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

[Philippe Mathieu-Daudé]

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 <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?
> 
> 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?