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From:   David Hildenbrand <david@redhat.com>
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Subject: Re: [PATCH RFC 0/4] mm: place pages to the freelist tail when onling and undoing isolation
Date:   Wed, 16 Sep 2020 21:31:21 +0200
Message-Id: <DAC9E747-BDDF-41B6-A89B-604880DD7543@redhat.com>
References: <5c0910c2cd0d9d351e509392a45552fb@suse.de>
Cc:     David Hildenbrand <david@redhat.com>, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, linux-hyperv@vger.kernel.org,
        xen-devel@lists.xenproject.org, linux-acpi@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Alexander Duyck <alexander.h.duyck@linux.intel.com>,
        Dave Hansen <dave.hansen@intel.com>,
        Haiyang Zhang <haiyangz@microsoft.com>,
        "K. Y. Srinivasan" <kys@microsoft.com>,
        Mel Gorman <mgorman@techsingularity.net>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Michal Hocko <mhocko@kernel.org>,
        Mike Rapoport <rppt@kernel.org>,
        Scott Cheloha <cheloha@linux.ibm.com>,
        Stephen Hemminger <sthemmin@microsoft.com>,
        Vlastimil Babka <vbabka@suse.cz>, Wei Liu <wei.liu@kernel.org>,
        Wei Yang <richard.weiyang@linux.alibaba.com>
In-Reply-To: <5c0910c2cd0d9d351e509392a45552fb@suse.de>
To:     osalvador@suse.de
Sender: linux-kernel-owner@vger.kernel.org
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> Am 16.09.2020 um 20:50 schrieb osalvador@suse.de:
>=20
> =EF=BB=BFOn 2020-09-16 20:34, David Hildenbrand wrote:
>> When adding separate memory blocks via add_memory*() and onlining them
>> immediately, the metadata (especially the memmap) of the next block will b=
e
>> placed onto one of the just added+onlined block. This creates a chain
>> of unmovable allocations: If the last memory block cannot get
>> offlined+removed() so will all dependant ones. We directly have unmovable=

>> allocations all over the place.
>> This can be observed quite easily using virtio-mem, however, it can also
>> be observed when using DIMMs. The freshly onlined pages will usually be
>> placed to the head of the freelists, meaning they will be allocated next,=

>> turning the just-added memory usually immediately un-removable. The
>> fresh pages are cold, prefering to allocate others (that might be hot)
>> also feels to be the natural thing to do.
>> It also applies to the hyper-v balloon xen-balloon, and ppc64 dlpar: when=

>> adding separate, successive memory blocks, each memory block will have
>> unmovable allocations on them - for example gigantic pages will fail to
>> allocate.
>> While the ZONE_NORMAL doesn't provide any guarantees that memory can get
>> offlined+removed again (any kind of fragmentation with unmovable
>> allocations is possible), there are many scenarios (hotplugging a lot of
>> memory, running workload, hotunplug some memory/as much as possible) wher=
e
>> we can offline+remove quite a lot with this patchset.
>=20
> Hi David,
>=20

Hi Oscar.

> I did not read through the patchset yet, so sorry if the question is nonse=
nse, but is this not trying to fix the same issue the vmemmap patches did? [=
1]

Not nonesense at all. It only helps to some degree, though. It solves the de=
pendencies due to the memmap. However, it=E2=80=98s not completely ideal, es=
pecially for single memory blocks.

With single memory blocks (virtio-mem, xen-balloon, hv balloon, ppc dlpar) y=
ou still have unmovable (vmemmap chunks) all over the physical address space=
. Consider the gigantic page example after hotplug. You directly fragmented a=
ll hotplugged memory.

Of course, there might be (less extreme) dependencies due page tables for th=
e identity mapping, extended struct pages and similar.

Having that said, there are other benefits when preferring other memory over=
 just hotplugged memory. Think about adding+onlining memory during boot (dim=
ms under QEMU, virtio-mem), once the system is up you will have most (all) o=
f that memory completely untouched.

So while vmemmap on hotplugged memory would tackle some part of the issue, t=
here are cases where this approach is better, and there are even benefits wh=
en combining both.

Thanks!

David

>=20
> I was about to give it a new respin now that thw hwpoison stuff has been s=
ettled.
>=20
> [1] https://patchwork.kernel.org/cover/11059175/
>=20