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Subject: Re: [PATCH v1 13/29] virtio-mem: factor out handling of fake-offline
 pages in memory notifier
To:     Wei Yang <richard.weiyang@linux.alibaba.com>
Cc:     linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        virtualization@lists.linux-foundation.org,
        Andrew Morton <akpm@linux-foundation.org>,
        "Michael S . Tsirkin" <mst@redhat.com>,
        Jason Wang <jasowang@redhat.com>,
        Pankaj Gupta <pankaj.gupta.linux@gmail.com>
References: <20201012125323.17509-1-david@redhat.com>
 <20201012125323.17509-14-david@redhat.com>
 <20201016071502.GM86495@L-31X9LVDL-1304.local>
 <20201016080046.GA43862@L-31X9LVDL-1304.local>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat GmbH
Message-ID: <17ca9bd8-bdc6-541c-b8f9-4076cb9a7322@redhat.com>
Date:   Fri, 16 Oct 2020 10:57:35 +0200
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>> Do we adjust the count twice?
>>
> 
> Ah, I got the reason why we need to adjust count for *unplugged* sub-blocks.

Exactly.

> 
>>> -		for (i = 0; i < nr_pages; i++) {
>>> -			page = pfn_to_page(pfn + i);
>>> -			if (WARN_ON(!page_ref_dec_and_test(page)))
> 
> Another question is when we grab a refcount for the unpluged pages? The one
> you mentioned in virtio_mem_set_fake_offline().

Yeah, that was confusing on my side. I actually meant
virtio_mem_fake_offline() - patch #12.

We have a reference on unplugged (fake offline) blocks via

1. memmap initialization, if never online via generic_online_page()

So if we keep pages fake offline when onlining memory, they

a) Have a refcount of 1
b) Have *not* increased the managed page count

2. alloc_contig_range(), if fake offlined. After we fake-offlined pages
(e.g., patch #12), such pages

a) Have a refcount of 1
b) Have *not* increased the managed page count (because we manually
decreased it)


-- 
Thanks,

David / dhildenb