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 10:44:40 +0200
From:   Oscar Salvador <osalvador@...e.de>
To:     Muchun Song <songmuchun@...edance.com>
Cc:     David Hildenbrand <david@...hat.com>, 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 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.


-- 
Oscar Salvador
SUSE Labs