[Devel] [PATCH vz7] Create debugfs file with virtio balloon usage information

[Alexander Atanasov]

Hello,

On 22.09.22 20:45, Konstantin Khorenko wrote:
> On 05.09.2022 16:01, Konstantin Khorenko wrote:

>>> +static int virtio_balloon_debug_show(struct seq_file *f, void *offset)
>>> +{
>>> +    struct virtio_balloon *vb = f->private;
>>> +    u64 inflated_kb = vb->num_pages << (VIRTIO_BALLOON_PFN_SHIFT - 10);
>>
>> /*
>>    * Balloon device works in 4K page units.  So each page is pointed 
>> to by
>>    * multiple balloon pages.  All memory counters in this driver are 
>> in balloon
>>    * page units.
>>    */
>> #define VIRTIO_BALLOON_PAGES_PER_PAGE (unsigned)(PAGE_SIZE >> 
>> VIRTIO_BALLOON_PFN_SHIFT)
>>
>> /* Size of a PFN in the balloon interface. */
>> #define VIRTIO_BALLOON_PFN_SHIFT 12
>>
>> Taking into account defines above, i think the proper code should be:
>>     /* "+2" here because we want the value in kilobytes */
>>     u64 inflated_kb = vb->num_pages << (PAGE_SHIFT - 
>> VIRTIO_BALLOON_PFN_SHIFT + 2);
>>
>> As it's said VIRTIO_BALLOON_PAGE cannot be larger than PAGE_SIZE,
>> so if we imagine VIRTIO_BALLOON_PFN_SHIFT is defined to, say, 10, the 
>> original code
>> "<< (VIRTIO_BALLOON_PFN_SHIFT - 10)" will definitely work wrong.
> 
> And please, address this comment as well.

num_pages is accounted in balloon pages, so to make the calculation we 
use only the VIRTIO_BALLOON_PFN_SHIFT.

So it is num_pages << (CONVERT TO BYTES - CONVERT TO KILO BYTES).

Using PAGE_SHIFT will be:
12 - 12 + 2 will do the same but if at some point they start to differ 
the calculation will go wrong because it is mixing units - host vs 
ballon page shifts.


-- 
Regards,
Alexander Atanasov