Re: [PATCHv7 02/14] mm: Add support for unaccepted memory

Vlastimil Babka
2022-08-05T13:39:01Z

On 8/5/22 14:09, David Hildenbrand wrote:
> On 05.08.22 13:49, Vlastimil Babka wrote:
>> On 6/14/22 14:02, Kirill A. Shutemov wrote:
>>> UEFI Specification version 2.9 introduces the concept of memory
>>> acceptance. Some Virtual Machine platforms, such as Intel TDX or AMD
>>> SEV-SNP, require memory to be accepted before it can be used by the
>>> guest. Accepting happens via a protocol specific to the Virtual Machine
>>> platform.
>>>
>>> There are several ways kernel can deal with unaccepted memory:
>>>
>>> 1. Accept all the memory during the boot. It is easy to implement and
>>> it doesn't have runtime cost once the system is booted. The downside
>>> is very long boot time.
>>>
>>> Accept can be parallelized to multiple CPUs to keep it manageable
>>> (i.e. via DEFERRED_STRUCT_PAGE_INIT), but it tends to saturate
>>> memory bandwidth and does not scale beyond the point.
>>>
>>> 2. Accept a block of memory on the first use. It requires more
>>> infrastructure and changes in page allocator to make it work, but
>>> it provides good boot time.
>>>
>>> On-demand memory accept means latency spikes every time kernel steps
>>> onto a new memory block. The spikes will go away once workload data
>>> set size gets stabilized or all memory gets accepted.
>>>
>>> 3. Accept all memory in background. Introduce a thread (or multiple)
>>> that gets memory accepted proactively. It will minimize time the
>>> system experience latency spikes on memory allocation while keeping
>>> low boot time.
>>>
>>> This approach cannot function on its own. It is an extension of #2:
>>> background memory acceptance requires functional scheduler, but the
>>> page allocator may need to tap into unaccepted memory before that.
>>>
>>> The downside of the approach is that these threads also steal CPU
>>> cycles and memory bandwidth from the user's workload and may hurt
>>> user experience.
>>>
>>> Implement #2 for now. It is a reasonable default. Some workloads may
>>> want to use #1 or #3 and they can be implemented later based on user's
>>> demands.
>>>
>>> Support of unaccepted memory requires a few changes in core-mm code:
>>>
>>> - memblock has to accept memory on allocation;
>>>
>>> - page allocator has to accept memory on the first allocation of the
>>> page;
>>>
>>> Memblock change is trivial.
>>>
>>> The page allocator is modified to accept pages on the first allocation.
>>> The new page type (encoded in the _mapcount) -- PageUnaccepted() -- is
>>> used to indicate that the page requires acceptance.
>>>
>>> Architecture has to provide two helpers if it wants to support
>>> unaccepted memory:
>>>
>>> - accept_memory() makes a range of physical addresses accepted.
>>>
>>> - range_contains_unaccepted_memory() checks anything within the range
>>> of physical addresses requires acceptance.
>>>
>>> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
>>> Acked-by: Mike Rapoport <rppt@linux.ibm.com> # memblock
>>> Reviewed-by: David Hildenbrand <david@redhat.com>
>>
>> Hmm I realize it's not ideal to raise this at v7, and maybe it was discussed
>> before, but it's really not great how this affects the core page allocator
>> paths. Wouldn't it be possible to only release pages to page allocator when
>> accepted, and otherwise use some new per-zone variables together with the
>> bitmap to track how much exactly is where to accept? Then it could be hooked
>> in get_page_from_freelist() similarly to CONFIG_DEFERRED_STRUCT_PAGE_INIT -
>> if we fail zone_watermark_fast() and there are unaccepted pages in the zone,
>> accept them and continue. With a static key to flip in case we eventually
>> accept everything. Because this is really similar scenario to the deferred
>> init and that one was solved in a way that adds minimal overhead.
>
> I kind of like just having the memory stats being correct (e.g., free
> memory) and acceptance being an internal detail to be triggered when
> allocating pages -- just like the arch_alloc_page() callback.

Hm, good point about the stats. Could be tweaked perhaps so it appears
correct on the outside, but might be tricky.

> I'm sure we could optimize for the !unaccepted memory via static keys
> also in this version with some checks at the right places if we find
> this to hurt performance?

It would be great if we would at least somehow hit the necessary code only
when dealing with a >=pageblock size block. The bitmap approach and
accepting everything smaller uprofront actually seems rather compatible. Yet
in the current patch we e.g. check PageUnaccepted(buddy) on every buddy size
while merging.

A list that sits besides the existing free_area, contains only >=pageblock
order sizes of unaccepted pages (no migratetype distinguished) and we tap
into it approximately before __rmqueue_fallback()? There would be some
trickery around releasing zone-lock for doing accept_memory(), but should be
manageable.