linux-kernel - Re: [PATCH v1 13/29] virtio-mem: factor out handling of fake-offline pages in memory notifier

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Message-ID: <20201018123733.GC50506@L-31X9LVDL-1304>
Date:   Sun, 18 Oct 2020 20:37:33 +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 13/29] virtio-mem: factor out handling of fake-offline
 pages in memory notifier

On Fri, Oct 16, 2020 at 10:57:35AM +0200, David Hildenbrand wrote:
>>> 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)
>

Yep, I got the reason now.

>
>-- 
>Thanks,
>
>David / dhildenb

-- 
Wei Yang
Help you, Help me