linux-kernel - Re: [PATCH v1 07/11] virtio-mem: Allow to offline partially unplugged memory blocks

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Message-ID: <69326f17-6762-dab1-0eca-c1c7d148b92a@redhat.com>
Date:   Tue, 10 Mar 2020 13:09:58 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        virtio-dev@...ts.oasis-open.org,
        virtualization@...ts.linux-foundation.org, kvm@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Michael S . Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        Oscar Salvador <osalvador@...e.de>,
        Igor Mammedov <imammedo@...hat.com>,
        Dave Young <dyoung@...hat.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Pavel Tatashin <pasha.tatashin@...een.com>,
        Stefan Hajnoczi <stefanha@...hat.com>,
        Vlastimil Babka <vbabka@...e.cz>
Subject: Re: [PATCH v1 07/11] virtio-mem: Allow to offline partially unplugged
 memory blocks

On 10.03.20 12:59, Michal Hocko wrote:
> 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");
> 

Makes sense -  I'll most probably convert this to a WARN_ON_ONCE.

Thanks!

-- 
Thanks,

David / dhildenb