linux-kernel - Re: [PATCH v1 05/29] virtio-mem: generalize check for added memory

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Message-ID: <be3dca3d-f5e1-c2d6-460b-04e666619db8@redhat.com>
Date:   Fri, 16 Oct 2020 12:32:50 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Wei Yang <richard.weiyang@...ux.alibaba.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        virtualization@...ts.linux-foundation.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Michael S . Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        Pankaj Gupta <pankaj.gupta.linux@...il.com>
Subject: Re: [PATCH v1 05/29] virtio-mem: generalize check for added memory

>>> Ok, I seems to understand the logic now.
>>>
>>> But how we prevent ONLINE_PARTIAL memory block get offlined? There are three
>>> calls in virtio_mem_set_fake_offline(), while all of them adjust page's flag.
>>> How they hold reference to struct page?
>>
>> Sorry, I should have given you the right pointer. (similar to my other
>> reply)
>>
>> We hold a reference either via
>>
>> 1. alloc_contig_range()
> 
> I am not familiar with this one, need to spend some time to look into.

Each individual page will have a pagecount of 1.

> 
>> 2. memmap init code, when not calling generic_online_page().
> 
> I may miss some code here. Before online pages, memmaps are allocated in
> section_activate(). They are supposed to be zero-ed. (I don't get the exact
> code line.) I am not sure when we grab a refcount here.

Best to refer to __init_single_page() -> init_page_count().

Each page that wasn't onlined via generic_online_page() has a refcount
of 1 and looks like allocated.

-- 
Thanks,

David / dhildenb