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

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Date:   Wed, 14 Nov 2018 23:49:15 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Matthew Wilcox <willy@...radead.org>
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 14.11.18 23:23, Matthew Wilcox wrote:
> 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.

Thank you for looking into this :)

> 
>> +++ 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?

BALLOON was/is documented as

"23 - BALLOON
    balloon compaction page
"

and only includes all virtio-ballon pages after the non-lru migration
feature has been implemented for ballooned pages. Since then, this flag
does basically no longer stands for what it actually was supposed to do.

To not break uapi I decided to not rename it but instead to add a new flag.

> 
> 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.
> 

Can we rename without breaking uapi?

-- 

Thanks,

David / dhildenb