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Date:   Tue, 10 Dec 2019 10:50:02 +0100
From:   Michal Hocko <mhocko@kernel.org>
To:     David Hildenbrand <david@redhat.com>
Cc:     Balbir Singh <bsingharora@gmail.com>, Baoquan He <bhe@redhat.com>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org, jgross@suse.com,
        akpm@linux-foundation.org
Subject: Re: [Patch v2] mm/hotplug: Only respect mem= parameter during boot
 stage
Message-ID: <20191210095002.GA10404@dhcp22.suse.cz>
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On Tue 10-12-19 10:36:19, David Hildenbrand wrote:
> On 10.12.19 10:24, Balbir Singh wrote:
> > 
> > 
> > On 10/12/19 7:44 pm, Baoquan He wrote:
> >> In commit 357b4da50a62 ("x86: respect memory size limiting via mem=
> >> parameter") a global varialbe global max_mem_size is added to store
> >                   typo ^^^
> >> the value parsed from 'mem= ', then checked when memory region is
> >> added. This truly stops those DIMM from being added into system memory
> >> during boot-time.
> >>
> >> However, it also limits the later memory hotplug functionality. Any
> >> memory board can't be hot added any more if its region is beyond the
> >> max_mem_size. System will print error like below:
> >>
> >> [  216.387164] acpi PNP0C80:02: add_memory failed
> >> [  216.389301] acpi PNP0C80:02: acpi_memory_enable_device() error
> >> [  216.392187] acpi PNP0C80:02: Enumeration failure
> >>
> >> From document of 'mem= ' parameter, it should be a restriction during
> >> boot, but not impact the system memory adding/removing after booting.
> >>
> >>   mem=nn[KMG]     [KNL,BOOT] Force usage of a specific amount of memory
> >> 	          ...
> >>
> >> So fix it by also checking if it's during boot-time when restrict memory
> >> adding. Otherwise, skip the restriction.
> >>
> > 
> > The fix looks reasonable, but I don't get the use case. Booting with mem= is
> > generally a debug option, is this for debugging memory hotplug + limited memory?
> 
> Some people/companies use "mem=" along with KVM e.g., to avoid
> allocating memmaps for guest backing memory and to not expose it to the
> buddy across kexec's. The excluded physical memory is then memmap into
> the hypervisor process and KVM can deal with that. I can imagine that
> hotplug might be desirable as well for such use cases.

If this is really the usecase (it makes some sense to me) then it should
be folded into the changelog. Because the real semantic is not really
clear as I've pointed out in the previous version of this patch [1].
The restriction to BOOT is documented since ever long before the memory
hotplug was a thing.

Thanks!

[1] Btw. it would have been much better if you posted the version 2 only
after all the feedback got discussed properly.

-- 
Michal Hocko
SUSE Labs