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[79.242.60.77]) by smtp.gmail.com with ESMTPSA id u8sm1796010wmq.45.2021.08.24.01.53.56 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 24 Aug 2021 01:53:57 -0700 (PDT) Subject: Re: [PATCH] softmmu/physmem: Improve guest memory allocation failure error message To: "Dr. David Alan Gilbert" 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> <628a7ee5-b88d-c043-2e67-67e791532c18@redhat.com> From: David Hildenbrand Organization: Red Hat Message-ID: <3d235b3e-9df1-de43-5d62-c214a4c44199@redhat.com> Date: Tue, 24 Aug 2021 10:53:56 +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: 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=170.10.133.124; envelope-from=david@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -55 X-Spam_score: -5.6 X-Spam_bar: ----- X-Spam_report: (-5.6 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.743, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, NICE_REPLY_A=-2.023, 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: Peter Maydell , QEMU Developers , Peter Xu , Paolo Bonzini , Igor Mammedov , Bin Meng , =?UTF-8?Q?Philippe_Mathieu-Daud=c3=a9?= Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 24.08.21 10:37, Dr. David Alan Gilbert wrote: > * David Hildenbrand (david@redhat.com) wrote: >> On 23.08.21 12:34, Philippe Mathieu-Daudé wrote: >>> On 8/23/21 12:24 PM, David Hildenbrand wrote: >>>> 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. >>> >>> What about simply giving a hint about memory overcommit and display >>> a link to documentation with longer description about how to check >>> and figure out this issue? >> >> That would be highly OS-specific -- for example, there is no memory >> overcommit under Windows. Sure, we could add a Linux specific hint, >> indication documentation. But I'm not sure if most end users stumbling into >> such an error+hint would be able to make sense of memory overcommit details >> (not to mention that they know what it even is) :) >> >> You can run into memory allocation issues with many applications. Let me >> give you a simple example >> >> t480s: ~ $ dd if=/dev/zero of=/dev/null ibs=100G >> dd: memory exhausted by input buffer of size 107374182400 bytes (100 GiB) >> >> So indicating the size of the failing allocation might be just good enough. >> For the other parts it's usually just "the way the OS was configured, it >> does not think it can allow this allocation". > > Does it also get complicated by the use of CGroup? Not in terms of memory overcommit AFAIU. cgroups only control actually memory consumption, not mmap() creation. > > Dave -- Thanks, David / dhildenb