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

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Date:   Tue, 19 Dec 2017 07:40:04 -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 15:25, <boris.ostrovsky@...cle.com> wrote:
> On 12/19/2017 03:23 AM, Jan Beulich wrote:
>>>>> On 18.12.17 at 23:22, <boris.ostrovsky@...cle.com> wrote:
>>> +	if (!xen_e820_table)
>>> +		return;
>> Not saying "out of memory" here is certainly fine, but shouldn't
>> there nevertheless be a warning, as failure to go through the
>> rest of the function will impact overall functionality?
> 
> Commit ebfdc40969f claims that these types of messages are unnecessary
> because allocation failures are signalled by the memory subsystem.

But the memory subsystem can't possibly provide an indication of
what will not work because of the failed allocation.

>>> +	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.

>>> +	/* 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.

Jan