linux-kernel - Re: [Xen-devel] [PATCH v2] xen/balloon: Mark unallocated host memory as UNUSABLE

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Date:   Tue, 19 Dec 2017 08:56:20 -0700
From:   "Jan Beulich" <JBeulich@...e.com>
To:     "Boris Ostrovsky" <boris.ostrovsky@...cle.com>
Cc:     <christian.koenig@....com>, <helgaas@...nel.org>,
        <xen-devel@...ts.xen.org>, "Juergen Gross" <jgross@...e.com>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [Xen-devel] [PATCH v2] xen/balloon: Mark unallocated host
 memory as UNUSABLE

>>> On 19.12.17 at 16:03, <boris.ostrovsky@...cle.com> wrote:
> On 12/19/2017 09:40 AM, Jan Beulich wrote:
>>>>> On 19.12.17 at 15:25, <boris.ostrovsky@...cle.com> wrote:
>>> On 12/19/2017 03:23 AM, Jan Beulich wrote:
>>>>> +	memmap.nr_entries = ARRAY_SIZE(xen_e820_table->entries);
>>>> Is it really reasonable to have a static upper bound here? As we
>>>> know especially EFI systems can come with a pretty scattered
>>>> (pseudo) E820 table. Even if (iirc) this has a static upper bound
>>>> right now in the hypervisor too, it would be nice if the kernel
>>>> didn't need further changes once the hypervisor is being made
>>>> more flexible.
>>> This is how we obtain the map in xen_memory_setup(). Are you suggesting
>>> that we should query for the size first?
>> That would be better, I think.
> 
> 
> I think we will first need to fix xen_memory_setup() to do that too and
> that would be a separate patch.
> 
> I am also not clear how this will work on earlier version of the
> hypervisor that didn't support querying for size. From what I am seeing
> in 4.4 we will get -EFAULT if the buffer is NULL.

That's not nice, I agree, but can be dealt with.

>>>>> +	/* Mark non-RAM regions as not available. */
>>>>> +	for (; i < memmap.nr_entries; i++) {
>>>>> +		entry = &xen_e820_table->entries[i];
>>>>> +
>>>>> +		if (entry->type == E820_TYPE_RAM)
>>>>> +			continue;
>>>> I can't seem to match up this with ...
>>>>
>>>>> +		if (entry->addr >= hostmem_resource->end)
>>>>> +			break;
>>>>> +
>>>>> +		res = kzalloc(sizeof(*res), GFP_KERNEL);
>>>>> +		if (!res)
>>>>> +			goto out;
>>>>> +
>>>>> +		res->name = "Host memory";
>>>> ... this. Do you mean != instead (with the comment ahead of the
>>>> loop also clarified, saying something like "host RAM regions which
>>>> aren't RAM for us")? And perhaps better "Host RAM"?
>>> Right, this is not memory but rather something else (and so "!=" is
>>> correct). "Unavailable host RAM"?
>> If you like to be even more specific than what I had suggested -
>> sure.
> 
> But did you want to have some changes in the preceding comment? Not sure
> I read your comment correctly.

Well, "non-RAM" is ambiguous in this context, so yes, I'd prefer it
to be clarified. Whether you use what I've suggested or something
else I don't care much.

Jan