linux-kernel - Re: [PATCH v4 2/2] mm: memory_hotplug: make hugetlb_optimize_vmemmap compatible with memmap_on

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Date:   Mon, 20 Jun 2022 10:56:03 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Oscar Salvador <osalvador@...e.de>,
        Muchun Song <songmuchun@...edance.com>
Cc:     akpm@...ux-foundation.org, corbet@....net, mike.kravetz@...cle.com,
        paulmck@...nel.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        duanxiongchun@...edance.com, smuchun@...il.com
Subject: Re: [PATCH v4 2/2] mm: memory_hotplug: make hugetlb_optimize_vmemmap
 compatible with memmap_on_memory

On 20.06.22 10:44, Oscar Salvador wrote:
> On Mon, Jun 20, 2022 at 04:29:11PM +0800, Muchun Song wrote:
>>>> Although it works, I think PageVmemmapSelfHosted() check for the 1st pfn's
>>>> vmemmap page is not always reliable.  Since we reused PG_owner_priv_1
>>>> as PG_vmemmap_self_hosted, the test is noly reliable for vmemmap page's
>>>> vmemmap page.  Other non-vmemmap page can be flagged with PG_owner_priv_1.
>>>> So this check can be false-positive. Maybe the following code snippet is
>>>> the solution.
>>>
>>> How could that happen for pages used for backing a vmemmap?
>>>
>>
>> It cannot happen for memmap_on_memory case. Howwver, it can happen for other
>> cases. E.g. the 1st pfn (of boot memory block) whose vmemmap page may be flagged
>> as PG_owner_priv_1 (if PG_swapcache is set). Then, the check is false-positive.
> 
> If this can really happen, which I am not that sure tbh, maybe a way out would be
> to just define a new page-type as we did in previous versions of memmap_on_memory.
> In that way we would not for flags, but for its type.
> 
> But as I said, I am not entirely sure about the potential fallout of what you mention.

We are talking about the memmap of a page, who's page content is the
memmap of pages actually exposed to other users (file-backed, anonymous,
whatsoever).

In other words, while setting PG_swapcache on the memmap of a page
exposed to the user is possible, it shouldn't be possible for the memmap
of the page "hosting" these memmaps.

I know, it's confusing and I keep confusing myself. I tried creating a
picture and it doesn't really clarify the situation :D

-- 
Thanks,

David / dhildenb