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Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.3.1 MIME-Version: 1.0 In-Reply-To: <20200204050643.20925-1-bhe@redhat.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 2.84 on 10.5.11.22 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 04.02.20 06:06, Baoquan He wrote: > In commit 357b4da50a62 ("x86: respect memory size limiting via mem= > parameter") a global varialbe max_mem_size is added to store > the value parsed from 'mem= ', then checked when memory region is > added. This truly stops those DIMMs from being added into system memory > during boot-time. > > However, it also limits the later memory hotplug functionality. Any > DIMM can't be hotplugged any more if its region is beyond the > max_mem_size. We will get errors like: > > [ 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 > > This will cause issue in a known use case where 'mem=' is added to > the hypervisor. The memory that lies after 'mem=' boundary will be > assigned to KVM guests. After commit 357b4da50a62 merged, memory > can't be extended dynamically if system memory on hypervisor is not > sufficient. > > So fix it by also checking if it's during boot-time restricting to add > memory. Otherwise, skip the restriction. > > And also add this use case to document of 'mem=' kernel parameter. > > Fixes: 357b4da50a62 ("x86: respect memory size limiting via mem= parameter") > Signed-off-by: Baoquan He > --- > v2->v3: > In discussion of v1 and v2, People have concern about the use case > related to the code change. So add the use case into patch log and > document of 'mem=' in kernel-parameters.txt. > > Documentation/admin-guide/kernel-parameters.txt | 13 +++++++++++-- > mm/memory_hotplug.c | 8 +++++++- > 2 files changed, 18 insertions(+), 3 deletions(-) > > diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt > index ddc5ccdd4cd1..b809767e5f74 100644 > --- a/Documentation/admin-guide/kernel-parameters.txt > +++ b/Documentation/admin-guide/kernel-parameters.txt > @@ -2533,13 +2533,22 @@ > For details see: Documentation/admin-guide/hw-vuln/mds.rst > > mem=nn[KMG] [KNL,BOOT] Force usage of a specific amount of memory > - Amount of memory to be used when the kernel is not able > - to see the whole system memory or for test. > + Amount of memory to be used in cases as follows: > + > + 1 for test; > + 2 when the kernel is not able to see the whole system memory; > + 3 memory that lies after 'mem=' boundary is excluded from > + the hypervisor, then assigned to KVM guests. I remember that there were more use cases, but forgot where that was documented :) I do wonder if we want to change that now without anybody complaining. Yes, I brought up a possible use case but don't know if it is relevant in practice (IOW, nobody complained yet :) ). Would like to get Michals opinion on this. -- Thanks, David / dhildenb