linux-kernel - Re: [PATCH RFC 2/6] mm: convert PG_balloon to PG

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Date:   Wed, 14 Nov 2018 14:23:21 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     David Hildenbrand <david@...hat.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org, devel@...uxdriverproject.org,
        linux-fsdevel@...r.kernel.org, linux-pm@...r.kernel.org,
        xen-devel@...ts.xenproject.org, Jonathan Corbet <corbet@....net>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Christian Hansen <chansen3@...co.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Michal Hocko <mhocko@...e.com>,
        Pavel Tatashin <pasha.tatashin@...cle.com>,
        Alexander Duyck <alexander.h.duyck@...ux.intel.com>,
        Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
        Miles Chen <miles.chen@...iatek.com>,
        David Rientjes <rientjes@...gle.com>
Subject: Re: [PATCH RFC 2/6] mm: convert PG_balloon to PG_offline

On Wed, Nov 14, 2018 at 10:17:00PM +0100, David Hildenbrand wrote:
> Rename PG_balloon to PG_offline. This is an indicator that the page is
> logically offline, the content stale and that it should not be touched
> (e.g. a hypervisor would have to allocate backing storage in order for the
> guest to dump an unused page).  We can then e.g. exclude such pages from
> dumps.
> 
> In following patches, we will make use of this bit also in other balloon
> drivers.  While at it, document PGTABLE.

Thank you for documenting PGTABLE.  I didn't realise I also had this
document to update when I added PGTABLE.

> +++ b/Documentation/admin-guide/mm/pagemap.rst
> @@ -78,6 +78,8 @@ number of times a page is mapped.
>      23. BALLOON
>      24. ZERO_PAGE
>      25. IDLE
> +    26. PGTABLE
> +    27. OFFLINE

So the offline *user* bit is new ... even though the *kernel* bit
just renames the balloon bit.  I'm not sure how I feel about this.
I'm going to think about it some more.  Could you share your decision
process with us?

I have no objection to renaming the balloon bit inside the kernel; I
think that's a wise idea.  I'm just not sure whether we should rename
the user balloon bit rather than adding a new bit.