Re: [PATCH] softmmu/physmem: Improve guest memory allocation failure error message

[Philippe Mathieu-Daudé]

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 <david@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?