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SubjectRe: [RFC PATCH v2 0/4] mm, memory_hotplug: allocate memmap from hotadded memory
On Tue, 22 Jan 2019 11:37:04 +0100
Oscar Salvador <> wrote:

> Hi,
> this is the v2 of the first RFC I sent back then in October [1].
> In this new version I tried to reduce the complexity as much as possible,
> plus some clean ups.
> [Testing]
> I have tested it on "x86_64" (small/big memblocks) and on "powerpc".
> On both architectures hot-add/hot-remove online/offline operations
> worked as expected using vmemmap pages, I have not seen any issues so far.
> I wanted to try it out on Hyper-V/Xen, but I did not manage to.
> I plan to do so along this week (if time allows).
> I would also like to test it on arm64, but I am not sure I can grab
> an arm64 box anytime soon.

Hi Oscar,

I ran tests on one of our arm64 machines. Particular machine doesn't actually have
the mechanics for hotplug, so was all 'faked', but software wise it's all the

Upshot, seems to work as expected on arm64 as well.
Tested-by: Jonathan Cameron <>

Remove currently relies on some out of tree patches (and dirty hacks) due
to the usual issue with how arm64 does pfn_valid. It's not even vaguely
ready for upstream. I'll aim to post an informational set for anyone else
testing in this area (it's more or less just a rebase of the patches from
a few years ago).

+CC Shameer who has been testing the virtualization side for more details on
that, and Robin who is driving forward memory hotplug in general on the arm64



> [Coverletter]:
> This is another step to make the memory hotplug more usable. The primary
> goal of this patchset is to reduce memory overhead of the hot added
> memory (at least for SPARSE_VMEMMAP memory model). The current way we use
> to populate memmap (struct page array) has two main drawbacks:
> a) it consumes an additional memory until the hotadded memory itself is
> onlined and
> b) memmap might end up on a different numa node which is especially true
> for movable_node configuration.
> a) is problem especially for memory hotplug based memory "ballooning"
> solutions when the delay between physical memory hotplug and the
> onlining can lead to OOM and that led to introduction of hacks like auto
> onlining (see 31bc3858ea3e ("memory-hotplug: add automatic onlining
> policy for the newly added memory")).
> b) can have performance drawbacks.
> I have also seen hot-add operations failing on powerpc due to the fact
> that we try to use order-8 pages when populating the memmap array.
> Given 64KB base pagesize, that is 16MB.
> If we run out of those, we just fail the operation and we cannot add
> more memory.
> We could fallback to base pages as x86_64 does, but we can do better.
> One way to mitigate all these issues is to simply allocate memmap array
> (which is the largest memory footprint of the physical memory hotplug)
> from the hotadded memory itself. VMEMMAP memory model allows us to map
> any pfn range so the memory doesn't need to be online to be usable
> for the array. See patch 3 for more details. In short I am reusing an
> existing vmem_altmap which wants to achieve the same thing for nvdim
> device memory.
> There is also one potential drawback, though. If somebody uses memory
> hotplug for 1G (gigantic) hugetlb pages then this scheme will not work
> for them obviously because each memory block will contain reserved
> area. Large x86 machines will use 2G memblocks so at least one 1G page
> will be available but this is still not 2G...
> I am not really sure somebody does that and how reliable that can work
> actually. Nevertheless, I _believe_ that onlining more memory into
> virtual machines is much more common usecase. Anyway if there ever is a
> strong demand for such a usecase we have basically 3 options a) enlarge
> memory blocks even more b) enhance altmap allocation strategy and reuse
> low memory sections to host memmaps of other sections on the same NUMA
> node c) have the memmap allocation strategy configurable to fallback to
> the current allocation.
> [Overall design]:
> Let us say we hot-add 2GB of memory on a x86_64 (memblock size = 128M).
> That is:
> - 16 sections
> - 524288 pages
> - 8192 vmemmap pages (out of those 524288. We spend 512 pages for each section)
> The range of pages is: 0xffffea0004000000 - 0xffffea0006000000
> The vmemmap range is: 0xffffea0004000000 - 0xffffea0004080000
> 0xffffea0004000000 is the head vmemmap page (first page), while all the others
> are "tails".
> We keep the following information in it:
> - Head page:
> - head->_refcount: number of sections
> - head->private : number of vmemmap pages
> - Tail page:
> - tail->freelist : pointer to the head
> This is done because it eases the work in cases where we have to compute the
> number of vmemmap pages to know how much do we have to skip etc, and to keep
> the right accounting to present_pages.
> When we want to hot-remove the range, we need to be careful because the first
> pages of that range, are used for the memmap maping, so if we remove those
> first, we would blow up while accessing the others later on.
> For that reason we keep the number of sections in head->_refcount, to know how
> much do we have to defer the free up.
> Since in a hot-remove operation, sections are being removed sequentially, the
> approach taken here is that every time we hit free_section_memmap(), we decrease
> the refcount of the head.
> When it reaches 0, we know that we hit the last section, so we call
> vmemmap_free() for the whole memory-range in backwards, so we make sure that
> the pages used for the mapping will be latest to be freed up.
> The accounting is as follows:
> Vmemmap pages are charged to spanned/present_paged, but not to manages_pages.
> I yet have to check a couple of things like creating an accounting item
> like VMEMMAP_PAGES to show in /proc/meminfo to ease to spot the memory that
> went in there, testing Hyper-V/Xen to see how they react to the fact that
> we are using the beginning of the memory-range for our own purposes, and to
> check the thing about gigantic pages + hotplug.
> I also have to check that there is no compilation/runtime errors when
> But before that, I would like to get people's feedback about the overall
> design, and ideas/suggestions.
> [1]
> Michal Hocko (3):
> mm, memory_hotplug: cleanup memory offline path
> mm, memory_hotplug: provide a more generic restrictions for memory
> hotplug
> mm, sparse: rename kmalloc_section_memmap, __kfree_section_memmap
> Oscar Salvador (1):
> mm, memory_hotplug: allocate memmap from the added memory range for
> sparse-vmemmap
> arch/arm64/mm/mmu.c | 10 ++-
> arch/ia64/mm/init.c | 5 +-
> arch/powerpc/mm/init_64.c | 7 ++
> arch/powerpc/mm/mem.c | 6 +-
> arch/s390/mm/init.c | 12 ++-
> arch/sh/mm/init.c | 6 +-
> arch/x86/mm/init_32.c | 6 +-
> arch/x86/mm/init_64.c | 20 +++--
> drivers/hv/hv_balloon.c | 1 +
> drivers/xen/balloon.c | 1 +
> include/linux/memory_hotplug.h | 42 ++++++++--
> include/linux/memremap.h | 2 +-
> include/linux/page-flags.h | 23 +++++
> kernel/memremap.c | 9 +-
> mm/compaction.c | 8 ++
> mm/memory_hotplug.c | 186 +++++++++++++++++++++++++++++------------
> mm/page_alloc.c | 47 ++++++++++-
> mm/page_isolation.c | 13 +++
> mm/sparse.c | 124 +++++++++++++++++++++++++--
> mm/util.c | 2 +
> 20 files changed, 431 insertions(+), 99 deletions(-)

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