Linux Virtualization - Mar 2020 - [PATCH v1 07/11] virtio-mem: Allow to offline partially unplugged memory blocks

On Mon 02-03-20 14:49:37, David Hildenbrand wrote:
[...]
> +static void virtio_mem_notify_going_offline(struct virtio_mem *vm,
> +					    unsigned long mb_id)
> +{
> +	const unsigned long nr_pages = PFN_DOWN(vm->subblock_size);
> +	unsigned long pfn;
> +	int sb_id, i;
> +
> +	for (sb_id = 0; sb_id < vm->nb_sb_per_mb; sb_id++) {
> +		if (virtio_mem_mb_test_sb_plugged(vm, mb_id, sb_id, 1))
> +			continue;
> +		/*
> +		 * Drop our reference to the pages so the memory can get
> +		 * offlined and add the unplugged pages to the managed
> +		 * page counters (so offlining code can correctly subtract
> +		 * them again).
> +		 */
> +		pfn = PFN_DOWN(virtio_mem_mb_id_to_phys(mb_id) +
> +			       sb_id * vm->subblock_size);
> +		adjust_managed_page_count(pfn_to_page(pfn), nr_pages);
> +		for (i = 0; i < nr_pages; i++)
> +			page_ref_dec(pfn_to_page(pfn + i));
Is there ever situation this might be a different than 1->0 transition?
-- 
Michal Hocko
SUSE Labs

On 10.03.20 12:43, Michal Hocko wrote:
> On Mon 02-03-20 14:49:37, David Hildenbrand wrote:
> [...]
>> +static void virtio_mem_notify_going_offline(struct virtio_mem *vm,
>> +					    unsigned long mb_id)
>> +{
>> +	const unsigned long nr_pages = PFN_DOWN(vm->subblock_size);
>> +	unsigned long pfn;
>> +	int sb_id, i;
>> +
>> +	for (sb_id = 0; sb_id < vm->nb_sb_per_mb; sb_id++) {
>> +		if (virtio_mem_mb_test_sb_plugged(vm, mb_id, sb_id, 1))
>> +			continue;
>> +		/*
>> +		 * Drop our reference to the pages so the memory can get
>> +		 * offlined and add the unplugged pages to the managed
>> +		 * page counters (so offlining code can correctly subtract
>> +		 * them again).
>> +		 */
>> +		pfn = PFN_DOWN(virtio_mem_mb_id_to_phys(mb_id) +
>> +			       sb_id * vm->subblock_size);
>> +		adjust_managed_page_count(pfn_to_page(pfn), nr_pages);
>> +		for (i = 0; i < nr_pages; i++)
>> +			page_ref_dec(pfn_to_page(pfn + i));
> 
> Is there ever situation this might be a different than 1->0 transition?
Only if some other code would be taking a reference. At least not from
virtio-mem perspective.

-- 
Thanks,

David / dhildenb

On Tue 10-03-20 12:46:05, David Hildenbrand wrote:
> On 10.03.20 12:43, Michal Hocko wrote:
> > On Mon 02-03-20 14:49:37, David Hildenbrand wrote:
> > [...]
> >> +static void virtio_mem_notify_going_offline(struct virtio_mem
*vm,
> >> +					    unsigned long mb_id)
> >> +{
> >> +	const unsigned long nr_pages = PFN_DOWN(vm->subblock_size);
> >> +	unsigned long pfn;
> >> +	int sb_id, i;
> >> +
> >> +	for (sb_id = 0; sb_id < vm->nb_sb_per_mb; sb_id++) {
> >> +		if (virtio_mem_mb_test_sb_plugged(vm, mb_id, sb_id, 1))
> >> +			continue;
> >> +		/*
> >> +		 * Drop our reference to the pages so the memory can get
> >> +		 * offlined and add the unplugged pages to the managed
> >> +		 * page counters (so offlining code can correctly subtract
> >> +		 * them again).
> >> +		 */
> >> +		pfn = PFN_DOWN(virtio_mem_mb_id_to_phys(mb_id) +
> >> +			       sb_id * vm->subblock_size);
> >> +		adjust_managed_page_count(pfn_to_page(pfn), nr_pages);
> >> +		for (i = 0; i < nr_pages; i++)
> >> +			page_ref_dec(pfn_to_page(pfn + i));
> > 
> > Is there ever situation this might be a different than 1->0
transition?
> 
> Only if some other code would be taking a reference. At least not from
> virtio-mem perspective.
OK, so that is essentially an error condition. I think it shouldn't go
silent and you want something like
	if (WARN_ON(!page_ref_sub_and_test(page)))
		dump_page(pfn_to_page(pfn + i), "YOUR REASON");

-- 
Michal Hocko
SUSE Labs