linux-kernel - Re: [PATCH v1] memory-hotplug.rst: complete admin-guide overhaul

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Date:   Tue, 8 Jun 2021 17:18:19 +0300
From:   Mike Rapoport <rppt@...nel.org>
To:     David Hildenbrand <david@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Oscar Salvador <osalvador@...e.de>,
        Michal Hocko <mhocko@...e.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Matthew Wilcox <willy@...radead.org>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Muchun Song <songmuchun@...edance.com>,
        Pavel Tatashin <pasha.tatashin@...een.com>,
        Jonathan Corbet <corbet@....net>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        linux-doc@...r.kernel.org
Subject: Re: [PATCH v1] memory-hotplug.rst: complete admin-guide overhaul

On Tue, Jun 08, 2021 at 03:04:19PM +0200, David Hildenbrand wrote:

...

> > > +--------------
> > > -You can offline a memory block by using the same sysfs interface that was used
> > > -in memory onlining::
> > > +We usually expect that a large portion of available system RAM will actually
> > > +be consumed by user space, either directly or indirectly via the page cache. In
> > > +the normal case, ZONE_MOVABLE can be used when allocating such pages just fine.
> > > -	% echo offline > /sys/devices/system/memory/memoryXXX/state
> > > +With that in mind, it makes sense that we can have a big portion of system RAM
> > > +managed by ZONE_MOVABLE. However, there are some things to consider when
> > > +using ZONE_MOVABLE, especially when fine-tuning zone ratios:
> > > -If offline succeeds, the state of the memory block is changed to be "offline".
> > > -If it fails, some error core (like -EBUSY) will be returned by the kernel.
> > > -Even if a memory block does not belong to ZONE_MOVABLE, you can try to offline
> > > -it.  If it doesn't contain 'unmovable' memory, you'll get success.
> > > +- Having a lot of offline memory blocks. Even offline memory blocks consume
> > > +  memory for metadata and page tables in the direct map; having a lot of
> > > +  offline memory blocks is not a typical case, though.
> > > +
> > > +- Memory ballooning. Some memory ballooning implementations, such as
> > > +  the Hyper-V balloon, the XEN balloon, the vbox balloon and the VMWare
> > 
> > So, everyone except virtio-mem? ;-)
> 
> Well, virtio-mem does not classify as memory balloon in that sense, as it
> only operates on own device memory ;)
> 
> virtio-balloon and pseries CMM support balloon compaction.
> 
> > I'd drop the names because if some of those will implement balloon
> > compaction they surely will forget to update the docs.
 
> I can do the opposite and mention the ones that already do. Some most
> probably will never support it.
> 
> "Memory ballooning without balloon compaction is incompatible with
> ZONE_MOVABLE. Only some implementations, such as virtio-balloon and pseries
> CMM, fully support balloon compaction."
 
Sounds good.
 
-- 
Sincerely yours,
Mike.