Date:   Wed, 26 Jun 2019 10:13:25 +0200
From:   Oscar Salvador <osalvador@...e.de>
To:     David Hildenbrand <david@...hat.com>
Cc:     akpm@...ux-foundation.org, mhocko@...e.com,
        dan.j.williams@...el.com, pasha.tatashin@...een.com,
        Jonathan.Cameron@...wei.com, anshuman.khandual@....com,
        vbabka@...e.cz, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 4/5] mm,memory_hotplug: allocate memmap from the added
 memory range for sparse-vmemmap

On Tue, Jun 25, 2019 at 10:49:10AM +0200, David Hildenbrand wrote:
> On 25.06.19 09:52, Oscar Salvador wrote:
> > Physical memory hotadd has to allocate a memmap (struct page array) for
> > the newly added memory section. Currently, alloc_pages_node() is used
> > for those allocations.
> > 
> > This has some disadvantages:
> >  a) an existing memory is consumed for that purpose
> >     (~2MB per 128MB memory section on x86_64)
> >  b) if the whole node is movable then we have off-node struct pages
> >     which has performance drawbacks.
> > 
> > a) has turned out to be a problem for memory hotplug based ballooning
> >    because the userspace might not react in time to online memory while
> >    the memory consumed during physical hotadd consumes enough memory to
> >    push system to OOM. 31bc3858ea3e ("memory-hotplug: add automatic onlining
> >    policy for the newly added memory") has been added to workaround that
> >    problem.
> > 
> > I have also seen hot-add operations failing on powerpc due to the fact
> > that we try to use order-8 pages. If the base page size is 64KB, this
> > gives us 16MB, and if we run out of those, we simply fail.
> > One could arge that we can fall back to basepages as we do in x86_64, but
> > we can do better when CONFIG_SPARSEMEM_VMEMMAP is enabled.
> > 
> > Vmemap page tables can map arbitrary memory.
> > That means that we can simply use the beginning of each memory section and
> > map struct pages there.
> > struct pages which back the allocated space then just need to be treated
> > carefully.
> > 
> > Implementation wise we reuse vmem_altmap infrastructure to override
> > the default allocator used by __vmemap_populate. Once the memmap is
> > allocated we need a way to mark altmap pfns used for the allocation.
> > If MHP_MEMMAP_{DEVICE,MEMBLOCK} flag was passed, we set up the layout of the
> > altmap structure at the beginning of __add_pages(), and then we call
> > mark_vmemmap_pages().
> > 
> > Depending on which flag is passed (MHP_MEMMAP_DEVICE or MHP_MEMMAP_MEMBLOCK),
> > mark_vmemmap_pages() gets called at a different stage.
> > With MHP_MEMMAP_MEMBLOCK, we call it once we have populated the sections
> > fitting in a single memblock, while with MHP_MEMMAP_DEVICE we wait until all
> > sections have been populated.
> 
> So, only MHP_MEMMAP_DEVICE will be used. Would it make sense to only
> implement one for now (after we decide which one to use), to make things
> simpler?
> 
> Or do you have a real user in mind for the other?

Currently, only MHP_MEMMAP_DEVICE will be used, as we only pass flags from
acpi memory-hotplug path.

All the others: hyper-v, Xen,... will have to be evaluated to see which one
do they want to use.

Although MHP_MEMMAP_DEVICE is the only one used right now, I introduced
MHP_MEMMAP_MEMBLOCK to give the callers the choice of using MHP_MEMMAP_MEMBLOCK
if they think that a strategy where hot-removing works in a different granularity
makes sense.

Moreover, since they both use the same API, there is no extra code needed to
handle it. (Just two lines in __add_pages())

This arose here [1].

[1] https://patchwork.kernel.org/project/linux-mm/list/?submitter=137061

-- 
Oscar Salvador
SUSE L3

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