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

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Date:   Fri, 16 Oct 2020 10:57:35 +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 13/29] virtio-mem: factor out handling of fake-offline
 pages in memory notifier

>> 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