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

From: David Hildenbrand <david@redhat.com>
To: "Philippe Mathieu-Daudé" <philmd@redhat.com>,
	"Peter Maydell" <peter.maydell@linaro.org>
Cc: QEMU Developers <qemu-devel@nongnu.org>,
	Peter Xu <peterx@redhat.com>,
	"Dr. David Alan Gilbert" <dgilbert@redhat.com>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Igor Mammedov <imammedo@redhat.com>,
	Bin Meng <bmeng.cn@gmail.com>
Subject: Re: [PATCH] softmmu/physmem: Improve guest memory allocation failure error message
Date: Mon, 23 Aug 2021 12:24:16 +0200	[thread overview]
Message-ID: <628a7ee5-b88d-c043-2e67-67e791532c18@redhat.com> (raw)
In-Reply-To: <1a63c2d2-7420-5fc1-1023-0504a67dc40b@redhat.com>

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.

-- 
Thanks,

David / dhildenb