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 <da...@redhat.com> >>>> 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?
