63c2d2-7420-5fc1-1023-0504a67dc40b@redhat.com> From: David Hildenbrand Organization: Red Hat Subject: Re: [PATCH] softmmu/physmem: Improve guest memory allocation failure error message Message-ID: <628a7ee5-b88d-c043-2e67-67e791532c18@redhat.com> Date: Mon, 23 Aug 2021 12:24:16 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.11.0 MIME-Version: 1.0 In-Reply-To: <1a63c2d2-7420-5fc1-1023-0504a67dc40b@redhat.com> Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=david@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=216.205.24.124; envelope-from=david@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -44 X-Spam_score: -4.5 X-Spam_bar: ---- X-Spam_report: (-4.5 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.746, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, NICE_REPLY_A=-0.959, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H2=-0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: QEMU Developers , Peter Xu , "Dr. David Alan Gilbert" , Paolo Bonzini , Igor Mammedov , Bin Meng Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 23.08.21 12:12, Philippe Mathieu-Daudé wrote: > On 8/23/21 11:29 AM, David Hildenbrand wrote: >> On 23.08.21 11:23, Peter Maydell wrote: >>> On Mon, 23 Aug 2021 at 09:40, David Hildenbrand wrote: >>>> Not opposed to printing the size, although I doubt that it will really >>>> stop similar questions/problems getting raised. >>> >>> The case that triggered this was somebody thinking >>> -m took a byte count, so very likely that an error message >>> saying "you tried to allocate 38TB" would have made their >>> mistake clear in a way that just "allocation failed" did not. >>> It also means that if a future user asks us for help then >>> we can look at the error message and immediately tell them >>> the problem, rather than going "hmm, what are all the possible >>> ways that allocation might have failed" and going off down >>> rabbitholes like VM overcommit settings... >> >> We've had similar issues recently where Linux memory overcommit handling >> rejected the allocation -- and the user was well aware about the actual >> size. You won't be able to catch such reports, because people don't >> understand how Linux memory overcommit handling works or was configured. >> >> "I have 3 GiB of free memory, why can't I create a 3 GiB VM". "I have 3 >> GiB of RAM, why can't I create a 3 GiB VM even if it won't make use of >> all 3 GiB of memory". >> >> Thus my comment, it will only stop very basic usage issues. And I agree >> that looking at the error *might* help. It didn't help for the cases I >> just described, because we need much more system information to make a >> guess what the user error actually is. > > Is it possible to get the maximal overcommitable amount on Linux? Not reliably I think. In the "always" mode, there is none. In the "guess"/"estimate" mode, the kernel takes a guess (currently implemented as checking if the mmap size <= total RAM + total SWAP). Committable = MemTotal + SwapTotal In the "never" mode: Committable = CommitLimit - Committed_AS However, the value gets further reduced for !root applications by /proc/sys/vm/admin_reserve_kbytes. Replicating these calculations in user space would be suboptimal IMHO. -- Thanks, David / dhildenb