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

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Date:   Sat, 17 Oct 2020 06:38:11 +0800
From:   Wei Yang <richard.weiyang@...ux.alibaba.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     Wei Yang <richard.weiyang@...ux.alibaba.com>,
        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

On Fri, Oct 16, 2020 at 12:32:50PM +0200, David Hildenbrand wrote:
>>>> 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, I see the logic.

    online_pages()
        move_pfn_range_to_zone()  --- 1)
	online_pages_range()      --- 2)

At 1), __init_single_page() would set page count to 1. At 2),
generic_online_page() would clear page count, while the call back would not.

Then I am trying to search the place where un-zero page count prevent offline.
scan_movable_pages() would fail, since this is a PageOffline() and has 1 page
count.

So the GUARD we prevent offline partial-onlined pages is

    (PageOffline && page_count)

And your commit aa218795cb5fd583c94f

mm: Allow to offline unmovable PageOffline() pages via MEM_GOING_OFFLINE

is introduced to handle this case.

That's pretty clear now.

>-- 
>Thanks,
>
>David / dhildenb

-- 
Wei Yang
Help you, Help me