Message-ID: <1e5776e4-d01e-fe86-57c3-1c3c27aae52f@redhat.com>
Date:   Thu, 1 Aug 2019 10:17:23 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Oscar Salvador <osalvador@...e.de>, akpm@...ux-foundation.org
Cc:     dan.j.williams@...el.com, pasha.tatashin@...een.com,
        mhocko@...e.com, anshuman.khandual@....com,
        Jonathan.Cameron@...wei.com, vbabka@...e.cz, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 0/5] Allocate memmap from hotadded memory

On 01.08.19 09:39, Oscar Salvador wrote:
> On Thu, Jul 25, 2019 at 06:02:02PM +0200, Oscar Salvador wrote:
>> Here we go with v3.
>>
>> v3 -> v2:
>>         * Rewrite about vmemmap pages handling.
>>           Prior to this version, I was (ab)using hugepages fields
>>           from struct page, while here I am officially adding a new
>>           sub-page type with the fields I need.
>>
>>         * Drop MHP_MEMMAP_{MEMBLOCK,DEVICE} in favor of MHP_MEMMAP_ON_MEMORY.
>>           While I am still not 100% if this the right decision, and while I
>>           still see some gaining in having MHP_MEMMAP_{MEMBLOCK,DEVICE},
>>           having only one flag ease the code.
>>           If the user wants to allocate memmaps per memblock, it'll
>>           have to call add_memory() variants with memory-block granularity.
>>
>>           If we happen to have a more clear usecase MHP_MEMMAP_MEMBLOCK
>>           flag in the future, so user does not have to bother about the way
>>           it calls add_memory() variants, but only pass a flag, we can add it.
>>           Actually, I already had the code, so add it in the future is going to be
>>           easy.
>>
>>         * Granularity check when hot-removing memory.
>>           Just checking that the granularity is the same.
>>
>> [Testing]
>>
>>  - x86_64: small and large memblocks (128MB, 1G and 2G)
>>
>> So far, only acpi memory hotplug uses the new flag.
>> The other callers can be changed depending on their needs.
>>
>> [Coverletter]
>>
>> This is another step to make memory hotplug more usable. The primary
>> goal of this patchset is to reduce memory overhead of the hot-added
>> memory (at least for SPARSEMEM_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) it is a 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.
>>
>> 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 hot-added memory itself. SPARSEMEM_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.
>> This feature is only usable when CONFIG_SPARSEMEM_VMEMMAP is set.
>>
>> [Overall design]:
>>
>> 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_ON_MEMORY 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().
>>
>> MHP_MEMMAP_ON_MEMORY flag parameter will specify to allocate memmaps
>> from the hot-added range.
>> If callers wants memmaps to be allocated per memory block, it will
>> have to call add_memory() variants in memory-block granularity
>> spanning the whole range, while if it wants to allocate memmaps
>> per whole memory range, just one call will do.
>>
>> Want to add 384MB (3 sections, 3 memory-blocks)
>> e.g:
>>
>> add_memory(0x1000, size_memory_block);
>> add_memory(0x2000, size_memory_block);
>> add_memory(0x3000, size_memory_block);
>>
>> or
>>
>> add_memory(0x1000, size_memory_block * 3);
>>
>> One thing worth mention is that vmemmap pages residing in movable memory is not a
>> show-stopper for that memory to be offlined/migrated away.
>> Vmemmap pages are just ignored in that case and they stick around until sections
>> referred by those vmemmap pages are hot-removed.
> 
> Gentle ping :-)
> 

I am not yet sure about two things:


1. Checking uninitialized pages for PageVmemmap() when onlining. I
consider this very bad.

I wonder if it would be better to remember for each memory block the pfn
offset, which will be used when onlining/offlining.

I have some patches that convert online_pages() to
__online_memory_block(struct memory block *mem) - which fits perfect to
the current user. So taking the offset and processing only these pages
when onlining would be easy. To do the same for offline_pages(), we
first have to rework memtrace code. But when offlining, all memmaps have
already been initialized.


2. Setting the Vmemmap pages to the zone of the online type. This would
mean we would have unmovable data on pages marked to belong to the
movable zone. I would suggest to always set them to the NORMAL zone when
onlining - and inititalize the vmemmap of the vmemmap pages directly
during add_memory() instead.

-- 

Thanks,

David / dhildenb

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