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

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Date:   Fri, 15 Dec 2017 16:33:40 +0100
From:   Juergen Gross <jgross@...e.com>
To:     Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        xen-devel@...ts.xen.org, linux-kernel@...r.kernel.org
Cc:     helgaas@...nel.org, christian.koenig@....com
Subject: Re: [PATCH] xen/balloon: Mark unallocated host memory as UNUSABLE

On 15/12/17 15:58, Boris Ostrovsky wrote:
> On 12/15/2017 09:47 AM, Juergen Gross wrote:
>> On 15/12/17 15:24, Boris Ostrovsky wrote:
>>>>> +
>>>>> +	hostmem_resource->start = max_addr;
>>>>> +	hostmem_resource->end = entry->addr + entry->size;
>>>>> +	for (; i < memmap.nr_entries; i++) {
>>>>> +		entry = &xen_e820_table->entries[i];
>>>>> +		if (entry->type == E820_TYPE_RAM)
>>>> Shouldn't that be != ?
>>> No, the idea here is to populate hostmem_resource with ranges already
>>> taken by things other than RAM, leaving memory regions as available for
>>> the balloon hotplug. This will allow us to use allocate_resource(),
>>> which searches for a free range, in the balloon driver.
>> But why says the comment "Host memory not allocated to dom0" then?
> 
> hostmem_resource is created starting from the end of dom0 RAM and ending
> at the end of host RAM. If these two are the same (or if the former is
> larger, which I don't think is possible) then there is nothing to do, as
> the hostmem_resource will be empty. That's what the comment is referring
> to.
> 
>> And why are you trying to allocate from this resource in case of
>> hotplugging memory (and fall back to iomem_resource in case of
>> failure)?
> 
> Because that area (end of dom0 RAM through end of host RAM) is not going
> to be used by anyone else and thus is available. That was the idea
> behind f5775e0b6116.

Aah, now I've got it.

Could you please add a comment like:

/* Mark non-RAM regions as not available. */

above the test? That would have helped me. :-)


Juergen